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QP34  .Sm5  1 907     A  manual  of  vetenna 


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COLLEGE  OF  PHYSICIANS 
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A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


A  MANUAL 


OF 


VETERINARY  PHYSIOLOGY 


COLONEL   F.   SMITH   C.B.,   C.M.G. 

Army  Veterinary  Staff 

fellow  of  the  royal  oolleoe  of  veterinary  surgeons 

fellow  of  the  institute  of  chemistry 

author  of  '  a  manual  of  veterinary  hygiene,'  etc. 


THIRD  EDITION 
COMPLETELY  REVISED  AND   IN  PARTS  RE-WRITTEN 


New  York 
WILLIAM  E.  JENKINS  CO. 

PUBLISHERS 

851-853  Sixth  Avenue 


1907 
{A//  Rights  Reserved) 


\^o 


TO    THE    MEMOKY   OF 

Sm  MICHAEL  FOSTEE 

K.C.B.,  M.A.,  M.D..  LL.D.,  D.C.L.,  F.R.S. 

THIS   ATTEMPT   TO    DEAL    WITH    A    BKANCH    OF    PHYSIOLOGY 

IS    DEDICATED 

IX    ACKNOWLEDGMENT    OF   THE 

ENCOURAGEMENT    AND   ASSISTANCE    HE    GAVE 

THE   AUTHOR 

IN     PROSECUTING     THE     STUDY     OF     VETERINAIIY     PHYSIOLOGY 


PREFACE  TO  THE  THIRD  EDITION 

Circumstances  beyond  my  concrol  have  delayed  the  re- 
vision of  this  manual.  During  the  twelve  years  which 
have  elapsed  since  the  last  edition  was  published  con- 
siderable additions  have  been  made  to  our  knowledge  of 
physiology.  This  has  necessitated  the  manual  being 
practically  rewritten ;  only  the  chapters  on  the  Senses, 
Locomotion,  and  the  Foot  stand  nearly  as  they  were ;  the 
others  have  been  partly  or  wholly  rewritten. 

This  book  is  essentially  a  veterinary,  and  not  a  com- 
parative, physiology.  It  treats  of  physiology  not  only  from 
its  theoretical  aspect,  but  from  the  point  of  view  of  clinical 
utility.  The  requirements  of  the  student  and  practitioner 
have  consequently  not  been  lost  sight  of,  and  every  oppor- 
tunity has  been  taken  in  the  text  to  point  out  the  clinical 
application  of  physiological  facts.  To  several  chapters  a 
special  pathological  appendix  is  added,  in  order  to  enforce 
the  lesson  that  pathology  is  only  physiology  out  of  health. 
In  the  chapter  on  the  Nervous  System  the  appendix  has 
been  omitted,  not  because  the  pathological  side  is  wanting 
in  interest,  but  for  the  reason  that  it  is  at  present  so 
defective  in  exactitude. 

As  in  previous  editions,  the  horse  has  been  taken  as  the 
type.  Though  he  offers  so  many  physiological  peculiarities 
and  differences  from  other  animals,  still  his  physiology 
among  quadrupeds  must  always  be  of  the  first  importance, 
and  of  the  greatest  practical  interest. 

By  the  process  of  elimination  and  compression,  room  has 
been  found  for  much  more  material  than  existed  in  the 
previous  editions,  without  adding  unduly  to  the  bulk  of 
the  book.  The  digestive  system,  owing  to  its  extraordinary 
importance  in  herbivora,  is  dealt  with  very  fully. 

My  cordial  thanks  are  due  to  Professor  Sherrington, 
F.R.S.,    who    has    again   very   kindly    read    the    Nervous 


viii  PKEFACE  TO  THE  THIRD  EDITION 

System,  and  contributed  some  new  matter  on  the 
'  Scratch  Reflex  '  and  '  Stepping '  in  the  dog. 

My  friend  Dr.  Sheridan  Lea,  F.Pt.S.,  has  taken  a  deep 
interest  in  the  production  of  this  edition.  As  an  old 
teacher  of  physiology  he  was  able  to  advise  me  of  those 
points  which  most  students  find  some  difficulty  in  grasping, 
and  he  has  rendered  the  text  of  these  portions  clearer  and 
more  accurate  by  his  careful  revision  and  additions.  He 
has  kindly  read  all  the  proofs,  amplified  the  chapter  on  the 
Muscular  System,  and  brought  the  final  chemical  chapter 
up  to  date.  I  am  glad  of  this  opportunity  of  thanking  him 
for  his  criticism  and  invaluable  assistance. 

Mr.  Goodall,  F.E.C.V.S.,  Christchurch,  and  Mr.  Leeney, 
M.R.C.V.S.,  Hove,  have  both  been  good  enough  to  supply 
me  with  information  for  the  chapter  on  Generation  and 
Development,  based  on  their  special  experience.  For  the 
information  contained  in  the  footnote  on  p.  608  I  am 
indebted  to  Mr.  Leach,  F.E.C.V.S.,  Newmarket. 

As  a  rule  no  references  have  been  made  in  the  text  to 
published  works  and  papers,  excepting  where  such  appeared 
desirable.  The  literature  of  the  subject  is  immense,  but  I 
must  not  omit  to  mention  the  help  I  have  obtained  from 
the  Manuals  and  Text-books  on  Physiology  published  by 
Professors  Halliburton,  Howell  (of  Baltimore),  Starling, 
Stewart  (of  Chicago),  and  from  Dr.  Leonard  Hill's  '  Recent 
Advances  in  Physiology  and  Bio-Chemistry.' 

The  number  of  figures  in  the  text  has  been  considerably 
increased,  some  of  them  being  new  and  original.  I  am 
indebted  to  Professor  Stewart  for  permission  to  use  many 
of  those  illustrating  his  'Manual  of  Physiology,'  to  Pro- 
fessor Cossar  Ewart,  F.R.S.,  for  the  figures  illustrating  the 
early  embryos  of  the  horse,  and  to  Messrs.  Macmillan  for 
the  use  of  certain  figures  in  Foster's  *  Text-book  of  Physi- 
ology '  and  Huxley's  '  Lessons  in  Elementary  Physiology.' 
My  thanks  are  also  due  to  Messrs.  Stahel  of  Wiirzburg,  for 
permission  to  publish  Fig.  159  from  Dr.  Michel's  paper  'Zur 
Kenntnis  der  Giirber'schen  Serum- Albumin-Krystalle.' 

F.  S. 

London,  August,  1907. 


PREFACE  TO   THE  SECOND   EDITION 

I  GREATLY  regret  the  considerable  delay  which  has  occurred 
in  the  production  of  this  edition,  but  it  has  been  unavoid- 
able ;  the  work  has  been  rewritten  in  order  to  admit  of  its 
scope  being  enlarged,  and  this  has  taken  longer  than  I 
anticipated. 

In  the  revision  of  the  sheets  I  have  received  great 
assistance  from  Professors  M'Kendrick,  Halliburton,  Hay- 
craft,  Sherrington,  and  Dr.  Waller.  Professor  Sherrington 
revised  the  whole  of  the  Nervous  System  and  supplied 
Figure  58.  Professor  Mettam,  of  the  Eoyal  Veterinary 
College,  Edinburgh,  kindly  wrote  the  chapter  dealing  with 
the  Development  of  the  Ovum,  while  to  Professor  Macqueen, 
of  the  London  Veterinary  College,  I  am  indebted  for  many 
useful  suggestions  and  valuable  criticism. 

To  all  these  gentlemen  I  offer  my  cordial  thanks ;  their 
corrections,  suggestions,  and  criticisms  have  been  of  the 
greatest  help,  and  cannot  fail  to  enhance  the  value  of  the 
book. 

As  in  the  first  edition,  I  have  avoided  dealing  with 
histology,  excepting  where  such  was  necessary  to  the  clear 
understanding  of  the  subj-^ct  under  consideration. 

After  due  deliberation,  I  determined  not  to  introduce  for 
the  present  the  metrical  system  of  weights  and  measures. 

The  number  of  illustrations  has  been  doubled,  and  for 
electrotypes  of  Ijlocks  I  am  indebted  to  Professor  Foster,  of 
Cambridge ;  Professor  M'Kendrick,  of  Glasgow  ;  Professor 
Hamilton,  of  Aberdeen  ;  and  Dr.  Waller,  of  London. 

The  rewriting  of  this  edition  has  unfortunately  necessi- 
tated an  increase  in  the  size  of  the  book. 

Woolwich, 

September^  1895. 


PEEFACE   TO   THE   FIRST   EDITION 

My  object  throughout  this  manual  has  been  to  condense 
the  information  as  much  as  possible,  for  which  purpose  I 
have  omitted  all  special  reference  to  the  physiology  of  the 
dog,  and  have  not  touched  upon  the  histology  of  the  tissues, 
or  methods  of  physiological  inquiry. 

The  reasons  for  these  omissions  are  obvious :  special 
canine  physiology  is  of  subordinate  interest  to  the  pro- 
fession, and  our  information  about  this  animal  is  so  com- 
plete, that  when  required  no  difficulty  is  experienced  in 
obtaining  it  from  human  text-books.  The  histology  of  the 
tissues  is  already  before  the  profession,  and  methods  of 
physiological  inquiry  are  only  needed  for  laboratory  work, 
for  which  purpose  this  book  is  not  intended. 

In  the  description  of  the  physiology  of  the  various  organs 
and  tissues  the  horse  is  necessarily  taken  as  the  type,  but 
the  ox,  sheep,  and  pig  are  dealt  with  wherever  their  special 
physiology  requires  it. 

It  was  my  original  intention  to  publish  nothing  until  I 
had  gone  over  the  field  of  equine  physiology,  but  I  found 
after  several  years  of  work,  that  the  information  I  had 
collected  was  a  mere  drop  in  the  ocean,  for  inquiries  of  this 
kind  are  necessarily  slow,  and  as  there  appeared  no  reason- 
able prospect  of  covering  within  the  space  of  one  life  the 
ground  I  had  mapped  out,  I  was  advised  that  only  good 
could  result  from  placing  on  record  what  little  we  know  of 
veterinary  physiology. 

I  have,  therefore,  ventured,  I  know  well  how  imperfectly, 
to  state  the  broad  facts  of  the  science,  so  as  to  render  them 
of  use  to  the  student  and  practitioner.  The  work  does  not 
pretend  to  be  anything  more  than  a  stepping-stone  to  the 
study  of  physiology ;  for  those  requiring  more  detailed 
information,  reference  must  be  made  to  the  various  text- 
books of  human  and  comparative  physiology  which  are 
available. 


xii  PREFACE  TO  THE  FIRST  EDITION 

Incomplete  as  the  work  is,  it  would  have  been  still  more 
so  but  for  the  assistance  I  have  received  from  my  friend 
Dr.  Sheridan  Lea,  F.R.S.,  of  Caius  College,  Cambridge, 
who,  at  great  personal  inconvenience,  has  kindly  read  and 
revised  nearly  all  the  sheets  as  they  passed  through  the 
press.  In  saying  this,  and  expressing  to  him  my  very 
great  indebtedness,  I  in  no  way  wish  to  shift  the  respon- 
sibility for  error  or  inaccuracy  which  may  exist,  l)ut  I  feel 
that  whatever  merit  the  book  possesses  is  entirely  due 
to  him. 

I  have  to  thank  Professor  Michael  Foster,  F.R.S.,  for 
the  loan  of  many  of  the  woodcuts  which  illustrate  this 
manual,  and  elsewhere  I  have  acknowledged  how  much  I 
owe  to  his  encouragement. 

To  my  friend  and  colleague,  Assistant-Professor  Butler, 
A.V.D.,  my  best  thanks  are  due  for  assistance  in  revising 
the  proofs,  and  in  the  preparation  of  the  index ;  to  Mr.  W. 
Hunting,  F.R.C.V.S.,  for  suggestions  on  the  chapter  dealing 
with  Locomotion  ;  and  to  Professor  M'Fadyean  for  the  loan 
of  two  woodcuts  illustrating  the  chapter  on  the  Foot. 

To  facilitate  the  study  of  locomotion,  I  have  had  the 
plates  so  arranged  as  to  face  as  nearly  as  possible  the 
letterpress  describing  the  movements. 

I  have  laid  under  contribution  Colin's  invaluable  *  Traite 
de  Physiologie  comparee  des  Animaux ' ;  Ellcnberger's 
'Physiologie  der  Haussiiugethiere  ' ;  Foster's,  M'Kendrick's, 
and  Landois  and  Stirling's  Text-books  of  Physiology; 
Gamgee's  translation  of  '  Hermann's  Physiology  ' ;  the 
same  author's  *  Physiological  Chemistry  of  the  Animal 
Body ' ;  Halliburton's  '  Text-book  of  Chemical  Physiology 
and  Pathology ' ;  Bunge's  '  Physiological  and  Pathological 
Chemistry ' ;  Meade  Smith's  '  Physiology  of  the  Domestic 
Animals,'  and  others  mentioned  in  the  text.  With  reference 
to  Dr.  Meade  Smith's  work,  I  regret  to  find  that  on  page  105 
I  have  inadvertently  given  the  title  as  '  A  Text-book  of 
Comparative  Physiology.' 

I  have  endeavoured  to  acknowledge  all  sources  of  in- 
formation, though  it  is  possible  that  in  drawing  from  such 
a  wide  area  I  may  have  omitted  in  places  to  do  so. 

Army  Veterinary  Schuol,  Aldershot, 
August,  1892. 


CONTENTS 


CHAPTER  PAGE 

I.    THE   BLOOD  ------  1 

II.    THE   HEART-                 -                 -                 -                 -  -  28 

III.  THE   BLOODVESSELS-                 -                 -                 -  -  55 

IV.  RESPIRATION                -                 -                 -                 -  -  84 
V.    DIGESTION    -                 -                 -                 -                 -  -  131 

VI.    THE    LIVER   AND   PANCREAS                  -                 -  -  218 

VIL    ABSORPTION                  -                 -                -                 -  -  242 

VIII.    DUCTLESS    GLANDS   AND    INTERNAL   SECRETIONS  -  264 

IX.    THE   SKIN     -                 -                 -                 -                 -  -  271 

X.    THE   URINE-                 .                 -                 .                 .  .  285 

XL    NUTRITION  -                 -                 -                 -                 -  -  314 

XII.    ANIMAL   HEAT             _                 .                 -                 -  .  336 

XIII.  THE   MUSCULAR   SYSTEM        -                 -                 -  -  352 

XIV.  THE   NERVOUS   SYSTEM           -                 -                 -  _  382 
XV.    THE   SENSES                 ....  -  454 

XVI.   THE   LOCOMOTOR   APPARATUS                -                 -  -  505 

XVII.    THE   FOOT    ------  537 

XVIII.    GENERATION    AND   DEVELOPIMENT      -                 -  -  577 

XIX.    GROWTH,    DECAY,    AND   DEATH            -                 -  -  623 

XX.   THE   CHEMICAL   CONSTITUENTS   OF   THE   BODY  -  632 


INDEX 


667 


Faliionliuit.  Centigrade. 

Water  bo/ls  . 


SI  2° 


Inches 



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10 

— 

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= 

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j 

32° 


A  COMPAKISON  OF    SOME  BEITISH  AND  METRIC 

UNITS. 


Degrees  Fahrenheit  =  |  C."  +  32. 
Degrees  Centigrade  =1;  (F.°  — 32). 


Length  - 


fl  inch  =  25*4  millimetres 
1  foot  =  304-8 
1  yard 

1  mile  =  1609-3  metres 
5  miles 


1  metre  =  1,000  millimetres  = 
1  centimetre  =  ^^g  metre  = 
1  kilometre  =  1,000  metres  = 
H  kilometres    -  -         = 


2-54  centimetres. 
30-48 
91-44 

1-609  kilometres. 

8  kilometres  (nearly). 

39-37  inches. 
0-39  inch. 
0-62  mile. 
5  miles  (nearly). 


(1  grain  =0-064  gramme. 
1  ounce  (avoir.)  =   28-35  grammes  =  457-5  grains. 


Weight  <; 


1  pound 
1  cwt. 
1  ton 


=  453-60 


1  kilogramme  =  1,000  grammes 

1  gramme 

1  milligramme  =  yuoo  gramme 


J  kilogramme  (approx.). 
=  50*8  kilogrammes. 
=  1,016 

=   22  pounds  (avoir.). 
=  15-432  grains. 
=  0-0154  grain. 


Capacity  < 


/ 1  fluid  ounce  =  28-4  cubic  centimetres. 
1  pint              =568-0       „  ,, 

1  gallon  =     4-54  litres. 

1  peck  =     9-08     „ 

1  bushel         =  36-32     „ 

1  cubic  inch  =  16-38  cubic  centimetres. 

1      „     foot    =  28-31  litres. 

1  litre  =  1,000  cubic  centimetres  =  1  cubic  decimetre  =  1-76 

pint  (imperial). 
1  cubic  centimetre  =  0-061  cubic  inch. 
1  cubic  metre  =  1,000  cubic  decimetres  =  35*3  cubic  feet. 


Work 


J 


1  foot-ton      =309-12 


ilki: 


logramme-metre  =  7*25  foot-pounds. 


1  unit  of  heat  (British)  =  heat  necessary  to  raise   1   pound  of  water 

through  1°  F. 
1  calorie  (Metric)  =  heat  necessary  to  raise  1  gramme  of  water  through 

r  c. 

Mechanical  equivalent  of  heat-unit     =772  foot-pounds. 

,,  ,,  calorie  =424  gramme-metres. 

,,  ,,  kilo-calorie  =424  kilogramme-metres. 


CORRIGENDA 

Page    19,  top  line, /or  '  action  '  read  '  difference.' 

59,  line  7  from  hottora,/or  'all'  read  'during.' 
94,  line  17  from  bottom, /or  '  occupy  '  read  '  occupying.' 
140,  line  19  from  top, /or  'Appendix  '  recul  '  Chapter  XX. 
171,  footnote,  'H.  J.  Brown  '  should  be  '  H.  T.  Brown.' 
185,  line  10  from  bottom,  the  semicolon  to  be  a  comma. 
225,  line  9  from  top,  after  '  such  '  insert  '  power.' 
337,  bottom  line,/o?"  'come'  read  'comes.' 
429,  line  5  from  bottom, /or  '  dilatation  '  read  'dilating.' 
439,  line  2  from  bottom,  for  '  cochlea  '  read  '  cochlear.' 
472,  line  8  from  bottom,  for  '  revolves  '  reaA  '  rotates. ' 
584,  line  18  from  top, /or  '  kreatine  '  recul  'creatine.' 
636,  line  15  from  toj),  delete  comma  after  '  all)umin.' 

636,  line  22  from  top,  delete  hyphen  after  '  blood.' 

637,  line  10  from  top,  for  '  albumose  '  read  '  albumoses.' 
637,  line  14  from  toj), /or  '  alchol'  recul  '  alcohol.' 


A  MANUAL  OF  VETERINARY 
PHYSIOLOGY 


CHAPTER   I 

'  THE  BLOOD 

The  special  functions  of  the  blood  are  to  nourish  all  the 
tissues  of  the  body,  and  thus  aid  in  their  growth  and  repair  ; 
to  furnish  material  for  the  purpose  of  the  body  secretions, 
to  supply  the  organism  with  oxygen,  without  which  life  is 
impossible,  and  finally  to  convey  from  the  tissues  tlie  pro- 
ducts of  their  activity.  To  enable  all  this  to  be  carried  out 
the  blood  is  constantly  in  circulation,  is  rapidly  renewed,  is 
instantaneously  purified  in  the  lungs  and,  by  means  of 
certain  channels,  is  placed  directly  in  communication  with 
the  nourishing  fluid  absorbed  from  the  intestines,  by  which 
it  is  constantly  repaired. 

Physical  Characters, — Blood  is  a  red,  opaque,  rather 
viscous  fluid,  the  tint  of  which  depends  upon  whether  it  is 
drawn  from  an  artery  or  a  vein  ;  in  the  former  it  is  of  a 
bright  scarlet  colour,  whilst  in  the  latter  it  is  of  a  purplish 
red.  The  colour  is  due  to  a  pigment  called  hitmoglobin 
contained  in  the  red  corpuscles.  Whether  the  colour  is 
scarlet,  as  in  blood  from  an  artery,  or  purplish,  as  from  a 
vein,  depends  on  the  difference  in  the  amount  of  oxygen 
with  which  the  hasmoglobin  is  combined. 

The  reaction  of  blood  is  alkaline ;  as  the  process  of 
coagulation  occurs  this  alkalinity  diminishes.  The  alkaline 
reaction  is  due  to  the  phosphate  and  bicarbonate  of  soda 
found  in  the  fluid ;  the  decreasing  alkalinity  observed  on 

1 


2      A  IMANUAL  OF  VETERINARY  PHYSIOLOGY 

standing  is  probably  due  to  the  formation  of  an  acid.  The 
alkahnity  of  the  blood  is  reduced  by  muscular  work,  owing 
to  the  production  of  an  acid  by  the  muscles. 

The  odour  of  blood  is  believed  to  be  due  to  a  volatile 
bod}^  of  the  fatty  acid  series.  The  blood  of  the  cat  and  dog 
has  a  peculiar  and  decidedly  disagreeable  smell ;  this  is 
not  observed  in  the  blood  of  the  horse  and  ox,  though  it  is 
said  that  the  odour  of  butyric  acid  can  always  be  obtained 
from  the  blood  of  the  latter  by  heating  it  with  sulphuric 
acid.  The  taste  of  blood  is  saltish,  due  to  the  amount  of 
sodium  chloride  it  contains. 

The  specific  gravity  varies  in  different  animals  :  in  the 
horse,  ox,  and  pig,  1060 ;  sheep,  1050-1058 ;  dog,  1050 
(Colin).  According  to  Hoppe-Seyler  the  specific  gravity 
of  the  liquor  sanguinis  of  the  horse  is  1027  to  1028,  and 
the  specific  gravity  of  the  cells  1105.  This  considerable 
difference  between  the  specific  gravity  of  the  cells  and 
the  liquor  sanguinis  in  the  horse,  accounts  for  the  rapid 
manner  in  which  the  cells  sink  in  horses'  blood  when 
drawn  from  the  body,  producing  during  the  process  of 
clotting  the  so-called  '  buffy  coat.' 

The  composition  of  the  blood  is  almost  absolutely  uniform 
so  far  as  the  presence  of  various  substances  is  concerned ; 
the  amount  of  these  substances,  however,  varies  in  animals 
of  different  classes.  The  source  from  which  the  blood  is 
taken  also  affects  its  composition ;  the  blood  from  an  artery 
does  not  represent  exactly  that  found  in  a  vein. 

Blood  consists  of  : 

1.  A  fluid  part,   Liquor  sanguinis  or  Plasma,  containing  in  solution 

proteids,  extractives,  mineral  matter  and  gases,  the  latter  in  a 
state  of  loose  chemical  combination. 

2.  Corpuscles.  a.  Eed  corpuscles. 

^.   White  corpuscles. 
7.  Platelets. 

The  Liquor  Sanguinis,  or  Plasma,  forms  about  66  per  cent, 
of  the  total  blood ;  it  is  an  albuminous  fluid  containing  a 
small  and  variable  amount  of  a  yellow  colouring  matter  of 


THE  BLOOD  3 

a  fatty  nature.  It  holds  in  solution  three  proteids — viz., 
fibrinogen,  serum  globulin  (paraglobulin),  and  serum 
albumin.  Eeeent  researches  have  shown  that  what  has 
always  been  regarded  as  serum  globulin  consists  in  reality 
of  two  proteids,  to  which  distinctive  names  have  been  given. 

It  is  a  simple  matter  to  separate  these  proteids  from 
plasma,  as  they  are  differently  acted  upon  by  neutral  salts. 
For  examjile,  fibrinogen  is  precipitated  by  half  saturation 
with  common  salt,  serum  globulin  is  precipitated  by 
saturation  with  magnesium  sulphate,  serum  albumin  may 
be  wholly  precipitated  by  ammonium  sulphate. 

During  the  life  of  the  blood  the  liquor  sanguinis  is  termed 
the  plasma,  but  after  it  has  been  shed  from  the  body  and 
coagulation  has  occurred,  the  liquid  residue  is  called  serum. 
Serum  is,  therefore,  plasma  which  is  modified  as  the  result 
of  coagulation,  and  as  this  latter  process  is  attended  by  the 
production  of  fibrin,  we  may  say  that  serum  is  plasma 
minus  the  fibrin-forming  elements.  Perhaps  the  nearest 
approach  to  pure  plasma  is  the  fluid  found  in  the  peri- 
cardium and  abdominal  cavity. 

The  fluid  effused  into  the  pleural  cavity  during  pleurisy  is 
plasma  to  start  with,  but  if  the  fibrin  in  it  becomes  thrown 
down  (forming  the  so-called  false  membranes),  the  remain- 
ing fluid  is  serum  which  is  no  longer  capable  of  clotting. 

The  Proteids  of  Serum  are  serum  globulin,  serum  albu- 
min, and  a  ferment  produced  as  the  result  of  coagulation. 
As  fibrinogen  is  used  up  in  the  process  of  coagulation  it  is 
not  found  in  serum,  but  a  proteid  known  as  fibrino-globulin 
appears,  though  in  small  quantities.  Fibrino-globulin  is 
produced  from  fibrinogen  during  the  process  of  fibrin 
formation.  In  the  following  table  a  comparison  is  made 
between  the  proteids  of  the  plasma  and  serum  : 

Proteids  of  the  Plasma.  Proteids  of  the  Serum. 

Fibrinogen.  Serum  globulin. 

Serum  globulin.  Serum  albumin. 

Serum  albumin.  Fibrin  ferment  (nucleo- 

proteid). 
Fibrino-globulin. 

1—2 


4      A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

It  has  been  shown  that  the  proportion  in  which  serum 
globulin  and  serum  albumin  exist  in  the  blood  varies  in 
different  animals.  In  the  horse  and  ox  the  globulins  are 
in  excess  of  the  albumins  ;  in  man  and  the  rabbit  this 
is  reversed.  Analyses  show  that  the  amount  of  total  proteids 
is  rather  more  uniform  than  is  that  of  the  different 
albumins  of  which  they  are  composed,  as  may  be  seen  from 
the  following  table,  which  represents  the  weight  of  proteid 
(grammes  in  100  c.c.)  of  blood  plasma  of  different  animals  : 


Total 

Serum 

Para- 

Proteids. 

A  Ibumin. 

globulin. 

Fibrinogen. 

Dog      -        - 

-       6-03 

3-17 

2-26 

0-60 

Sheep  - 

-       7-29 

3-83 

3-00 

0-46 

Horse  - 

-       8-04 

2-80 

4-79 

0-45 

Pig       - 

-       8-05 

4-42 

2-98 

0-65 

Fibrinogen  is  the  precursor  of  fibrin,  a  substance  of 
which  we  shall  have  more  to  say  when  dealing  with  coagu- 
lation ;  it  is  found  in  blood  plasma,  but  not  in  the  serum, 
since  it  is  converted  into  fibrin  during  the  process  of 
clotting ;  it  also  exists  in  the  fluids  exuded  into  the  cavity 
of  the  chest,  pericardium,  etc. 

Corpuscles. — Blood  examined  under  the  microscope  is 
found  to  consist  of  an  enormous  number  of  bodies  termed 
corpuscles  floating  in  the  liquor  sanguinis.  These  corpuscles 
are  of  two  kinds,  red  and  white ;  the  former  are  the  more 
numerous,  the  latter  are  the  larger. 

The  Red  Corpuscles  constitute  3B  per  cent,  or  one-third  of 
the  total  blood.  Viewed  under  the  microscope,  they  are 
found  to  be  biconcave  discs,  circular  in  shape,  and  possess- 
ing no  nucleus  (Plate  I.) ;  they  are  soft,  flexible,  elastic 
bodies,  capable  of  having  their  shape  readily  altered  by 
pressure,  which  enables  them  to  pass  along  the  finest 
capillaries.  The  colour  of  a  single  corpuscle  is  yellow,  but 
when  heaped  together  they  are  red,  and  thus  give  the 
colour  to  the  blood. 

In  all  mammals  excepting  the  Camel  tribe  the  red  cells 
are  circular ;  in  all  vertebrates  below  mammals  they  are  bi- 
convex, oval,  and  nucleated.     The  corpuscles  vary  in  size 


THE  BLOOD  5 

in  different  animals,  as  may  be  seen  in  the  diagram  (Fig.  1). 
When  a  drop  of  blood  is  shed,  the  red  cells  at  first  move 
quite  freely  each  over  the  other.  In  a  short  time  they 
tend  apparently  to  become  sticky,  and  when  this  state  is 
reached  they  have  a  tendency  to  lie  in  long  rows,  with  their 
flat  surfaces  in  close  contact,  resembling  the  appearance 
of  a  pile  of  pennies.  This  condition  is  not  marked  in 
horses'  blood. 

A  red  blood-cell  is  composed  of  a  spongy  stroma,  holding 
in  its  meshes  the  red  colouring  matter.     The  stroma  or 


ELcnha-ni-  ■0  09lt.mm 

V Man  0  077 

[_.-,    -Goat     ,  OOn 

7_J Musk-deer  ■  00 ZS 


Fig.  1. — Diagram   shov.ing  Relative   Size  of  Eed  Corpuscles  of 
Various  Animals  (Stewart). 

framework  of  the  corpuscle  consists  chiefly  of  nucleo- 
albumin,  and  also  contains  lecithin,  cholesterin,  and  salts ; 
the  red  colouring  matter  consists  of  an  albuminous  crystal- 
line substance,  hemoglobin,  which  forms  no  less  than  90 
to  94  per  cent,  of  the  total  solid  matter  of  the  dried 
corpuscle. 

The  number  of  corpuscles  in  the  blood  is  determined 
approximately  either  by  the  method  of  Gowers  or  Malassez. 
The  principle  on  which  these  methods  are  based  is  the  same 
— a  known  quantity  of  blood  is  diluted  with  a  known  bulk  of 
artificial  serum  and  thoroughly  mixed  ;  of  this  a  small  drop 
is  placed  in  a  counting-chamber,  which  is  ruled  into  squares, 
and  examined  under  the  microscope.  The  blood  cells  occu- 
pying the  squares  are  counted,  as  may  readily  be  done, 
and  the  mean  of  them  taken.  In  the  horse  the  mean 
number  of  red  blood  corpuscles  per  cubic  millimetre  is 
7,212,500,  and  in  the  ox  5,073,000.  Taking  the  amount 
of  blood  in  the  horse  as  66  lbs.  (50  pints  or  29  litres), 


6       A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

this  gives  204,113,750,000,000  as  the  approximate  number 
of  red  cells  in  the  l)ody  (Ellenberger).*  It  is  evident  that 
a  loss  of  water  from  the  blood  means  a  larger  relative  pro- 
portion of  red  cells  present,  while  an  excess  of  water  by 
diluting  the  blood  would  show  a  loss  of  red  cells ;  thus  the 
number  of  the  red  cells  is  increased  by  sweating,  by  the 
excretion  of  water  from  the  bowels  and  kidneys,  and  by 
starvation,  while  it  is  diminished  by  pregnancy  and  copious 
draughts  of  water.  But  apart  from  these  conditions,  it 
is  undoubted  that  an  actual  increase  or  decrease  in  the 
number  of  red  cells  may  occur,  this  numerical  variation 
being  especially  marked  in  some  diseases.  The  shape  of 
the  red  cell  is  affected  by  the  amount  of  fluid  in  the 
plasma — if  the  latter  be  artificially  concentrated  water 
diffuses  from  the  corpuscle  to  the  plasma,  and  in  conse- 
quence it  shrinks  and  becomes  wrinkled.  If  the  plasma  be 
diluted  the  red  cells  swell.  A  "9  per  cent,  solution  of  sodium 
chloride  causes  the  corpuscles  neither  to  shrink  nor  swell ; 
this  strength  is  known  as  '  physiological  salt  solution,'  and 
may  be  employed  for  the  purpose  of  transfusion. 

Each  red  cell  offers  a  certain  absorbing  surface  for  oxygen, 
which,  if  calculated  on  the  total  number  of  corpuscles,  is 
something  enormous,  being  equal  for  the  horse  to  a  square 
having  a  side  of  180  yards.  The  opacity  of  blood  is  due  to 
the  red  cells  reflecting  light  as  the  result  of  their  peculiar 
shape ;  if  the  cells  be  destroyed  either  by  freezing  and 
thawing  the  blood  alternately,  or  by  the  passage  through  it 
of  electric  shocks,  or  by  the  addition  of  certain  agents  such 
as  chloroform,  ether,  bile  salts,  water,  tannic  or  boric 
acids,  etc.,  the  haemoglobin  becomes  liberated  from  the 
broken-up  cell  and  stains  the  naturally  yellow  plasma  of  a 
red  colour.  Further,  the  destruction  of  the  corpuscles 
leads  to  the  blood  becoming  transparent  or,  as  it  is  termed, 
*  laky.' 

The  greater  part  of  the  red  cell  consists,  as  already 
stated,  of  haemoglobin,  a  substance  possessing  a  remarkable 
affinity  for  oxygen ;  this  it  obtains  at  the  lungs  and  leaves 
*  '  Physiologie  der  Haussiiugethiere.' 


THE  BLOOD  7 

behind  it  in  the  tissues.  The  hasmoglobin  of  the  red  cells, 
therefore,  exists  in  two  states,  one  in  which  it  is  charged 
with  oxygen  called  oxy-haemoglobin,  and  the  other  in  which 
it  has  lost  its  oxygen  and  is  known  as  reduced  htemoglobin. 
The  process  of  oxidation  in  the  lungs  and  reduction  in  the 
tissues  is  constantly  occurring  at  every  cycle  of  the  circula- 
tion, with  the  ultimate  result  that  the  red  blood  disc  gets 
worn  out  and  dies.  In  this  condition  it  is  cast  off  from  the 
system,  being  got  rid  of  through  the  medium  of  the  liver, 
and  also,  probably,  destroyed  in  the  spleen  and  elsewhere. 
When  the  red  cells  die  their  haemoglobin  is  set  free,  and 
decomposed  into  an  iron-free  residue  from  which,  probably, 
all  the  pigments  of  the  body  are  formed,  especially  those  of 
the  bile. 

The  production  of  red  cells  is  a  matter  of  extreme 
rapidity,  as  may  be  witnessed,  for  example,  after  haemor- 
rhage ;  the  seat  of  their  formation  is  in  the  red  marrow  of 
bones,  where  they  are  formed  from  certain  nucleated  cells ; 
there  are  several  varieties  of  cells  in  the  red  marrow,  and 
it  is  not  quite  definitely  settled  which  of  these  furnish  the 
red  blood  cells.  All  other  seats  of  formation  are  doubtful, 
though  it  should  be  mentioned  that  the  formation  of  red 
cells  from  blood  platelets  in  the  blood  stream  has  been  put 
forward  as  of  possible  occurrence.  In  the  embryo  the 
future  red  cells  for  a  certain  period  are  nucleated  and 
contain  no  ha3moglobin,  but  these  are  gradually  replaced 
by  non-nucleated  haemoglobin-holding  corpuscles  before 
birth.  It  is  interesting  to  observe  that  both  in  the  embryo 
and  in  the  adult  the  red  cells  are  derived  from  a  nucleated 
precursor. 

Blood  Platelets  are  small  bodies  one-quarter  the  size  of 
a  red  cell,  which  have  been  observed  in  the  circulating 
blood,  but  can  also  be  seen  immediately  after  the  blood  is 
shed.  They  have  been  supposed  by  some  to  be  the  pre- 
cursors of  the  red  cells,  but  this  point  is  not  as  yet  settled. 

We  have  mentioned  that  the  red  colouring  substance 
haemoglobin  is  retained  in  the  pores  of  the  stroma  of  the 
red  cells,  and  with  this  we  must  now  deal. 


8      A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

Haemoglobin  is  a  most  remarkable  substance.  It  is  a 
pi'oteid,  distinguished  from  the  majority  of  the  other  mem- 
bers of  its  class  by  the  comparative  ease  with  which  it  may 
be  obtained  in  a  crystalline  form,  whilst,  on  the  other 
hand,  its  behaviour  in  a  dialyser  is  not  that  of  a  crystalloid 
but  of  a  colloid.  It  is  one  of  the  most  complex  substances 
in  organic  chemistry,  containing  C,  H,  0,  N,  S,  and  Fe, 
and  its  molecule  is  an  enormous  one,  the  molecular  weight 
being  quoted  at  13,000  to  14,000.  Crystals  of  haemoglobin 
when  seen  in  bulk  are  of  a  dark-red  or  bluish-red  colour  ; 
they  are  extremely  soluble  in  water,  the  solution  being 
dichroic — viz.,  green  by  reflected  and  bluish-red  by  trans- 
mitted light.  Hftmoglobin  is  remarkable  as  being  the 
most  important  proximate  constituent  of  the  body  contain- 
ing iron,  the  amount  being  about  "4  per  cent.  The  source 
of  the  iron  is  not  settled,  but  there  is  an  organic  iron-con- 
taining substance  in  food  known  as  luematogcn,  belonging 
to  the  nucleo-albumin  group,  which  possibly  furnishes  it. 
Its  formation  from  inorganic  iron  is  probably  of  doubtful 
occurrence. 

The  total  amount  of  h£emoglobin  in  a  horse's  body  is 
about  8'8  lbs.  (4  kilogrammes),  and  the  amount  of  iron 
contained  in  this  is  about  257  grains  (17  grammes).  This 
calculation  is  based  on  the  assumption  that  the  amount  of 
blood  in  the  body  is  6(i  lbs. 

In  the  dried  red  blood  cells  haemoglobin  exists  in  the 
proportion  of  90  to  94  per  cent.,  in  the  corpuscle  under 
normal  conditions  it  represents  32  per  cent,  of  its  weight, 
while  in  the  total  blood  of  the  horse  it  forms  13*15  per 
cent.,  in  the  ox  9*96  per  cent.,  sheep  1034  per  cent., 
pig  12-7  per  cent.,  and  dog  9'77  per  cent.  (EUenberger).* 
The  younger  the  animal  the  less  hsemoglobin  ;  males  have 
more  than  females,  and  castrated  animals  more  than 
entires  (G.  Miiller).t 

Haemoglobin  has  a  remarkable  affinity  for  oxygen,  and 
the  ordinary  laws  relating  to  the  absorption  of  gases  by 
fluids  and  solids  do  not  apply — as  we  shall  see  later  when 
*  '  Pbysiologie  der  Haussaugethiere.'  f  Ibid. 


THE  BLOOD  9 

dealing  with  Eespiration — to  the  absorption  of  oxygen  by 
hfemoglobin.  According  to  Bohr,  haemoglobin  can  absorb 
carbon  dioxide,  which  combines  with  the  globulin  portion 
of  the  molecule. 

We  have  mentioned  that  when  haemoglobin  is  charged 
with  oxygen  it  is  spoken  of  as  oxy-hiemoglobin  ;  when  it 
has  discharged  its  oxygen,  which  it  is  capable  of  doing  with 
considerable  facility,  it  is  described  as  reduced  hemoglobin, 
or  simply  as  haemoglobin.  As  oxy-haemoglobin  it  is  charged 
with  oxygen  in  the  capillaries  of  the  lungs,  brought  back  to 
the  heart  and  distributed  all  over  the  body ;  in  the  tissues 
it  gives  up  its  oxygen,  and  as  partially  reduced  haemoglobin 
is  brought  back  by  the  veins  to  the  heart  for  distribution 
to  the  lungs,  where  it  renews  its 
oxidized  condition.  Haemoglobin  is 
never  completely  reduced  in  the  body 
excepting  in  the  last  stage  of  asphyxia. 

Oxy  -  haemoglobin  crystallises  in 
some  animals,  horse,  cat,  dog,  and 
guinea-pig,  with  facility  ;  in  others, 
ox,  sheep,  and  pig,  with  difficulty. 
The  crystals  are  generally  rhombic 
plates  and  prisms,  but  the  form 
differs  according  to  the  animal  Fu'-  2.— Crystals  of 
(Fig.    2).     Keduced   hemoglobin  can  i^rNrrSgrnS 

only  be  crystallised  with  great  difficulty  Rel;     c,    Guinea- 

in  an  atmosphere  free  from  oxygen.  ^^^  (Stewart). 

When  examined  by  the  spectroscope  the  two  hemoglo- 
bins produce  quite  distinctive  spectra,  by  which  they  may 
be  readily  recognised.  To  put  the  matter  broadly,  oxy- 
hemoglobin gives  two  well-marked  dark  absorption  bands 
or  shadows  in  the  green  portion  of  the  spectrum,  one  band 
being  wide,  the  other  narrow  ;  while  reduced  hemoglobin 
gives  one  wide  single  band  in  nearly  the  same  position 
(Fig.  3).  Oxy-hemoglobin  may  readily  be  reduced  to 
hemoglobin  by  the  addition  of  Stokes's  Fluid  (an  alkaline 
solution  of  ferrous  tartrate). 

Oxygen  and  hemoglobin  are  so  lightly  bound  together 


10    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

that  they  are  readily  separated  ;  oxygen  is  given  off  if  the 
blood  be  placed  in  a  vacuum  or  boiled,  or  if  it  be  brought 
into  contact  with  indifferent  gases  such  as  nitrogen  and 
hydrogen ;  it  is  the  facility  with  which  haemoglobin  parts 
with  its  oxygen  which  enables  the  tissues  to  obtain  it. 

Haemoglobin   forms    certain   compounds    with    oxygen, 
carbon  monoxide,  and  nitric  oxide  : 

With  oxygen  it  forms  oxy-hsemoglobin  and  methaemoglobin. 
,,      carbon  monoxide  it  forms  CO  haemoglobin. 
,,     nitric  oxide  ,,  NO  „ 


B     ( 

:             D                J 

:                 I 

■ 

Oxy-ha3- 
moglobin.               | 

1 

Hrt-mo-                     1 
gliibiii                      I 
(reduced).               i 

1 

Fig.  3. — Blood  Spkctka  (Waller). 

Oxy-ha3moglobin  we  have  dealt  with  ;  the  others,  in  a 
work  of  this  kind,  can  only  receive  a  short  notice  at  our 
hands,  though  the  subject  is  full  of  interest. 

Methsemoglobin  is  produced  by  allowing  blood  to  be  ex- 
posed to  the  air  until  it  becomes  brown  in  colour  and  acid 
in  reaction  ;  or  it  may  be  prepared  by  the  action  of  acids  or 
alkalies  on  oxy-htemoglobin.  This  substance  separates 
from  its  oxygen  with  difficulty,  and  gives  a  three-banded 
spectrum.  Methicmoglobin  does  not  occur  normally  in 
the  body,  but  may  be  found  in  the  urine  whenever  a 
sudden  breaking  down  of  red  corpuscles  occurs,  as,  for 
example,  in  the  so-called  azoturia  of  the  horse. 

Carboxy-hsemoglobin. — In  this  compound  the  oxygen  is 
replaced  by  carbon  monoxide,  which  forms  a  stable  com- 
pound with  the  haemoglobin  and  is  not  displaced  on  breath- 
ing oxygen  ;  hence  the  rapidly  fatal  results  of  this  form  of 
poisoning.  The  blood  of  people  who  have  died  from  CO 
poisoning  is  of  a  cherry-red  colour,  and  yields  the  spectrum 
of  CO-haemoglobin — viz.,  two  bands  very  much  like  those 


THE  BLOOD  11 

of  oxy-haemoglobin,  though  somewhat  darker  and  situated 
slightly  nearer  to  the  violet  end  of  the  spectrum.  These 
two  bands  are  unaltered  by  Stokes's  Fluid.  Nitric  oxide 
haemoglobin  in  many  respects  resembles  CO-h?emoglobin. 

Haemoglobin  is  easily  decomposed  either  by  boiling  or 

the  addition  of  alkalies,  acids,  or  acid  salts ;  in  either  case 

it  splits  up  into  a  substance  containing  the  iron,  known 

as  haematin,  and  a  proteid  substance  or  substances  termed 

globin.      Haematin   in    the    dry   state    strongly   resembles 

iodine  in  appearance  ;  it  has  a  metallic  lustre,  a  blue-black 

colour,  is  not  crystallisable,  and  yields,  when  pulverised,  a 

dark  brown  powder  which  contains  882  per  cent,  of  iron. 

Hciematin    is    a    remarkably   stable    substance,    and    the 

colouring  matter  presents  a  distinctive  spectrum  both  in  an 

acid  and  alkaline  solution.     Alkaline  solutions  of  htematin 

can  take  up  and  give  off  oxygen  like  haemoglobin.     When 

htematin  is  treated  with  glacial  acetic  acid  and  common 

salt  it  yields  haemin,  which,  when  examined  microscopically, 

is  found  to  consist  of  prismatic  crystals,  dark,  or  nearly 

black  in  colour.     Haemin  crystals  may  be  readily  produced 

by  warming  the  dried  blood  with  a  drop  of  glacial  acetic 

acid  on  a  slide  ;  this  is  used  as  a  microscopical  test,  but  it 

is  said  that  from  the  blood  of  the  ox  and  pig  haemin  can 

only  be  obtained  in  very  irregular  crystalline  masses. 

When  reduced  haemoglobin  is  decomposed  by  acids  or 
alkalies,  oxygen  being  carefully  excluded,  it  yields  haemo- 
chromogen,  a  substance  presenting  a  definite  spectrum  and 
thus  a  ready  means  of  detecting  old  blood-stains.  Haema- 
toporphyrin  is  obtained  by  the  action  of  strong  sulphuric 
acid  on  haematin,  which  thereby  loses  its  iron  ;  haematopor- 
phyrin  is  really  haematin  from  which  the  iron  has  been 
removed  ;  it  is  isomeric  with  bilirubin. 

Hydrobilirubin  is  obtained  by  the  action  of  reducing 
agents  on  h^matin ;  it  very  closely  resembles  urobilin,  a 
pigment  found  in  urine. 

Haematoidin  (Fig.  4)  is  found  in  old  blood-clots  and  in 
the  ovary ;  it  is  a  crystalline  iron-free  product  derived 
from  haematin,  and  gives  the  same  reaction  with  nitrous 


12     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

acid  as  bile  pigment,  viz.,  a  play  of  colours.     Haematoidin 
is,  in   fact,   chemically  identical  with    bilirubin,    and   the 

name  is  now  of  interest  merely  as 
indicating  the  close  genetic  relation- 
ship of  the  pigments  of  bile  to  the 
colouring  matter  of  blood.  Not- 
withstanding this  close  relation- 
ship, it  has  not  as  yet  been  found 
possible  to  convert  hfematin  into 
bilirubin.     The    nearest   approach 

l^L^^;^^Z).     to  W1"-»W"   i«   i^on-free  h«mati„ 

(hffimatoporphyrin).  Again,  both 
haematin  and  bilirubin  may  be  made  to  yield  an  identical 
product  (hydro-bilirubin)  ;  this  product  closely  resembles 
urobilin,  a  pigment  found  in  the  urine,  and  urobilin  beyond 
all  doubt  is  derived  from  bilirubin  in  the  digestive  canal, 
under  the  influence  of  putrefactive  organisms. 

The  White  Corpuscles,  also  termed  leucocytes,  are  found 
in  blood,  lymph,  pus,  connective  tissue,  etc.  ;  they  exist  in 
blood  in  the  proportion  of  1  in  300  to  1  in  700,  the  propor- 
tion varying  according  to  the  vessel  from  which  the  blood 
is  examined.  In  the  splenic  artery  there  are  very  few,  in 
the  splenic  vein  they  are  exceedingly  numerous.  Blood 
which  has  been  removed  from  the  vessels  contains  but  few, 
for  the  reason  that  they  are  probably  broken  down  during 
the  formation  of  librin. 

The  white  corpuscle  is  somewhat  larger  than  the  red  ;  it 
consists  of  a  granular-looking  protoplasm,  within  which  is 
a  nucleus ;  the  nucleus  shows  no  sign  of  a  nuclear  network, 
which  is  a  distinguishing  difference  between  the  white  cell 
and  its  very  close  ally  the  lymph  cell.  The  granular 
condition  of  the  corpuscle  is  due  to  minute  particles  of  fat, 
proteid,  and  probably  other  substances,  which  are  on 
their  way  either  to  or  from  the  tissues,  probably  both. 
There  are  at  least  five  varieties  of  colourless  corpuscles : 
(1)  The  poly  nuclear,  which  are  very  numerous  and  consist 
of  a  cell  containing  two  or  three  nuclei  united  by  fine 
threads ;   (2)  hyaline  leucocytes,  relatively  few  in  number. 


THE  BLOOD 


13 


Containing  a  single  nucleus  and  more  protoplasm ;  (3) 
cosinophile  cells,  consisting  of  large  masses  of  granular 
protoplasm  with  a  simple  or  lobed  nucleus :  the  granules 
stain  deeply  with  eosin  ;  (4)  lymphocytes  derived  from  the 
lymphatic  glands  containing  a  large  spherical  nucleus  and 
limited  protoplasm  ;  (5)  hasophile  leucocytes,  which  are  very 
rare  and  distinguished  by  staining  with  basic  dyes  and  a 
methylene  blue  (Plate  I.j. 

The  white  corpuscles  are  capable  of  undergoing  changes 
in  shape ;  the  movements  known  as  amcehoiel  are  exhibited 
by  projections  shooting  out  from  the  surface  and  being  again 
retracted  (Fig.  5).  The  amoeboid  movements  are  destroyed 
by  heat    or   by    shocks   from   an   induction   coil.      These 


Fig.  5. — Amceboid  Movement. 
A,  B,  C,  D,  Successive  changes  in  the  form  of  an  amceba  (Stewart). 


changes  in  shape  assist  materiallj'  in  the  passage  of  the 
corpuscles  through  the  walls  of  the  vessels  into  the  tissues- 
The  process  is  termed  diapedesis ;  within  moderation  it  is  a 
perfectly  normal  phenomenon,  though  under  inflammatory 
and  other  disturbing  influences  it  becomes  greatly  ex- 
aggerated. The  white  corpuscle  has  the  power  of  taking  up 
into  its  interior  small  particles  of  colouring  matter,  bacteria, 
etc.,  the  importance  of  which  will  presently  be  alluded  to. 

The  white  corpuscles  contain  about  10  per  cent,  of  solids. 
The  cell  protoplasm  consists  of  proteids  belonging  to  the 
globulin  and  nucleo-proteid  groups,  while  the  nucleus  con- 
sists of  nuclein  which  is  remarkable  as  being  a  very  stable 
substance  and  also  as  containing  phosphorus.    The  nucleo- 


14     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

proteid  obtained  from  the  protoplasm  is  probably  the  pre- 
cursor of  the  fibrin  ferment.  Besides  these  we  have  the 
complex  fatty  body  lecithin,  cholesterin,  glycogen  (especially 
in  the  horse),  salts  of  potash  and  phosphates,  the  latter 
being  probably  derived  from  the  phosphorus-containing 
compounds. 

The  origin  of  the  white  corpuscles  is  from  the  lymphatic 
system,  from  which  they  enter  the  blood  stream  through 
the  large  lymphatic  channels  opening  into  the  vena  cava  at 
the  junction  of  the  two  jugular  veins.  The  hyaline  corpuscles 
are  derived  from  the  lymphocytes,  the  polynuclear  and 
eosinophile  produce  themselves  in  the  blood  stream  by  cell 
division.  The  white  corpuscles,  as  well  as  the  red,  are 
constantly  being  used  up  and  as  constantly  replaced. 
They  also  possess  the  power  of  passing  through  the  walls 
of  the  vessels  into  the  surrounding  tissues,  from  which  they 
are  removed  by  the  lymjDh  channels,  and  so  find  their  way 
back  to  the  blood.  No  doubt  many  corpuscles  leave  the 
blood  the  destruction  of  which  we  are  unable  to  account 
for,  but  it  is  suggested  that  by  their  death  they  influence 
the  composition  of  the  blood  plasma,  as  in  this  fluid  their 
component  parts  must  become  dissolved  after  their  death. 

During  the  life  of  the  white  corpuscle  great  activity 
prevails ;  it  is  constantly  giving  up  and  taking  in  material 
which  must  affect  the  composition  of  the  plasma.  It  is 
known  that  the  white  cell  possesses  the  power  of  digesting 
certain  substances,  both  solid  and  liquid.  The  researches 
of  Metschnikoff  have  paved  the  way  towards  a  better  under- 
standing of  the  probable  manner  in  which  protection  against 
certain  diseases  is  obtained.  He  has  shown  that  the  white 
cells  take  up  the  bacteria  into  their  interior  and  digest 
them  ;  it  is  really  a  fight  between  bacteria  and  leucocytes. 
The  protection  afl'orded  to  the  system  by  the  white  blood 
cells  is,  therefore,  not  the  least  important  of  the  functions 
performed  by  them,  and  whether  they  accomplish  this  duty 
thoroughly  or  imperfectly  depends  largely  on  the  composi- 
tion of  the  blood  plasma  (see  p.  26). 

Coagulation. —  We  are  now  brought   to   a   consideration 


THE  BLOOD 


15 


of  the  subject  of   blood-clotting,  a  process  by  which  the 
naturally  fluid  blood  becomes  converted  into  a  solid. 

If  blood  be  drawn  from  the  body  and  left  at  rest,  it  will 
be  found  within  a  few  minutes  to  have  undergone  the  process 
of  clotting.  The  fluid  first  becomes  a  jelly  and  then  a  firm 
clot  or  crassamentum,  taking  a  complete  cast  of  the  vessel 
in  which  it  is  placed,  and  so  firm  in  consistence  that  it  may 
be  inverted  without  any  blood  being  lost.  In  a  short  time 
the  clot  begins  to  contract,  and  by  so  doing  squeezes  out  a 
fluid  known  as  serum  (Fig.  6).  This  gradually  accumulates, 
and  as  it  becomes  abundant  the  clot  sinks.  The  blood  of 
the  horse  is  remarkable  for  the  slow  rate  at  which  coagulation 
occurs,  and  the  red  cells,  being 
specifically  heavier  than  the 
plasma,  have  time  to  fall  in  the 
fluid  before  the  process  is  com- 
pleted. The  result  of  this  is 
that  the  upper  solid  layer  is 
considerably  decolourised,  form- 
ing the  so-called  bufFy  coat, 
which  though  natural  to  the 
blood  of  the  horse,  is  indicative 
in  other  animals  of  the  presence 
of  an  inflammatory  process  in 
the  system. 

We  have  here  closely  followed 
the  account  given  by  human  physiologists  of  the  coagula- 
tion of  the  blood  in  the  horse,  but  the  appearance  described 
is  by  no  means  invariable.  Coagulation  in  this  animal  is 
often  complete  in  less  than  five  minutes,  when,  of  course,  no 
bufiy  coat  forms,  and  we  are  inclined  to  believe  that  rapid 
coagulation  and  non-buffy  coat  are  the  rule  rather  than  the 
exception  ;  we  have  repeatedly  observed  the  blood  of  the 
horse  clot  so  rapidly  as  to  be  almost  instantaneous.  One 
thing  in  connection  with  horse's  blood  is  undoubted,  and 
that  is  that  coagulation  is  more  easily  slowed  or  prevented 
by  cold  and  neutral  salts  than  it  is  in  the  blood  of  any  other 
warm-blooded  animal     May  it  not  be  that  some  confusion 


Fig.  6. — Diagram   of  Clot 
WITH     BuFFY    Coat 

(Stewart). 

V,  Lower  portion  of  clot  with 
red  corpuscles ;  iv,  white 
corpuscles  in  upper  layer 
of  clot ;  c,  cupped  upper 
surface  of  clot ;  s,  serum. 


1()     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

has  thus  arisen,  and  we  have  come  to  regard  this  abnormally 
easy  slowing  of  clotting  by  cold  and  salts,  as  if  it  were 
markedly  a  characteristic  of  horse's  blood  as  it  clots 
naturally  ? 

According  to  Nasse,  the  average  time  occupied  in  coagu- 
lation is  as  follows : 


Pig 
Sheep 
Dog 
Ox  - 
Horse 


i  to    1^  minutes. 

-L  11 

1     ,,     3 

O      „    lO  „ 

5     ,,  13  „ 


In  our  experience  the  extreme  time  mentioned  for  the  horse 
is  exceptionally  long. 

If  the  clot  be  examined  microscopically  it  is  found  to 
consist  of  fine  fibrils,  entangled  in  which  are  the  blood 
corpuscles ;  if  the  fibrin  produced  be  washed  completely 
free  from  blood,  its  appearance  is  well  described  by  its 
name. 

If  instead  of  allowing  the  blood  to  clot  spontaneously 
it  be  whipped  with  a  rod  or  bunch  of  twigs,  or,  as  we  say,  is 
'  defibrinated,'  the  fibrin  separates  rapidly  and  coats  the 
rod,  while  no  coagulation  in  the  remaining  fluid  can  occur. 
The  power  of  spontaneous  clotting  lies,  then,  in  the  pro- 
duction of  fibrin. 

These  changes  may  be  graphically  represented  thus : 


Blood. 


Blood. 


f  Plasma. 


^  Corpuscles. 


Plasma. 


vCorpuscles. 


Gn  Clotting. 

J  Serum. 

\  Fibrin. 

(Red. 

-White.  Clot. 

(Blood  platelets.) 

When  Whipped. 

f  Fibrin. 

\  Serum.  \ 

(Red.  [ 

\  White.  I 

(Blood  platelets./ 


Defibrinated  blood. 


THE  BLOOD  17 

Fibrin  is  a  yellowish-white,  stringy-looking,  bulky  mass. 
It  may  be  dissolved  by  dilute  hydrochloric  acid,  forming 
acid-albumin  or  syntonin,  also  by  dilute  alkalies  with  the 
production  of  alkali-albumin,  and  by  the  prolonged  action 
of  neutral  salts,  with  the  formation  of  globulins.  Its  bulky 
appearance  would  lead  to  the  belief  that  it  exists  in  blood 
in  large  quantities ;  it  is  found,  however,  to  be  by  weight 
relatively  small.  In  human  blood  its  proportion  is  "2  per 
cent. ;  sheep,  "2  to  "3  per  cent. ;  ox,  "3  to  '4  per  cent. ; 
horse,  '4  per  cent. ;  pig,  "4  to  '5  per  cent.  ;  dog  '2  per 
cent. 

The  Cause  of  Coagulation  has  kept  physiologists  busy  for 
many  years,  and  even  at  the  present  time  the  matter  has 
by  no  means  been  settled.  The  theory  most  generally 
accepted  is  that  of  Hammarsten — viz.,  that  clotting  is  due 
to  the  conversion  of  a  fluid  fibrinogen  into  a  solid  fibrin, 
under  the  influence  of  a  ferment. 

If  blood  be  prevented  from  coagulating  plasma  can  be 
obtained,  and  this  plasma,  depending  upon  the  agents  used 
in  its  production,  will  teach  us  the  main  facts  of  coagulation. 
If  it  be  obtained  by  cooling  the  blood,  then  the  plasma 
will  clot  spontaneously  by  allowing  the  temperature  to  rise ; 
if  the  plasma  be  obtained  by  previously  mixing  the  blood 
with  a  definite  amount  of  magnesium  or  sodium  sulphate, 
or  common  salt,  clotting  can  be  obtained  by  diluting  it.  If 
it  be  obtained  by  acting  on  blood  with  oxalates,  then 
clotting  can  be  brought  about  on  the  addition  of  a  lime 
salt,  and  if  it  be  peptone  plasma  (see  p.  20),  simple  dilution 
will  cause  it  to  clot.  The  clot  formed  by  the  plasma  coagu- 
lating is  precisely  the  same  as  that  formed  by  the  blood 
coagulating ;  it  is  of  course  colourless. 

If  the  above  plasmas  be  acted  upon  by  adding  common 
salt  to  saturation,  a  precipitate  of  fibrinogen  occurs ;  it  is  a 
proteid  belonging  to  the  globulin  group,  and  has  previously 
been  alluded  to.  If  this  precipitate  be  re-dissolved  by 
diluting  the  fluid  and  allowed  to  stand,  it  clots  spon- 
taneously. If  a  solution  of  pure  fibrinogen  be  prepared, 
it  does  not  clot  spontaneously,  but  it  may  be  made  to  do  so 

2 


18     A  MANUAL  OF  YETEPJNAllY  PHYSIOLOGY 

l^y  the  addition  of  a  drop  of  serum  or  the  washings  of  a 
blood-clot. 

The  interpretation  of  all  this  is  that  the  substance  which 
brings  about  coagulation  of  the  Idood  is  contained  in 
the  plasma.  This  substance  is  fibrinogen,  but  fibrinogen 
will  not  work  alone :  it  requires  a  very  small  quantity  of 
something  else,  and  this  something  has  been  termed  the 
fibrin  ferment.  It  is  called  a  ferment  inasmuch  as  a  very 
small  amount  of  it  is  capable  of  acting  on  an  indefinitely 
large  amount  of  fibrinogen,  and  that  its  action  is  closely 
dependent  on  temperature. 

The  ferment  is  known  as  thrombin ;  it  does  not  exist  as 
such  in  the  living  blood,  l)ut  can  readily  be  obtained  from 
blood  which  is  shfid.  Thrombin  can  be  injected  into  the 
general  circulation  without  producing  any  ill  efi'ect.  Ex- 
perimental inquiry  shows  that  there  is  in  the  blood  a 
precursor  of  thrombin  spoken  of  as  pro-thrombin,  and  that 
under  the  influence  of  calcium  salts  the  pro-thrombin  is 
converted  into  thrombin.  In  fact,  under  the  influence  of 
calcium  salts,  any  of  the  tissues  of  the  body,  especially 
lymphatic  glands,  will  provide  a  thrombin.  The  substance 
from  which  this  thrombin  is  obtained  is  known  as  nucleo- 
alljumin,  and  if  nucleo-alljumin  be  injected  into  the  blood- 
stream clotting  at  once  occurs. 

Chemically  very  little  is  known  of  the  l;lood  ferment 
beyond  the  fact  that  heating  to  181°  F.  (55°  C.)  destroys 
it.  The  substance  from  which  it  is  formed,  pro-thrombin, 
is  rich  in  phosphorus  and  contains  nuclein.  Histologically 
pro-thrombin  appears  to  be  identical  with  blood  platelets, 
and  the  latter  may  be  observed,  when  repairing  the 
damaged  wall  of  a  bloodvessel,  to  plug  it  with  a  substance 
resembling  fibrin  in  appearance.  It  is,  in  fact,  by  this 
means  that  haemorrhage  gradually  tends  to  cease. 

Circumstances  influencing  Coagulation. — It  is  a  matter  of 
common  observation,  that  after  death  the  coagulation  of 
blood  in  the  vessels  is  a  slow  process,  though  by  exposure 
to  air  clotting  is  almost  at  once  produced.  At  one  time  it 
was  supposed  that  the  air  in  some  way  influenced  this,  but 


THE  BLOOD 


19 


it  has  been  shown  that  the  action  is  due  rather  to  the 
influence  on  the  blood  exerted  by  the  wall  of  the  vessels. 
The  jugular  vein  of  a  horse  may  be  included  between  liga- 
tures and  excised  (Fig.  7),  yet  the  blood  will  remain  fluid 
in  it  for  one  or  two  days,  though  suspended  in  such  a  way 
as  to  be  left  freely  exposed  to  the  air  ; 
nevertheless  on  removal  from  the  vein 
clotting  at  once  occurs. 

When  bloodvessels  are  injured  during 
life,  or  when  pathological  changes  occur 
in  the  blood,  coagulation  inside  the 
vessels  will  take  place  :  this  is  due  to 
the  influence  exercised  over  the  blood 
by  the  injured  vascular  wall  which  acts 
as  a  foreign  body. 

Clotting  in  shed  blood  may  be  retarded 
or  hastened  by  certain  conditions.  The 
blood  of  a  horse  received  into  a  vessel  so 
constructed  as  to  expose  it  to  a  freezing 
temperature  may  be  kept  fluid  for  an 
indefinite  period,  though  coagulation 
will  at  once  occur  when  the  tempera- 
ture is  allowed  to  rise.  Clotting  is 
delayed  by  the  addition  to  the  blood  of 
the  neutral  salts  of  the  alkalies  and 
alkaline  earths,  and  of  ammonium. 
The  addition  of  dilute  acetic  acid  or  the 
passage  of  a  current  of  carbonic  acid 
entirely  prevents  it,  by  precipitating  the 
fibrinogen.  On  adding  to  blood  even 
a  weak  solution  of  potassium  oxalate, 
calcium  oxalate  is  formed  and  clotting  prevented,  as  without 
the  calcium  salt  no  fibrin  can  be  produced.  It  has  been 
shown  that  the  administration  of  citric  acid  to  the  living 
animal  renders  the  blood  uncoagulable,  and  this  it  effects 
by  binding  up  the  lime  salts.  Conversely,  the  addition  of 
lime  salts  to  the  blood  increases  the  coagulability  of  the 
fluid. 

2—2 


Fig.  7.  —  Jugular 
Vein  of  the 
Horse  ligatured 
and  removed 
FROM  THE  Body, 
known  as  the 
'Living  Test- 
Tube'  Experi- 
ment. 

1,  Plasma;  2,  red 
corpuscles  which 
have  settled,  and 
in  the  upper  layer 
of  which  the  white 
corpuscles  are  en- 
tangled. 


•20    A  MANUAL  OF  VETEEINAllY  PHYSIOLOGY 

In  the  curdling  of  milk  by  the  addition  of  rennet,  the 
presence  of  a  calcium  salt  is  also  absolutely  essential.  The 
similarity  between  milk  curdling  and  blood  clotting  has 
been  recognised  for  many  years.  If  peptone  be  injected 
into  the  blood  of  the  dog  such  blood  will  not  clot,  and 
much  the  same  effect  is  produced  if,  instead  of  peptone,  an 
extract  of  the  ordinary  medicinal  leech  be  used.  The 
peptone  blood  refuses  to  clot  not  on  account  of  the  peptone 
as  such,  but  for  the  reason  that  the  peptone  causes  a 
secretion  from  the  liver  of  a  substance  which  prevents 
coagulation.  The  leech  extract  acts  by  destroying  the 
ferment. 

By  rapidly  heating  blood  to  133°  F.  (56°  C),  the  fibrin- 
forming  substances  are  precipitated,  and  clotting  is  pre- 
vented. It  is  also  considerably  retarded  if  the  blood  as  it 
is  shed  from  the  living  animal  is  collected  in  a  vessel  the 
walls  of  which  are  thinly  coated  with  oil.  The  layer  of  oil 
apparently  acts  as  an  *  inert '  substance  towards  the  blood, 
thus  resembling  the  inner  wall  of  an  uninjured  bloodvessel. 
The  shape  of  the  collecting  vessel  has  an  influence  over 
coagulation,  clotting  being  much  slower  in  a  smooth  deep 
vessel  than  in  a  rough  shallow  one. 

The  Extractives  of  the  blood  are  fats,  cholesterin,  lecithin, 
creatin,  urea,  hippuric  acid,  uric  acid,  and  grape  sugar,  all 
in  small  and  varying  quantities.  Fats  occur  as  neutral 
fats,  olein,  stearin,  and  palmitin  ;  the  peculiar  fat  lecithin 
occurs  only  in  very  small  proportions.  The  amount  of  fat 
in  the  blood  during  digestion  is  '4  to  "6  per  cent. ;  in  fast- 
ing animals,  '2  per  cent.  ;  in  dogs  fed  on  a  fatty  diet  it  may 
reach  1'25  per  cent.,  and  may  give  the  serum  a  milky 
appearance.  There  is  twice  as  much  fat  in  the  serum  of 
recently  fed  horses  as  in  the  serum  of  those  kept  starving. 
Other  extractives  such  as  soaps  are  found  to  the  extent  of 
•05  to  •!  per  cent.  ;  urea,  '02  to  '04  per  cent.  ;  sugar, 
•1  to  '15  per  cent. 

The  characteristic  Difference  between  Arterial  and  Venous 
Blood  is  that  the  former  contains  more  oxygen  and  less  car- 
bonic acid,  though  there  is  always,  in  fully  arterialised  blood, 


THE  BLOOD 


21 


about  twice  as  much  carbon  dioxide  as  there  is  of  oxygen 
(see  p.  24).  Arterial  blood  also  contains  more  water, 
fibrinogen,  extractives,  salts,  and  sugar,  fewer  blood- 
corpuscles,  and  less  urea ;  its  temperature  is,  on  the 
average,  1  C.  lower.  The  dark  colour  of  venous  blood 
is  not  due  to  the  greater  amount  of  COo  it  contains,  but 
to  the  diminution  of  oxygen  in  the  red  blood-cells.  The 
alteration  in  colour  effected  by  the  addition  of  reagents 
and  gases  to  blood  is  probably  due  partly  to  alterations  in 
the  shape  of  the  corpuscles  themselves,  which  become  more 
concave  on  the  addition  of  oxygen  and  less  concave  on 
its  removal,  and  also  to  the  fact  that  oxy-hsemoglobin  is 
brighter  in  colour  than  reduced  hiiemoglobin. 

The  Salts  of  the  blood  are  divided  between  the  plasma 
and  the  corpuscles.  The  distribution  of  these  is  not  the 
same  in  all  animals ;  in  the  horse  and  pig,  for  example, 
sodium  only  exists  in  the  plasma  and  none  in  the  corpuscles, 
whereas  in  the  ox  and  dog  both  corpuscles  and  plasma 
contain  it.  Sodium  chloride  is  the  most  abundant  salt 
of  the  blood,  potassium  chloride  and  sodium  carbonate 
come  next,  and  lastly  phosphates  of  calcium,  magnesium 
and  sodium.  The  chief  inorganic  substance  of  the  cells  is 
potassium  phosphate.  Iron  is  found  in  haemoglobin  but 
not  in  the  plasma.  The  following  table  from  Bunge  bears 
on  the  question  of  the  salts  of  the  blood  in  different  animals  : 


1,000  grammes  of  Cor- 

1,000  grammes  o^ 

f  Serum 

l^uscles  contain 

contain 

K. 

Na. 

CI. 

K. 

Na. 

CI. 

Horse 

4-92 

0 

1-93 

-27 

4-43 

3-75 

Ox     -       - 

■747 

2  093 

1-635 

•254 

4-351 

3-717 

Pig    -       - 

5-543 

0 

1-504 

•273 

4-272 

3-611 

The  use  of  the  salts  is  to  assist  in  secretion,  repair,  and 
disintegration.  The  growth  of  the  solid  tissues  of  the 
body  absolutely  depends  on  the  inorganic  material  supplied 
by  the  blood.     Water  free  from  salts  is  destructive  to  pro- 


22     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

toplasm ;  no  doubt,  therefore,  one  important  function  of  the 
salts  in  the  blood  is  to  maintain  the  vitality  of  the  tissues. 
Sodium  chloride  is  here  especially  valuable,  and  its  exten- 
sive presence  in  blood  (60  per  cent,  to  90  per  cent,  of  the 
total  amount  of  ash)  corresponds  to  its  importance.  As 
the  blood  is  simply  the  carrier  of  the  salts,  and  the  only 
means  by  which  the  tissues  can  obtain  them,  it  by  no  means 
follows  that  all  the  mineral  matter  found  in  it  is  essential 
to  its  own  repair  and  constitution. 

The  Temperature  of  the  Blood  in  the  different  domestic 
animals  varies  from  100°  E.  to  105°  E.  (37'8°  C.  to 
40'54°  C),  the  warmest  blood  in  the  body  being  found  in 
the  hepatic  veins. 

The  Quantity  of  Blood  in  the  Body  cannot  be  determined 
by  mere  direct  bleeding  alone.  After  all  the  blood  is 
drained  off,  the  vessels  require  to  be  washed  out,  and  the 
quantity  of  blood  in  the  water  estimated  by  the  colour 
present ;  the  body  has  then  to  be  minced  and  macerated, 
and  the  quantity  of  blood  in  this  estimated  by  the  colour  test, 
comparison  being  made  with  a  standard  solution  of  blood. 

By  Haldane  and  Lorrain  Smith's  carbon  monoxide  process 
the  amount  of  blood  in  the  living  animal  may  be  calculated. 
The  essential  steps  in  this  process  are  to  estimate  first 
colorimetrically  the  percentage  of  haemoglobin  in  the  blood, 
and  then  the  extent  to  which  this  is  saturated  by  breathing 
a  measured  volume  of  carbon  monoxide.  In  this  way  the 
total  capacity  of  the  blood  for  carbon  monoxide  may  be 
ascertained,  and  the  carbon  monoxide  capacity  being  the 
same  as  the  oxygen  capacity,  the  volume  of  the  blood  may 
be  readily  calculated. 

Sussdorf  *  puts  the  proportion  which  the  weight  of  the 
blood  bears  to  the  body  weight  as  follows  : 


Horse     - 

Jg  =  6"6  per  cent,  of  the  body  weight. 

Ox 

1  —7-71 

Sheep     - 

A  =  8-01 

Pig         - 

A  =  4-6 

Dog        - 

TT  to  Ts  =  ^'^  to  91  per  cent,  of  the  body  weight. 

*  EUenberger's  '  Physiologie  der  Haussaugethiere.' 


THE  BLOOD 


23 


The  same  observer  gives  the  amount  of  blood  in  the  body 
of  the  horse  at  QG  lbs.,  or  nearly  50  pints. 

The  Distribution  of  Blood  in  the  Body  (Fig.  8)  is  believed 
to  be  as  follows  : 

About  one-fourth  in  the  heart,  lungs,  hirge  vessels,  and  veins. 
,,  ,,  liver. 

,,  ,,  skeletal  muscles. 

„  „  other  organs. 

It  is  probable  that  in  the  horse  the  liver  would  contain  less 
than  one-fourth  the  bulk  of  blood,  while  the  skeletal 
muscles    would   contain   more.     Under  certain   conditions 


Liver  29-5 

Muscles         29-2 

GreatVes%ei%Heartklungs  2  27 
'Bones     ,  8-2 

Jnu  stint  siigtnitctl  organs  6-3 
Skin  Zl 

Kidneys  IS 

/Verve  Centres  L  2 
Sjileen.  OZ^ 


YiG.  8. — Diagram  to  illustrate  the  Distribution  of  the  Blood 
IN  THE  Various  Organs  of  a  Kabbit,  after  Eanke's 
Measurements  (Stewart). 

The  numbers  are  percentages  of  the  total  blood. 

the  abdominal  veins  are  capable  of  containing  the  whole  of 
the  blood  in  the  body.  When  an  organ  is  active  it  receives 
more  blood  than  when  in  a  state  of  rest ;  this  increase  has 
been  variously  estimated  at  from  30  to  50  per  cent. 

The  Gases  of  Blood, — The  blood  gases  are  obtained  by 
introducing  the  fluid  into  a  Toricellian  vacuum,  the  in- 
strument used  to  obtain  it  being  a  mercury  pump.  In  a 
vacuum  the  blood  froths  up  and  gives  off  its  gases,  which 
are  then  collected  and  analysed. 

The  gases  are  oxygen,  carbon  dioxide,  and  nitrogen. 
The  proportion  of  these  found  depends  upon  whether  the 
blood  be  taken  from  an  artery  or  a  vein  ;  in  the  former  the 
oxygen  is  present  in  larger  amount  than  in  the  latter,  and 
the  carbon  dioxide  is  less.  The  nitrogen  is  in  both  cases 
practically  the  same. 


24     A  MANUAL  OF  VETEPJNAKY  PHYSIOLOGY 

At  a  pressure  of  30  inches  (760  mm.)  of  the  barometer 
and  a  temperature  of  32°  F.  (0°  C),  the  following  gases  are 
found  in  100  volumes  of  blood  : 


Arterial, 

Venous. 

Oxygen 

20, 

12 

Carbonic  acid 

40 

45 

Nitrogen 

2 

2 

62  59 

The  exact  amount  of  gas  varies  ;  the  above  can  only  be 
taken  as  convenient  averages. 

Oxygen  exists  in  arterial  blood  in  the  proportion  of  about 
20  per  cent,  per  volume  of  the  blood,  whilst  in  venous  blood 
it  is  found  to  vary  within  wide  limits,  depending  upon  the 
vessel  from  which  it  is  taken,  and  the  activity  of  the  part 
at  the  time  of  its  collection.  In  the  blood  of  asphyxia  it 
is  nearly  absent. 

It  will  be  remembered  that  by  far  the  greater  part  of 
the  oxygen  was  stated  to  be  in  combination  with  the 
haemoglobin  of  the  red  blood-corpuscles ;  in  fact  the  pro- 
portion of  oxygen  in  the  blood  bears  a  definite  relation  to 
the  amount  of  iron  contained  by  the  haemoglobin.  It  has 
been  determined  that  15|  grains  (1  gramme)  of  haemoglobin 
are  capable  of  absorbing  "095  cubic  inch  (1"55  c.c.)  of 
oxygen.  Whatever  oxygen  the  serum  of  blood  contains  is 
simply  absorbed ;  the  amount  held  in  solution  is  therefore 
small.  Oxygen  chemically  united  with  hfemoglolnn  is  quite 
independent  of  the  laws  which  regulate  the  absorption  of 
gases  (see  Eespiration). 

Besides  the  vacuum  of  the  air-pump,  various  chemical 
substances  have  the  power  of  deoxidizing  the  blood-cells ; 
such  reducing  substances  are  ammonium  sulphide,  sul- 
phuretted hydrogen,  some  iron  salts,  etc.  Blood  exposed 
to  the  air  loses  oxygen,  due  to  the  production  of  reducing 
substances  formed  as  the  result  of  decomposition. 

The  Carbon  Dioxide  in  arterial  blood  is  about  40  per  cent. ; 
in  venous  blood  it  varies,  depending  on  the  vessel  from 
which  the  blood  is  drawn.    The  COo  is  principally  combined 


THE  BLOOD  25 

with  the  sodium  carbonate  in  the  plasma  of  the  blood, 
though  Bohr  considers  the  htiemoglobin  is  also  a  carrier. 

The  Nitrogen  in  the  blood  is  small  in  amount,  about 
2  vols,  per  cent. ;  it  does  not  vary  in  arterial  or  venous 
blood,  as  in  both  cases  it  is  simply  absorbed  by  the  plasma. 

Composition  of  the  Blood. — Eeviewing  the  various  analyses 
which  have  been  published  of  the  blood  of  animals,  the 
following  represents  the  average  composition  of  the  tiuid  : 

The  Plasma. 

Water  -  -  -  90  parts  per  cent. 

Proteids  -  -  -       8  or  9  parts. 

Fats  -  -  -  -      -1 

Extractives  -  -  -  "4  ,, 

Salts  -  -  -      '8  ,, 

The  Corpiiscles. 

These  represent  from  one-third  to  half  the  weight  of  the 
blood  and  consist  of : 

Water  -  -     64  parts  per  cent. 

Solids    -  -     35     „      consisting  of  32  per  cent,  haemo- 

globin, "1  per  cent,  proteids. 
Salts     -  -       1     ;, 

Taking  the  blood  as  a  whole  the  following  represents 
approximately  its  composition  in  every  100  parts : 

Water  - 
Solids   - 


Defensive  Mechanisms  of  the  Body. — If  the  serum  of  one  animal 
be  injected  into  another  of  a  different  species,  it  may  cause  the  corpuscles 
to  break  up  (lice molij sis)  and  haemoglobin  to  appear  in  the  urine.  This 
destructive  effect  is  found  to  occur  whether  the  blood  be  injected  into 
the  circulation,  or  merely  added  to  the  foreign  blood  in  vitro.  The 
action  is  termed  globulicidal,  and  it  can  be  abolished  by  previously 
heating  the  added  serum  to  132°  F.  (55°  C).  Not  only  will  the  serum 
of  one  blood  destroy  the  corpuscles  of  another,  but  it  will  also  destroy 


81  parts. 
19     „ 

'  Haemoglobin     ■ 
Proteids   - 

-     13  parts. 
■       4     „ 

Salts 

-       1     „ 

Extractives 

■      -6     „ 

26     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

certain  bacteria  (bacteriolysis),  and  the  effect  on  these  is  greatly 
increased,  if  the  animal  furnishing  the  serum  has  previously  been 
treated  with  an  intravenous  injection  of  similar  bacteria.  These  facts 
have  opened  a  field  of  therapeutics  still  in  its  infancy  and  endowed 
with  great  possibilities. 

The  substance  produced  in  the  blood  which  acts  as  a  protective  is 
known  as  an  antibody ;  it  is  a  defensive  mechanism  of  the  greatest 
importance.  An  antibody  is  not  necessarily  the  result  of  bacterial 
activity,  it  may  be  produced  in  a  blood  by  the  injection  of  almost  any 
proteid,  and  the  serum  so  obtained  is  capable  of  precipitating  that 
particular  proteid  from  solution  and  no  other ;  such  a  body  is  known 
as  a  prcecipitin. 

An  animal  may,  by  carefully  graduated  doses  of  virus,  be  rendered 
completely  immune  to  a  dose  sufficiently  large  to  kill  many  hundreds 
of  unprotected  animals.  The  blood  serum  of  the  one  so  protected  may 
be  employed  in  the  treatment  of  others  naturally  infected  or  unpro- 
tected ;  such  a  scrum  may  be  both  curative  and  protective,  an  example 
of  which  is  rinderpest  serum,  or  it  may  onlj'  be  protective,  as  in 
tetanus. 

Another  defensive  mechanism  of  physiological  value  is  ^j/m^o- 
cytosis.  No  one  can  possibly  doubt  the  difference  in  the  resisting 
power  to  disease  of  '  fit '  over  '  unfit '  animals,  nor  the  greater 
protection  afforded  by  maturity  as  compared  with  youth.  These 
facts  assure  the  perpetuation  of  the  species  and  are  probably 
intimately  connected  with  this  question.  When  referring  to  phago- 
cytosis (p.  14)  we  stated  that  the  thoroughness  with  which  the 
phagocytes  did  their  work  depended  upon  the  composition  of  the 
blood  plasma.  It  would  appear  that  it  does  not  matter  much  from 
what  source  the  leucocytes  are  derived,  they  are  all  capable  of 
turning  out  equally  good  work  if  the  blood  plasma  contains  sufficient 
of  a  substance  which  acts  upon  the  bacteria,  and  renders  them  more 
easily  eaten  by  the  leucocytes  (Wright).  The  nature  of  the  substance 
is  unknown,  but  it  would  appear  to  act  chemically  on  the  bacteria,  and 
render  them  an  easy  prey  to  the  leucocytes  ;  it  does  not  act  upon  nor 
stimulate  the  leucocytes.  This  substance  is  known  as  an  opsonin,  and 
it  is  probable  that  there  are  several  varieties  in  the  plasma,  each 
having  its  own  particular  microbic  infection  to  deal  with.  Opsonins 
must  not  be  confused  with  bacteriolysins,  agglutinins  (anti-bodies  which 
agglutinate  bacteria)  or  antitoxins,  from  which  they  are  quite  distinct. 

The  Blood  in  Disease. — The  blood  plays  two  distinct  parts  in 
disease,  it  is  a  carrier  and  distributer  of  infection  to  the  body  cells, 
and  further  it  may  itself  undergo  profound  pathological  change. 

All  the  specific  infective  diseases  of  animals  are  spread  through  the 
body  by  means  of  the  blood  stream.  It  is  true  that  the  initial  source 
of  entry  may  be  an  allied  passage — the  lymph  stream — but  it  is  by 


THE  BLOOD  27 

means  of  the  blood  that  the  final  and  complete  invasion  of  the  body  is 
effected.  Nor  does  the  observation  apply  to  specific  diseases  only ;  if 
we  take  two  such  opposite  conditions  as  anthrax  and  poisoning  by 
arsenic,  it  is  the  blood  in  each  case  which  is  responsible  for  the  dis- 
tribution of  the  infecting  agent. 

The  blood  tissue  itself  may  be  the  seat  of  disease ;  micro-organisms 
may  live  and  multiply  in  the  plasma  and  infect  the  whole  body  as  in 
anthrax.  Some  of  the  organisms  may  be  so  small  as  to  be  ultra- 
microscopic,  and  in  connection  with  this  question  some  of  the  most 
acute  and  fatal  infectious  diseases  of  animals  are  caused  by  organisms 
of  this  class,  of  which  rinderpest,  foot  and  mouth  disease,  rabies,  and 
African  '  horse  sickness '  are  examples.  Still,  in  spite  of  the  fact  that 
these  microbes  have  not  been  seen  their  existence  is  undoubted,  the 
best  evidence  of  which  is  that  some  of  them  are  sufficiently  large  to 
be  caught  in  the  pores  of  a  filter,  leaving  the  filtrate  sterile.  Other 
organisms  attack  the  blood  cells,  for  example  the  important  group  of 
Trj'panosomes,  the  malaria  parasite,  the  organism  of  Texas  fever,  and 
such  like.  In  these  cases  the  product  of  red  cell  destruction  may  show 
itself  by  the  discoloured  urine  and  is  evident  in  the  tissues,  for  example 
the  liver  and  spleen. 

Compared  with  the  red  corpuscles  the  white  are  seldom  affected  witli 
disease,  but  there  are  certain  affections  of  the  spleen  associated  with 
a  great  increase  in  their  number. 

There  are  other  conditions  affecting  the  blood,  for  instance  Purpura, 
which  cannot  be  attributed  to  parasitic  agency.  In  this  disease,  either 
from  defects  in  the  blood  or  vessel-wall,  htemori-hage  takes  place  into 
the  tissues.  No  organ  appears  to  be  able  to  escape,  though  probably 
the  subcutaneous  and  muscular  tissues  are  the  most  frequent  seat  of 
the  hsemorrhage. 

Quite  as  strange  and  obscure  is  the  dietetic  disease  of  equines 
known  as  haemoglobinuria,  in  which  the  animal  in  the  middle  of 
work  suddenly  falls  'paralysed,  the  urine  becomes  coffee-coloured  and 
loaded  with  methtemoglobin,  in  consequence  of  the  destruction  of  the 
red  cells.  What  the  destructive  agent  is,  is  at  present  unknown,  but  it 
is  probably  one  of  the  poisonous  products  of  proteid  disintegration, 
which  will  be  found  dealt  with  in  the  chapter  on  digestion. 

Blood-letting  in  the  treatment  of  disease  was  at  one  time  so 
universal  that  it  came  to  be  regarded  as  the  '  sheet-anchor  '  of  life, 
and  animals  were  regularly  bled  in  order  to  l-eep  tliem  well.  '  Blood- 
letting '  was  killed  by  abuse  ;  it  is  a  question  whether  the  pendulum 
has  now  travelled  too  far  in  the  other  direction,  and  the  employment 
of  a  physiological  means  in  the  treatment  of  disease  been  too  long 
neglected. 


CHAPTER   II 

THE  HEART 

The  blood  in  the  body  has  to  be  kept  in  constant  motion, 
so  that  the  tissues  which  are  depending  upon  it  for  their 
vitality  may  be  continuously  supplied,  and  also  in  order 
that  the  impure  fluid  resulting  from  the  changes  in  the 
tissues  may  be  rapidly  and  effectually  conveyed  to  those 
organs  where  its  purification  is  carried  out. 

The  heart  is  the  organ  which  pumps  the  blood  over  the 
body,  not  only  distributing  it  to  the  tissues,  but  forcing  it 
on  from  these  back  to  the  heart  again,  to  be  prepared  for 
redistribution.  It  may  be  described  as  a  hollow  muscle 
divided  into  two  compartments,  usually  known  as  right  and 
left,  but  in  quadrupeds  really  anterior  and  posterior,  each 
compartment  being  divided  into  an  upper  half  or  auricle, 
and  a  lower  or  ventricle.  Opening  into  the  auricles  are 
large  veins  which  convey  the  blood  back  to  the  heart,  while 
from  the  ventricles  other  vessels,  arteries,  take  their  origin 
for  the  conveyance  of  blood  from  the  heart.  The  auricles 
and  ventricles  are  separated  by  a  valvular  arrangement, 
and  the  two  sides  of  the  heart  are  separated  by  a  muscular 
partition  (Fig.  9). 

So  far  the  general  arrangement  of  both  right  and  left 
sides  is  much  the  same,  each  having  to  receive  and  then  to 
get  rid  of  a  certain  quantity  of  blood  sent  into  it.  But  the 
blood  sent  into  the  right  side  of  the  heart  is  very  different 
from  that  received  by  the  left,  and  with  this  difference  we 
must  for  a  moment  deal.  The  whole  of  the  impure  or 
venous  blood  in  the  body  is  brought  into  the  right  side  of 
the  heart  for  the  purpose  of  being  distributed  to  the  lungs 

28 


THE  HEAET 


29 


where  it  is  purified  ;  into  the  left  heart  this  arterial  or 
purified  blood  is  brought  back  from  the  lungs  for  distribu- 
tion to  the  body.  The  passage  of  the  impure  or  venous 
blood  from  the  right  side  of  the  heart  through  the  lungs  to 
the  left  side  is  known  as  the  Pulmonic  circulation;  that  of 
the  blood,  thus  purified,  through  the  body  and  back  to  the 
right  side  of  the  heart  is  called  the  Systemic  circulation 
(Fig.  10). 

Mention  has  been  made  of  valves  in  the  cavities  of  the 
heart ;   they  are  found   on  both    sides   separating   auricle 


Fig.  9. — Diagram  of  the  Circulation  through  the  Heart. 

and  2,  The  venae  cavse ;  3,  right  auricle ;  4,  right  ventricle ; 
T),  pulmonary  artery  ;  6,  6,  pulmonary  veins  ;  7,  left  auricle ; 
8,  left  ventricle  ;  9,  aorta  dividing  into  anterior  and  posterior. 
The  arrows  represent  the  direction  taken  by  the  blood  stream. 


from  ventricle,  and  are  known  as  the  right  auriculo-ventri- 
cular  or  tricuspid  valve,  and  the  left  auriculo-ventricular 
or  mitral  valve.  Besides  these,  valves  are  found  in  the 
vessels  arising  from  the  ventricles,  viz.,  in  the  pulmonary 
artery  and  the  aorta  ;  these  valves,  pulmonary  and  aortic, 
are  known  as  the  semi-lunar  valves.  No  valves  are  found 
guarding  the  entrance  of  the  vessels  (veins)  into  the  auricles. 


30     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

In  order  to  understand  the  function  of  these  valves,  which 
play  such  an  important  part  in  the  physiology  of  the  heart, 
it  is  necessary  that  we  should  briefly  detail  the  course  which 
the  blood  takes  from  the  time  it  enters  the  right  auricle, 
until  it  completes  the  round  of  the  circulation  and  finds 
itself  at  this  auricle  again. 

Course  of  the  Circulation. — The  venous  blood  from  the 
whole  of  the  body  flows  into  the  right  auricle  by  means  of 
the  anterior  and  posterior  vense  cav?e  ;  it  passes  from  here 
through  the  tricuspid  valve  into  the  right  ventricle ;  from 


Fig.  10. — Diagram  of  the  Circulation  of  the  Blood. 

1,  The  heart ;  2,  anterior,  3,  posterior  aorta  ;  4,  anterior  vena  cava  ; 
5,  pulmonary  artery  ;  6,  pulmonary  veins  ;  7,  mesenteric  arteries ; 
8,  intestinal  capillaries  ;  9,  portal  vein  ;  10,  the  liver,  the  veins 
from  wliich  open  into  (12)  the  posterior  vena  cava  ;  11,  the 
circulation  through  the  hind  extremities ;  13,  the  circulation 
through  the  kidney. 

the  right  ventricle  it  travels  to  the  lungs  by  means  of  the 
pulmonary  artery,  where,  having  been  exposed  to  the  action 
of  the  air  and  become  greatly  changed  in  its  gaseous  com- 
position, it  returns  to  the  heart  by  means  of  the  pulmonary 
veins,  emptying  itself  into  the  left  auricle.  It  now  passes 
through  the  auriculo-ventricular  opening  into  the  left 
ventricle,  and  thence  into  the  aorta  to  be  pumped  all  over 
the  body,  being  distributed  by  means  of  the  arteries  and 
capillaries ;  it  is  then  collected  by  the  veins,  and  eventually 
brought  back  to  the  heart  to  undergo  afresh  its  distribution 
to  the  lungs  and  body  (Fig.  10). 

The  use  of  the  valves  is  to  allow  of  and  to  insure  the 
transference  of  blood  from  auricles  to  ventricles,  and  from 


THE  HEART  31 

the  ventricles  to  the  aorta  and  pulmonary  artery  without 
any  chance  of  regurgitation.  This  they  do  in  virtue  of  the 
fact  that  they  are  so  constructed  and  arranged  as  to  open 
only  in  that  direction  towards  which  the  blood  has  to  l)e 
sent. 

Position  of  the  Heart. — The  heart  occupies  a  position  in 
the  middle  line  of  the  chest,  being  enclosed  in  a  sac,  the 
pericardium,  and  suspended  from  the  spine  by  its  aortic 
vessels.  Its  base  is  uppermost,  its  apex  nearly  touches  the 
sternum,  and  the  organ  occupies  in  the  horse  a  position 
corresponding  to  the  third,  fourth,  fifth,  and  sixth  ribs.  It 
is  between  the  fifth  and  sixth  ribs,  at  their  sternal  insertion, 
where  the  impulse  or  '  beat '  of  the  heart  may  be  felt  in  the 
horse.  Its  other  relations  are  with  the  diaphragm  which 
is  just  behind  the  apex,  but  with  which  it  has  no  structural 
connection.  On  its  right  side  is  the  right  lung,  and  on 
its  left  part  of  the  left  lung  ;  there  is  a  triangular  notch  in 
the  left  lung  of  the  horse  which  exposes  the  left  ventricle, 
and  allows  it  to  make  its  impulse  felt  against  the  chest 
wall.  The  anterior  face  of  the  heart  is  formed  by  the  right 
auricle  and  ventricle,  the  posterior  by  the  left  auricle  and 
ventricle. 

Heart  Muscle. — The  heart  is  an  involuntary  muscle,  but 
does  not  conform  histologically  to  the  involuntary  muscular 
fibres  met  with  in  other  parts  of  the  body.  The  muscle  is 
red  in  appearance ;  microscopically  its  fibres  are  short, 
striated  both  in  a  cross  and  longitudinal  direction,  possess 
no  sarcolemma,  and  anastomose  freel3^  The  network 
formed  by  the  fibres  of  the  heart  is  a  most  distinctive 
feature  of  cardiac  muscle.  The  contractile  tissue,  though 
spoken  of  as  a  fibre,  is  in  reality  a  quadrilateral  nucleated 
cell.  In  some  animals,  sheep  and  ox  in  particular,  cells  of 
a  peculiar  kind  are  found  immediately  beneath  the  endo- 
cardium ;  they  are  polyhedral  in  shape,  containing  proto- 
plasm and  a  nucleus,  and  are  surrounded  by  striated  fibres  ; 
they  are  called  the  cells  of  rurkinje. 

The  arrangement  of  the  fibres  of  the  heart  is  peculiar ; 
the  fibres  of  the  auricle  are  quite  distinct  from  those  of  the 


3'2     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

ventricle,  and  both  are  arranged  in  layers.  Two  layers  exist 
in  the  auricle,  circular  and  longitudinal,  the  circular  fibres 
being  continued  around  the  entrance  of  the  veins,  whilst  in 
the  ventricle  several  layers  exist  of  oblique,  longitudinal, 
and  circular  fibres.  Owing  to  the  peculiar  direction  in  which 
the  oblique  fibres  run  a  somewhat  spiral  arrangement 
results.  It  has  been  shown  that  the  auricles  and  ventricles, 
though  separated  by  a  fibrous  ring,  are  yet  connected  by 


Fig.  11. — Left  Ventricle  of  Horse  exposed  to  show  Mitral 

Valve. 
1,  Portion  of  valve  ;  2,  columncc  carnece,  on  the  upper  surface  of  which 
are  found  the  muscwli  xmpillares,  to  which  the  cJiordce  tendinece 
are  attached. 


bands  of  altered  muscular  tissue  which  pass  through  the 
ring. 

The  cavities  of  the  heart  are  lined  by  the  endocardium 
which  is  reflected  over  the  valves  ;  this  membrane  in  the 
left  auricle  of  the  horse  is  of  a  peculiar  grey  colour. 

Certain  fibrous  rings  are  found  in  the  heart  where  the 
valves  are  situated,  and  to  which  these  obtain  a  firm 
attachment.  The  ring  surrounding  the  aortic  opening  in 
the  ox  has  constantly  in  its  substance  one  or  more  pieces  of 
bony  tissue  ;  this  is  also  common  in  the  horse. 


THE  HEAET 


33 


Valves  of  the  Heart. — The  auricido-rentncidar  valves  are 
made  up  of  fibrous  membrane,  in  which  a  small  proportion 
of  muscular  fibre  is  found  close  to  the  attached  border. 
The  mitral  or  hicusjnd  valve  in  the  horse  consists  of  one 
large  distinct  segment,  and  several  smaller  ones  united  to 
form  a  second  ;  the  tricuspid  consists  of  three  segments, 
one,  much  larger  than  the  others,  being  placed  opposite  to 


Fig.  12. — Diagram  to  illustrate  the  Action  of  the  V.a.lves  of 
THE  Heart  (Huxley). 

In  A  the  auricle  is  contracting,  ventricle  dilated,  mitral  valve  open, 
semi-lunar  valves  closed.  In  B  the  auricle  is  dilated,  ventricle 
contracting,  mitral  valve  closed,  semi-lunar  valves  open.  Anr., 
auricle;  vent.,  ventricle  ;  v.,  i-.,  vein  ;  a.,  aorta  ;  m.,  mitral  valve  ; 
s.,  semi-lunar  valve.  Note  the  manner  in  which  the  papilhe  have 
shortened  in  B,  in  order  to  compensate  for  the  ventricular  walls 
approximating. 

that  portion  of  the  ventricle  which  leads  to  the  pulmonary- 
artery. 

The  free  edges  of  all  the  valves  are  held  in  position  by 
large  and  small  tendinous  cords  {chorda  tendinece)  composed 
of  fibrous  tissue,  which  are  inserted  into  musculi  papillares 
found  on  the  internal  surface  of  the  ventricle ;  the  cords 
from  one  papilla  do  not  all  pass  to  one  segment  of  the  valve, 
but  to  two  or  three  (Fig.  11).  The  function  of  the  papillae  is 
to  restrain  the  valves  from  flapping  back  into  the  auricle 

3 


U     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


during  the  contraction  of  the  ventricle,  and  this  they 
accomplish  by  gradually  shortening  as  the  walls  of  the 
ventricle  approximate ;  compensating  by  their  shortening 
for  the  movement  of  the  ventricular  wall  and  thus  exerting 
traction  on  the  cords  (Fig.  12).  Other  bands  pass  from  one 
side  of  the  ventricle  to  the  opposite  wall ;  they  are  called 
moderator  hands,  and  their  function  is  to  restrain  the  ven- 
tricular wall  from  undue  distension. 

The  valvular  flaps  meet  in  the  most  perfect  apposition 
when  the  ventricles  contract,  their  edges  are  inverted,  and 

the  sides  of  the  valves  curl 
in  and  lie  so  close  to  their 
fellows  that  nothing  can 
escape  upwards  into  the 
auricles  (Fig.  13).  This  may 
be  readily  demonstrated  in 
the  dead  heart  by  tying  the 
aorta  and  pulmonary  veins, 
and  introducing  into  the  left 
auricle  a  tube  which  admits 
a  powerful  jet  of  water  ;  the 
left  side  of  the  heart  dis- 
tends and  hardens,  and  at 
last  water  forces  its  way  out 
of  the  hole  in  the  auricle 
through  which  the  tube  is 
inserted.  If  the  auricle  be  now  opened,  the  ventricle  is 
found  cut  off  from  view  by  a  tense  membranous  parachute- 
like dome,  convex  towards  the  auricle,  which  is  the 
mitral  valve  in  position  ;  not  a  drop  of  water  will  escape 
from  the  ventricle,  though  the  heart  be  turned  upside 
down,  and  it  requires  some  little  force  to  depress  the 
valve. 

The  semi-lunar  or  sigmoid  valves,  which  guard  the 
entrance  into  the  aorta  and  pulmonary  artery,  are  com- 
posed of  fibrous  tissue,  and  possess  at  the  centre  of  each 
segment  a  small  hard  body,  corpus  Arantii,  which  is 
particularly  marked  in  the  aorta. 


Fig.  13. — Tricuspid  Valve  in 
CLOSED  Position  seen  from 
THE  Auricle. 

Note  the  cracks  in  the  surface, 
which  represent  where  the 
margin  of  the  valves  meet  and 
fold  in  against  each  other  like 
the  lips  of  a  toothless  mouth. 


THE  HEART  35 

Movements  of  the  Heart. — If  the  exposed  mammalian  heart 
be  watched  at  work,  a  great  deal  may  be  learned  of  its 
action.  It  will  be  observed  that  both  auricles  contract 
together  and  both  ventricles  together ;  further  that  certain 
changes  in  shape  occur.  The  contraction  of  either  auricle 
or  ventricle  is  spoken  of  as  its  systole,  while  the  subsequent 
relaxation  is  described  as  its  diastole. 

Prior  to  any  heart  movement  occurring  the  roots  of  the 
veins  entering  the  auricles  contract  through  the  medium 
of  the  circular  fibres  surrounding  them  ;  this  contraction 
next  sweeps  over  the  auricles  which  are  drawn  downwards 
towards  the  ventricles,  the  auricular  appendage  taking  an 
active  part  in  expelling  its  contents.  The  ventricles  then 
contract,  but  more  slowly,  and  as  they  do  so  they  shorten, 
become  more  circular  in  shape  and  owing  to  the  oblique 
direction  of  the  muscular  fibres  above  described,  there  is 
a  twisting  or  squeezing  of  the  ventricular  walls.  The 
contraction  of  the  ventricles  does  not  begin  at  the  apex,  as 
might  be  supposed,  but  at  the  base,  and  extends  from  there 
to  the  apex.  Further,  there  is  no  apex  beat,  for  the  apex 
does  not  move  unless  the  pericardium  be  opened. 

The  contraction  of  the  ventricles  is  succeeded  by  a  pause, 
during  which  the  heart  is  in  a  state  of  relaxation. 

A  Cardiac  Cycle  is  the  term  used  to  describe  the  changes 
which  occur  in  the  heart,  during  the  time  which  elapses 
between  one  contraction  or  relaxation  of  the  auricle,  and 
the  one  which  immediately  succeeds  it. 

We  may  take  the  moment  when  the  blood  is  entering  the 
auricles  from  the  venae  cavne  and  pulmonary  veins  as  the 
most  convenient  point  to  start  from.  This  flow  is  brought 
about  by  the  pressure  of  blood  in  the  veins,  which  though 
low  is  yet  higher  than  that  in  the  auricles.  Further,  the  flow 
into  the  right  heart  is  assisted  by  gravity,  particularly  the 
blood  in  the  anterior  vena  cava,  while  even  in  the  posterior 
vena  cava  this  is  by  no  means  a  negligible  quantity.  There 
is  likewise  an  aspiration  in  the  veins  produced  by  a  relaxa- 
tion of  the  walls  of  the  auricle  after  the  previous  contraction, 
and  an  aspiration  in  the  thorax  the  result  of  inspiration, 

3—2 


36     A  MANUAL  OF  YETEEINARY  PHYSIOLOGY 

which  gives  rise  to  a  negative  pressure  in  the  veins  leading 
to  the  heart  (see  p.  91). 

The  auricles  being  now  full,  a  wave  of  contraction  which 
first  appears  at  the  vessels  leading  into  them,  passes  over 
these  chambers,  which  by  a  sudden  sharp  and  brief  contrac- 
tion empty  their  contents  into  the  ventricles.  The  systole 
of  the  auricle  produces  a  backward  positive  wave  in  the 
vessels  leading  into  it,  and  this  wave  passing  through  the 
anterior  vena  cava,  shows  itself  in  the  jugulars  of  the  horse 
by  a  distinct  pulsation  at  the  root  of  the  neck. 

The  auricular  contraction  forces  the  blood  into  the 
ventricles,  which  have  been  partly  filling  during  the  time 
the  auricles  were  distending,  and  the  final  filling  of  the 
ventricle  by  the  auricular  systole  forces  up  the  auriculo- 
ventricular  valves,  which  bulge  into  the  auricle  under  the 
increasing  pressure  to  which  the  ventricular  contents  are 
exposed.  The  ventricles  give  a  prolonged  contraction,  and 
owing  to  the  spiral  arrangement  of  their  muscular  fibres 
exhibit  a  peculiar  movement.  The  pressure  which  now 
exists  in  the  ventricles  is  greater  than  that  in  the  vessels 
leading  from  them,  and  as  the  auriculo-ventricular  valves 
cannot  be  thrown  open  upwards  into  the  auricles  owing  to 
their  chords  tendinese,  the  semi-lunar  valves  are  forced 
open,  and  the  stream  of  blood  passes  into  the  aorta  and 
pulmonary  artery.  At  the  moment  the  ventricles  contract, 
the  heart  slightly  rotates  around  its  vertical  axis  from  left 
to  right,  while  the  left  ventricle  hardens  and  makes  its 
impulse  felt  against  the  chest  wall.  The  impulse  is  syn- 
chronous with  the  closure  of  the  auriculo-ventricular  valves, 
and  the  forcing  open  of  the  sigmoid  valves.  The  systole  of 
the  ventricle  produces  a  dull,  booming,  prolonged  sound, 
which  is  brought  about  by  the  muscular  contraction  of  its 
walls,  and  probably,  at  the  same  time,  partly  by  a  vibration 
of  the  auriculo-ventricular  valves  ;  the  sound  is  known  as 
the  first  sound  of  the  heart. 

The  blood  now  rushes  into  the  aorta  and  pulmonary 
artery,  and  the  elastic  resistance  of  these  arteries  being 
brought  into  play,  the  fluid  has  a  tendency  to  regurgitate 


THE  HEAET  37 

towards  the  ventricles ;  by  this  process  the  semi-lunar 
valves  are  closed,  the  closure  being  accompanied  by  the 
second  sound  of  the  heart,  which  is  short  and  sharp.  This 
sound  is  due  to  the  sudden  tension  of  the  membranous 
flaps  of  the  valves  at  the  moment  of  their  closure,  which 
gives  rise  to  vibrations. 

The  semi-lunar  valves  are  mechanically  most  perfect. 
The  thin  margins  on  either  side  of  the  corpora  Arantii  are 
closely  pressed  together,  the  corpora  Arantii  filling  up  the 
centre,  and  not  a  drop  of  blood  passes  back  into  the 
ventricles.  These  valves  do  not  lie  back  close  against  the 
arterial  wall  during  the  exit  of  blood  from  the  ventricle, 
but  stand  out  in  the  stream,  probably  being  kept  there  by 
reflux  currents.  They  form  a  triangular  orifice  with  curved 
sides. 

The  arterial  trunks  which  during  the  systole  of  the 
ventricles  elongate  and  curve,  now  at  the  diastole  shrink 
and  shorten,  and  so  bring  the  base  of  the  heart  back  to 
its  former  place.  The  force  of  aortic  reflux  is  not  wholly 
expended  on  the  valves,  but  largely  on  the  muscular  pads 
to  which  the  valves  are  attached ;  to  admit  of  this  the 
diameter  of  the  aorta  is  much  greater  than  the  opening 
out  of  the  ventricle. 

It  is  not  until  the  semi-lunar  valves  are  firmly  closed 
that  the  ventricles  begin  to  relax  ;  this  they  now  do,  and  the 
relaxation  of  the  walls  produces  a  negative  pressure,  viz., 
a  pressure  below  that  of  the  atmosphere,  which  in  the  dog 
has  been  measured  at  from  1  to  2  inches  (25  to  50  mm.)  of 
mercury.  This  negative  pressure  opens  the  auriculo-ven- 
tricular  valves,  the  blood  flows  in  from  the  auricle,  while 
the  auricle  and  ventricle,  neither  contracting  nor  dilating, 
assume  a  passive  condition  during  a  period  known  as  the 
pause.  Throughout  the  pause  blood  is  flowing  into  the 
auricles  from  the  pulmonary  veins  and  vense  cavfe,  and 
into  the  ventricles  from  the  auricles ;  towards  the  close  of 
the  pause  the  auricles  contract,  and  the  whole  process  is 
repeated. 

We  have  thus  the  contraction  of  the  auricles,  the  con- 


38     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

traction  of  the  ventricles,  and  the  pause.  The  time  each 
of  these  occupies  has  been  determined  for  the  horse,  by 
Chauveau  and  Marey,  by  means  of  a  cardiac  sound.  The 
value  of  the  periods  they  give  us  is  as  follows :  auricular 
systole,  two-tenths  of  a  second,  ventricular  systole,  four- 
tenths,  and  pause,  four-tenths  of  a  second.  We  cannot 
accept  the  value  of  these  periods  as  correct,  owing  to  the 
fact  that  they  cause  the  horse  to  have  a  pulse  of  60  to  the 
minute,  which  is  distinctly  abnormal ;  36  to  40  beats  per 
minute  is  the  normal  rate. 

A  complete  cycle  of  the  horse's  heart  occurs,  on  an 
average,  once  in  every  1*5  seconds ;  but  the  time  value 
of  the  various  phases  which  make  up  this  period  can- 
not be  exactly  stated.  No  matter  how  fast  the  heart  is 
beating,  the  frequency  depends  not  on  the  duration  of  the 
ventricular  systole,  but  on  the  length  of  the  subsequent 
pause. 

Summary  of  Events  occurring  duri)ig  a  Cardiac  Cycle. — 
Dividing  the  events  into  three  periods,  and  starting  with 
the  contraction  of  the  auricles,  the  following  is  a  summary 
of  the  changes  occurring  in  the  heart : 

1st  Period. — The  contraction  of  the  auricles  completes 
the  filling  of  the  ventricles. 

%id  Period. — The  ventricles  contract,  the  auriculo- 
ventricular  valves  are  closed,  the  aortic  and  pulmonary 
valves  open,  blood  is  pumped  into  the  aorta  and  pulmonary 
artery,  the  impulse  of  the  heart  is  made  against  the  wall 
of  the  chest,  the  first  sound  is  produced,  the  auricles  fill 
with  blood,  and  the  whole  is  followed  by  a  short  pause. 

"^rd  Period. — The  aortic  and  pulmonary  valves  close,  the 
auriculo-ventricular  valves  open,  the  second  sound  of  the 
heart  is  produced,  diastole  of  both  auricles  and  ventricles 
occurs,  followed  by  a  long  pause,  during  which  blood  flows 
into  all  the  chambers. 

The  impulse  of  the  heart,  to  which  we  have  previously 
referred  as  being  felt  externally  between  the  fifth  and  sixth 
ribs,  is  not  given  by  the  apex,  but  by  the  lower  half  of  the 
left  ventricle.     There  is  no  such  thing  as  an  apex  beat ; 


THE  HEART  39 

the  apex  practically  does  not  move  as  long  as  the  heart  is 
retained  within  the  pericardium,  but  if  the  latter  be  opened, 
the  apex  is  tilted  forward  with  each  contraction.  The 
Use  of  the  Pericardium  is  to  prevent  over-distension  of  the 
heart. 

The  Action  of  the  Valves  of  the  Heart  during  a  cardiac 
cycle  is  peculiar  and  interesting.  We  have  described  how 
the  auriculo-ventricular  curtains  are  floated  up  as  the 
ventricles  fill,  and  how  with  increased  pressure  they  come 
together,  being  prevented  by  the  chordse  tendinese,  and  the 
muscular  pillars  to  which  these  are  attached,  from  being 
pushed  back  into  the  auricle.  Further,  experimental  in- 
quiry has  determined  that  when  the  ventricular  systole  is 
at  its  height,  these  valves  bulge  upwards  into  the  auricles, 
assuming  a  concave  surface  towards  the  ventricle ;  this 
appears  to  be  especially  the  case  in  the  horse. 

The  pulmonary  valves,  and  probably  the  aortic,  not  only 
meet  at  their  free  border  but  actually  overlap.  Chauveau 
states  that  he  has  tried  experimentally  to  hold  back  one  of 
the  pulmonary  valves,  but  the  others  have  applied  them- 
selves so  closely  around  the  finger  that  it  was  impossible  to 
produce  a  patent  opening.  In  the  aorta  it  is  probable  that 
overlapping  does  not  occur  to  the  same  extent,  and  here 
the  corpora  Arantii  are  of  value.  When  the  sigmoid  valves 
are  not  acting  they  still  lie  in  the  blood  stream,  and  not 
against  the  wall  of  the  vessel  as  was  at  one  time  supposed  ; 
nor  do  those  in  the  aorta  cover  the  openings  of  the  coronary 
arteries. 

It  is  generally  believed  that  both  the  aortic  and  pul- 
monary valves  are  closed  by  the  regurgitation  of  the  blood ; 
but  it  has  been  pointed  out  that  as  the  blood  is  leaving  both 
ventricles,  it  is  streaming  through  orifices  which  at  that 
time  are  mere  chinks,  owing  to  the  pads  of  muscle  which 
take  their  origin  from  all  sides  of  the  mouth  of  the  vessels. 
Vortices  are  thus  created  in  the  space  between  the  arterial 
root  and  the  edge  of  the  valves.  These  vortices  tend  to 
press  the  edges  of  the  valves  together,  and  the  valves  con- 
sequently close  the  moment  the  blood   actually  ceases  to 


40    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

stream  through  the  narrow  crevice.  In  this  way  there  is 
no  regurgitation,  as  the  valves  are  closed  before  the  recoil 
of  the  aorta.  If  this  explanation  be  correct,  the  second 
sound  of  the  heart  must  be  considered  as  due  to  the  sudden 
tension,  and  not  the  closure,  of  the  aortic  valves  at  the 
time  of  the  aortic  recoil. 

The  Cardiac  Sounds  are  really  four  in  number,  but  as 
they  are  in  pairs  we  recognise  only  two.  The  first  sound 
is  a  long  booming  one,  due  to  the  muscle-sound  of  the  con- 
tracting ventricle,  assisted,  probably,  by  the  simultaneous 
vibrations  of  the  auriculo-ventricular  valves.  The  second 
sound  is  due  to  the  sudden  tension  of  the  aortic  and 
pulmonary  valves  at  the  moment  of  their  closure,  which 
gives  rise  to  vil^rations.  It  is  a  short  sound,  and  its  source 
has  been  clearly  proved  by  hooking  back  the  valves,  which 
causes  the  sound  to  cease.  The  two  sounds  are  reproduced 
by  the  words  '  luhb  chlpp.'' 

Intra-Cardiac  Pressure. — The  internal  pressure  exercised 
by  the  walls  of  the  heart  on  the  blood,  is  ascertained  by 
means  of  an  instrument  termed  a  cardiac  sound,  first  used 
by  Chauveau  and  Marey.  It  is  a  double  tube  having  at  its 
extremity  two  elastic  balls ;  the  air  in  these  is  compressed 
when  the  cavities  contract,  and  the  compression  moves  a 
lever  placed  in  connection  with  a  recording  surface.  The 
instrument  is  passed  into  the  right  heart  through  the 
jugular  vein,  one  ball  being  in  the  auricle,  the  other  in  the 
ventricle.  It  is  stated  that  its  presence  causes  no  in- 
convenience to  the  animal,  due  to  the  fact  that  no  sentient 
nerves  are  supplied  to  the  lining  membrane  of  the  blood- 
vessels or  even  to  the  heart.  A  tracing  so  obtained  from 
the  heart  of  the  horse  is  shown  in  Fig.  14  ;  the  curves  do 
not  indicate  the  force  of  the  stroke,  but  only  the  differences 
in  intra-cardiac  pressure  at  each  instant  of  one  contrac- 
tion. It  is  seen  from  the  tracing  that  the  auricles  contract 
first,  followed  by  the  ventricles.  The  contraction  of  the 
former  is  sharper  and  shorter  than  of  the  latter,  which  is 
slower,  maintained  for  some  time,  and  then  falls. 

Observations  on  the  intra-cardiac  pressure  show  that  it  is 


THE  HEART 


41 


greatest  at  the  beginning  of  contraction,  and  then  gradu- 
ally falls ;  whilst  a  negative  pressure  occurs  during  the 
period  of  diastole,  and  is  brought  about  by  the  relaxation 
of  the  walls  of  the  heart  during  the  long  pause.  This 
relaxation  causes  a  sucking  action  which  assists  in  filling 
the  heart  with  blood. 

Though  both  ventricles  deliver  the  same  amount  of  blood 
the   pressure   in    each    cavity    is    different,    owing   to    the 


Right 
Auricle. 


■■■■■■■■■■■■■■■■■■■■■■■■I 

■!«!■!■»■■■■■■■■!■■■■■ 

liiliiaBDniaBinivEinnHaai 


Right 
Ven- 
tricle 


Apex  nf 

the 

Heart 


aHBBBaaaBaiaia 
RgaiaKnwiBBBBBa 


^BliaaBBBBBBBBia^ 

BB»iiaBBBBr~ 

mBB! 


iBBBBBaiflBBaBBBBBBBBBBBBBBn 

BBBBBBBlliaBBBBBBBBBBBBBBBBr 


Fict.  14. — Simultaneous  Tracings  from  the  Intekior  of  the  Uight 
Heart  of  the  Horse,  after  Chauveau  and  Marey  (M'Kendrick). 

Each  horizontal  Hne  equals  i\  second,  the  vertical  lines  indicate 
pressure  ;  the  vertical  dotted  lines  mark  coincident  points  in  the  three 
movements. 

The  auricular  curve  is  a,  b,  c,  the  ventricular  curve  is  c',  d' ,  e' ,  f.  The 
auricle  contracts  sharply,  relaxes  rapidly,  and  is  followed  by  the 
contraction  of  the  ventricle  which  is  maintained  with  certain 
oscillations  for  about  three-tenths  of  a  second,  and  then  relaxes ; 
the  pause  follows  at  /,  /'. 

The  oscillations  seen  at  d,  d',  d",  and  e',  e',  e",  are  believed  to  indicate 
vibrations  of  the  tricuspid  valve. 


differences  in  the  resistance  to  be  overcome  in  the  systemic 
and  pulmonic  circulations.  The  systolic  pressure  in  the 
left  ventricle  of  the  horse  is  equal  to  a  column  of  blood 
from  9  to  14  feet  (2-4  to  43  metres)  in  height,  or  178  to 


42     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

318  mm.  of  mercury,  and  in  the  right  ventricle  is  equal 
to  Ih  feet  of  blood  ('46  metre),  or  34  mm.  of  mercury  in 
height.  In  the  dog  the  intra-cardiac  pressure  has  been 
estimated  as  follows  : 

Left  ventricle      -         -     7  inches  (180  mm.)  of  mercury. 
Aorta-         -         -         -     6^     „       (160  mm.) 
Right  ventricle    -         -     If     ,,         (45  mm.)  „ 

The  Cardiograph. — The  impulse  of  the  heart  against  the 
chest  wall  is  graphically  obtained  by  means  of  the  cardio- 
graph, of  which  there  are  many  forms.  Their  essential 
construction  consists  of  a  button  applied  externally  to 
the  chest  wall,  which  is  pressed  upon  by  each  impulse  of 
the  heart,  and  so  conveys  the  movement  to  an  elastic  air- 
chamber,  which  transmits  it  to  a  recording  lever.  By 
this  means  we  obtain  a  graphic  representation  of  the  heart's 
impulse,  but  there  are  many  difficulties  in  obtaining 
reliable  records  with  this  instrument. 

The  cardiograph  demonstrates  that  the  aortic  valves 
close  slightly  before  the  pulmonary. 

Capacity  of  Heart. — The  quantity  of  blood  in  the  heart 
can  only  be  ascertained  approximately ;  measuring  the 
capacity  of  the  chambers  is  no  guide.  Munk  states  that 
the  capacity  of  the  ventricle  in  a  horse  weighing  880  lbs.  is 
about  1'76  pints  (1  litre),  equivalent  to  2*25  lbs.  (1  kilo)  of 
blood ;  each  ventricle  contains  one-thirtieth  of  the  total 
blood,  so  that  when  both  contract  one-fifteenth  of  the  total 
blood  is  ejected.  Colin  gives  the  capacity  of  the  left 
ventricle  of  the  horse  at  1*76  pints,  and  states  that  at  each 
systole  two-thirds  or  three-fourths  of  this  quantity  are 
injected  into  the  aorta,  viz.,  1*17  pints  (670  c.c.)  to 
1'36  pints  (772  c.c.) ;  the  left  ventricle  at  each  contrac- 
tion, according  to  this  observer,  forces  into  the  aorta  about 
one-twenty-fifth  of  the  total  blood  of  the  body.  It  is  said 
by  Colin,  that  in  the  horse  the  ventricles  do  not  empty 
themselves  completely  at  each  systole,  and  this  appears  to 
be  the  case  in  all  animals. 

Each  side  of  the  heart  must  hold  the  same  quantity  of 


THE  HEART  43 

blood,  for  it  is  evident  the  amount  of  blood  leaving  the 
heart  must  be  equal  to  the  amount  entering  it. 

"Work  of  the  Heart. — This  may  be  calculated  if  we  know 
the  amount  of  blood  being  discharged  from  the  heart  at 
each  stroke,  and  the  pressure  against  which  it  is  propelled. 
The  amount  pumped  out  at  each  systole  of  the  ventricle 
is  liable  to  great  variation,  at  least  such  are  the  results  of 
experiments  on  the  dog.  It  is  obvious  that  the  right 
ventricle  does  less  work  than  the  left,  for  the  reason  that 
it  has  to  pump  the  same  volume  of  blood  against  a  much 
smaller  pcnplieval  resistance  ;  it  has  been  said  indeed  that 
the  right  heart  does  one  quarter  the  work  of  the  left. 

If  we  take  the  amount  of  blood  pumped  at  each  stroke 
into  the  aorta  of  the  horse  at  about  2-25  lbs.  (1  kilo)  in 
weight,  and  the  pressure  under  which  it  is  forced  upwards 
as  equivalent  to  a  column  of  blood  10  feet  in  height,  then 
the  work  of  the  left  ventricle  at  each  stroke  is  equal  to 
22-5  lbs.  raised  one  foot  high,  or  for  24  hours,  allowing  the 
work  of  the  right  heart  to  be  one-fourth  that  of  the  left, 
1,539,000  foot  pounds.  This  amounts  to  about  one-thirtieth 
of  a  horse  power  per  diem  ;  Munk  places  it  at  one-thirty- 
sixth  of  a  horse  power.  If  the  amount  of  blood  expelled 
by  the  left  ventricle  at  each  stroke  be  equal  to  2*25  lbs., 
then  in  a  state  of  repose  the  entire  blood  in  the  body  of 
a  horse  passes  through  the  heart  in  about  thirty  beats,  or 
in  45  seconds.  Munk  says  that  in  the  horse  the  entire 
blood  passes  through  the  heart  in  50  seconds,  in  the  ox  in 
40  seconds,  and  in  the  dog  in  20  seconds. 

Since  the  amount  of  work  performed  by  the  heart  is 
increased  during  exercise,  the  above  calculations  are  for 
a  horse  in  a  state  of  repose. 

The  term  Blood  Pressure  is  frequently  used  in  the 
physiology  of  the  circulatory  system.  It  is  one  we  shall 
have  to  consider  in  detail  when  we  come  to  speak  of  the 
bloodvessels  ;  but  it  should  be  clearly  understood  that  the 
condition  is  due  to  the  amount  of  blood  pumped  into  the 
vessels  by  the  heart,  and  the  pressure  which  results  from 
this  depends  principally  on  the  rate  at  which  that  which 


44     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

is  in  front  of  it  in  the  vessels  escapes  into  the  veins.  If 
the  arterioles  are  contracted  so  that  the  amount  passing 
into  the  veins  is  reduced  in  quantity,  then  a  larger  bulk 
of  blood  will  be  between  the  pump  and  its  outlets,  and  the 
blood  pressure  rises  ;  if,  on  the  other  hand,  the  blood  is 
passing  rapidly  through  the  relaxed  arterioles  into  the 
veins  the  blood  pressure  falls.  When  the  amount  poured 
into  the  venous  system  in  any  given  time  is  equivalent 
to  that  pumped  into  the  arterial  system  during  the  same 
time  (which  is  the  normal  condition),  the  pressure  is 
described  as  being  constant. 

The  above  facts  may  be  tabulated  as  follows  : 

When  the  heart  is  more  active  the  blood  pressure  rises. 

,,  ,,  less         „  ,,  ,,  falls. 

When  the  arterioles  contract  the  blood  pressure  rises. 

,,  ,,  dilate  ,,  ,,  falls. 

The  heaviest  work  the  heart  performs  is  in  overcoming 
the  resistance  offered  by  the  minute  bloodvessels  or 
arterioles  ;  only  a  very  small  part  of  the  heart's  work 
is  expended  on  producing  blood  velocity.  This  question  of 
peripheral  resistance  will  shortly  be  considered  in  detail. 

The  number  of  heart  beats  in  different  animals,  and  the 
conditions  influencing  it,  are  more  conveniently  considered 
in  the  next  chapter,  see  p.  70. 

Nervous  Mechanism.  —  The  heart  is  said  to  possess  no 
sensory  nerves ;  it  may  be  handled,  pinched,  pricked,  or 
otherwise  injured  without  provoking  the  least  sign  of  pain 
on  the  part  of  the  animal.  Colin's  experiments  in  this 
direction  on  horses  appear  quite  conclusive.  Not  only  is 
it  considered  that  the  external  surface  is  insensible  to  pain, 
but  the  internal  surface  also  ;  for,  as  previously  noted,  the 
experimental  introduction  of  foreign  bodies  into  the  cavities 
of  the  heart  appears  to  produce  no  pain.  Under  patho- 
logical conditions  the  results  are  otherwise ;  foreign  bodies, 
so  common  in  the  heart  of  the  cow,  cause  great  suffering, 
therefore,  there  must  be  sensory  nerves,  though  normally 
their  excitability  is  probably  low. 


THE  HEAET  45 

The  nerves  supplying  the  heart  are  the  pneumogastrics, 
or  vagus  nerves,  and  the  sympathetics ;  the  function  of 
these  is  diametrically  opposite.  The  pneumogastric  has 
a  restraining,  or,  as  it  is  termed,  inhibitory  effect  over 
the  movements  of  the  heart;  the  sympathetic  has  an 
accelerating  or  augmentinfi  effect.  Histologically  the  two 
nerves  differ  greatl}'  in  structure,  the  pneumogastric  being 
a  meduUated,  whilst  the  sympathetic  is  a  non-medullated 
nerve. 

The  inhibitory  fibres  found  in  the  vagus  are  derived 
from  the  internal  branch  of  the  spinal  accessory,  which 
joins  the  vagus  within  the  skull,  and  travelling  with  this 
nerve  reaches  the  heart  by  its  cardiac  branches.  The 
accelerator  nerves  arise  from  the  spinal  cord,  by  the  inferior 
roots  of  the  second  and  third  dorsal  nerves  and  probably 
of  others  ;  they  pass  through  the  sympathetic  ganglia,  and 
reach  the  inferior  cervical  ganglion,  from  which  they  are 
distributed  to  the  heart.     (See  Fig.  15.) 

If  the  Vagus  Nerve  in  the  neck  be  gently  stimulated  the 
rate  of  the  heart  heat  is  slowed  and  the  force  of  the  heat 
reduced.  Either  of  these  effects  may  occur,  or  they  may 
be  combined.  If  instead  of  stimulating  gently  a  strong 
stimulation  be  applied,  the  heart  stops  in  diastole.  Strong 
stimulation  may  be  applied  to  the  vagus  of  the  cat  without 
stopping  the  heart,  but  in  the  dog  even  weak  stimulation 
may  cause  it  to  cease  beating. 

The  above  action  of  the  vagus  is  spoken  of  as  inhihitory ; 
it  controls  or  inhibits  the  heart  beat.  Experiment  shows 
that  while  both  auricles  and  ventricles  are  affected  by  this 
action  of  the  vagus,  yet  the  effect  appears  to  be  more 
marked  upon  the  auricles  than  on  the  ventricles.  The 
effects  of  the  vagus  on  the  heart  are  often  better  demon- 
strated through  one  nerve,  frequently  the  right,  than  its 
fellow,  and  this  is  explained  by  saying  there  are  more 
inhibitory  fibres  in  one  nerve  than  in  the  other. 

If  one  vagus  be  cut  the  rate  of  the  heart  beat  is  slightly 
increased,  if  both  be  cut  the  rate  is  greatly  increased,  and 
the  blood  pressure  rises ;  the  reason  why  the  beats  are  in- 


46 


A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


G.Tr.Vg.  Gan- 
glion on  the 
trunk  of  the  3Tj^Vq 
vagus ;  the  black  '  '  ^ 
line  through  it 
is  the  internal 
brancli  of  the 
spinal  accessory. 


Vg.  The  vagus  Vri 
nerve. 


nc. ,  nc.  The 
cardiac  bran- 
ches of  the  vagus 
conveying 
hibitory  fibres 
to  the  heart. 


Inhibitory  fibres  from 
Ac  .spinal  accessory  entering 
the  vagus. 


G.Th.*  and 
G.Th.5  Fourth 
and  fifth  thora- 
cic ganglia  on 
.sympathetic 
chain. 


C.Sy.  The  cervical  sym- 
pathetic nerve. 


G.C.     Inferior       cervical 
ganglion. 


Asb.  Subclavian  artery. 


An.Y.  Annulus  of  Vieus- 
D.I  .sens. 

G.St.  Ganglion  stellatum. 


D.II.,  D.III.  The  inferior 
roots  of  the  second  and 
third  spinal  nerves,  passing 
by  means  of  r.c.  the  ramus 
commuuicans  to  tlie  gang- 
lion stellatum.  These  are 
the  augmentor  fibres,  pass- 
ing both  by  the  annulus 
[]  ]][  and  inferior  cervical  gang- 
lion to  the  heart  by  nc. 
nc. 


The  dotted  line  in  certain 
thoracic  nerves,  D.I.,  D.I.V., 
and  D.V.,  indicate  that 
they  ma3'  contribute  aug- 
mentor fibres  to  the  sym- 
pathetic. 


Fig.   15. — Diagrammatic    Kepresentation  of   the    Cardiac   Inhibitory   and 
Augmentor  Fibres  in  the  Dog  (Foster). 

The  upper  portion  of  the  figure  shows  the  inhibitory,  the  lower  the  augmentor 

fibres, 


THE  HEART  47 

creased  in  frequency  is  that  the  inhibitory  action  of  the 
vagus  is  removed,  and  the  antagonistic  nerve,  the  sympa- 
thetic, has  things  all  its  own  way.  If  now  the  cut  end  of 
the  vagus  be  stimulated  impulses  are  sent  out  which  call 
into  existence  the  inhibitory  action,  and  the  heart  beats 
become  fewer  and  more  feeble. 

If  an  artery  be  placed  in  communication  with  a  recording 
apparatus  and  the  vagus  stimulated,  a  tracing  such  as  that 


Fig.  16. — Tracing  showing  the  Influence  of  Stimulating  the 
Vagus  Nerve  ;  Fall  of  Blood  Pressure  due  to  Arrest  of 
THE  Heart.     From  a  Rabbit  (Foster). 

cT  marks  on  the  signal  line  when  the  current  is  thrown  into,  and  y  shut 
off  from  the  vagus.  The  time  marker  below  marks  seconds,  a 
corresponds  in  point  of  time  with  x ;  the  heart  does  not  at  once 
cease  to  beat.  The  first  beat  b  occurs  a  short  time  after  shutting 
off  the  current.  The  notches  in  the  tracing  are  the  beats  of  the 
heart. 

seen  in  Fig.  16  is  obtained.  The  inhibitory  effect  is  not 
obtained  immediately  the  stimulus  is  applied ;  at  least  one 
beat  may  occur  before  the  heart  stops,  and  in  the  same  way 
the  beats  do  not  return  immediately  the  stimulus  is  with- 
drawn. The  effect  of  this  on  the  blood  pressure  is  seen  in 
Fig  16,  where  the  drop  in  the  curve  is  due  to  a  fall  in  blood 
pressure  the  result  of  cardiac  inhibition,  while  it  rises  by 
leaps  and  bounds  shortly  after  the  stimulus  is  withdrawn. 

The  inhibitory  power  of  the  vagus  is  lost  if  atrojnn  be 
applied  to  the  heart  or  injected  into  the  circulation,  owing 


48     A  MANUAL  OF  VETERINAKY  PHYSIOLOGY 

to  its  nerve  endings  in  the  heart  being  jDaralysed.  Minute 
doses  of  this  alkaloid  are  sufficient  to  prevent  stoppage  of 
the  heart's  beat  by  stimulation  of  the  vagus.  The  action 
of  atropin  is  counteracted  by  muscarin  or  'physostigmin,  both 
of  which  produce  a  remarkable  slowing  effect  on  the  heart, 
even  causing  it  to  stop,  behaving,  in  fact,  very  much  like 
vagus  stimulation. 

The  inhibitory  action  of  the  vagus  on  the  heart  is  under 
the  control  of  a  centre  in  the  medulla ;  the  exact  extent 
and  position  of  this  is  not  known,  but  it  is  situated  close  to 
the  origin  of  the  vagus.  The  centre  is  spoken  of  as  the 
cardio-mhihitorji,  it  is  bilateral,  and  from  it  the  inhibitory 
fibres  which  pass  down  the  vagus  obtain  their  origin.  This 
centre  is  in  action  during  the  whole  life  of  the  animal ;  its 
constant  action  is  known  as  tonic  activity,  and  its  function 
is  to  keep  a  rein  on  the  heart ;  the  tighter  the  rein  is  held 
the  slower  the  heart  beat  becomes,  the  slacker  the  rein  the 
quicker  the  beat.  As  to  whether  the  rein  shall  be  tight, 
moderate,  or  slack,  depends  upon  the  afferent  impressions 
carried  to  the  centre  from  the  periphery,  and  impulses 
carried  in  this  way  and  passed  out  through  another  channel 
are  described  as  reficx  impulses.  If  the  central  ends  of 
sensory  nerves  be  stimulated  the  heart  may  slow  down  ; 
painful  stimulation  of  any  sensory  surface,  a  blow  on  the 
abdomen,  an  accident,  sudden  fright,  or  in  the  human  sub- 
ject a  sickening  sight,  may  reflexly  slow  the  heart  through 
the  above  centre.  The  centre  is  also  excited  by  carbonic 
acid,  since  venous  blood  circulating  through  it  slows  the 
beat.  It  is  probable  that  the  tonic  activity  of  the  centre 
throughout  life  is  a  reflex  tonus,  viz.,  is  not  due  to  impulses 
originating  in  the  centre,  but  to  the  centre  always  being 
stimulated  through  a  continuous  inflow  of  sensory  im- 
pressions. A  rise  in  blood  pressure  causes  a  slowing  of 
the  beat  (Marey's  law),  which  is  a  good  example  of 
reflex  inhibition  effected  through  the  cardio-inhibitory 
centre. 

In  the  dog,  cardiac  inhibition  is  slightly  increased  during 
expiration,  so  that  in  this  animal  the  heart  beats  slower 


THE  HEART  49 

during  expiration  than  during  inspiration ;  the  effect  is 
aboUshed  b}'  section  of  the  vagi  (see  Figs.  18  and  19). 

The  Sympathetic  nerve  is  the  augmentor  nerve  of  the 
heart ;  it  accelerates  the  beat,  and  is  consequently  the  an- 
tagonist of  the  vagus.  When  stimulated  the  rate  of  beat  is 
increased,  and  in  some  cases  not  only  the  rate  of  beat  but 
its  force.  Finally  in  a  third  group  of  cases  the  force  and 
not  the  rate  is  increased.  The  explanation  of  these  differ- 
ences on  stimulation  is  considered  to  be  that  the  sympa- 
thetic contains  two  sets  of  fibres — (1)  the  accelerators, 
which  increase  the  rate  of  beat,  and  (2)  the  aug mentors, 
which  produce  a  more  forcible  beat.  These  may  act  sepa- 
rately or  in  combination. 

If  the  sympathetic  nerves  on  both  sides  be  cut,  the  heart 
rate  is  decreased  owing  to  the  influence  of  the  uncontrolled 
vagus.  This  view  of  the  effect  of  the  divided  sympathetics 
has  not  always  existed  ;  at  one  time  it  was  held  that  division 
of  the  sympathetic  led  to  no  effect  upon  the  rate  of  the  beat, 
from  which  it  was  reasoned  that  the  influence  of  the  sympa- 
thetic, unlike  the  vagus,  was  only  occasionally  in  operation. 
A  centre  in  the  medulla  controls  the  operations  of  the 
sympathetic  ;  it  is  known  as  the  accelerator  centre,  and  it  is 
believed  that,  like  the  inhibitory  centre,  it  is  in  a  state  of 
tonic  and  constant  activity. 

The  two  antagonistic  forces  above  described  are  con- 
stantly at  work  on  the  heart :  the  inhibitory  through  the 
vagus  slowing  the  rate,  the  accelerator  through  the  sympa- 
thetic quickening  the  rate.  Whichever  of  these  effects  is  at 
any  given  moment  most  needed,  is  brought  into  play  by 
impulses  from  the  centres  in  the  medulla. 

The  vagus  is  the  protecting  nerve  of  the  heart.  It  is 
commonly  observed  after  its  stimulation  and  the  consequent 
inhibition,  that  on  recovery  there  is  an  improvement  either 
in  the  rate  or  force  of  the  heart  beats.  Gaskell  concludes 
from  this  that  inhibition  is  due  to  a  building  up,  anabolism, 
of  the  muscular  tissue  brought  about  by  the  vagus,  and 
resulting  in  an  improvement  in  the  condition  of  the 
heart.   Conversely,  he  regards  the  sympathetic  as  a  katabolic 

4 


50     A  MANUAL  OF  VETERlNAllY  PHYSIOLOGY 

nerve,  viz.,  one  bringing  about  tissue  destruction.  During 
muscular  contraction,  as  we  shall  learn  later,  there  is  a 
breaking  down  of  the  complex  muscle  elements  into  simpler 
bodies,  with  the  production  of  heat  and  energy.  In  the 
case  of  the  heart  muscle  this  may  be  hastened  through  the 
agency  of  the  sympathetic  nerve. 

The  Depressor  Nerve. — The  nervous  mechanisms  con- 
sidered up  to  this  point  are  concerned  in  bringing  about 
some  modified  action  of  the  heart,  under  the  guiding  influ- 
ence of  a  nerve  centre  in  the  medulla.  We  have  now  to 
consider  the  case  where  a  nerve  running  from  tJic  heart ,io 
the  medulla  is  engaged  in  a  regulative  action  which,  unlike 
that  of  the  vagus  or  sympathetic,  is  not  a  direct  action  on 
the  heart  itself,  luit  is  brought  to  l)ear  indirectly  on  the 
heart  through  the  instrumentality  of  the  vascular  (arterial) 
system.  This  nerve  is  the  depressor.  It  originates  in  the 
heart,  some  say  in  the  walls  of  the  aorta,  and  runs  up  the 
neck  as  a  separate  branch  in  the  horse,  cat,  and  rabbit,  but 
in  other  animals  it  is  contained  in  the  trunk  of  the  vagus. 
It  joins  the  suj^erior  laryngeal  nerve,  and  finally  reaches  a 
centre  in  the  medulla  which  regulates  the  movements  of  the 
bloodvessels  of  the  body,  known  as  the  vasomotor  centre. 
The  heart  in  this  way  is  placed  directly  in  communica- 
tion with  the  centre  which  presides  over  the  vascular 
system,  a  centre  by  whose  varying  activities  the  arteries  of 
the  body  are  made  smaller  (constricted)  or  larger  (dilated), 
according  to  the  needs  of  the  system.  If  the  heart  is 
labouring  and  its  muscular  structure  becoming  weakened, 
impulses  pass  up  the  depressor  to  the  vasomotor  centre, 
resulting  in  im^julses  being  sent  out  which  cause  the  abdo- 
minal arteries  to  dilate  and  hold  more  blood.  By  this 
means  the  peripheral  resistance  is  diminished,  the  blood 
pressure  falls,  and  the  heart  is  eased,  since  it  now  has  less 
work  to  do  in  ejecting  its  contents. 

If  the  depressor  nerve  be  divided  no  effect  follows ;  if 
the  end  in  contact  with  the  heart  be  stimulated  there  is  no 
result,  but  if  the  central  or  upper  end  be  stimulated,  the 
blood  pressure  falls  (see  Fig.  28,  p.  79). 


THE  HEART  51 

Cause  of  the  Heart  Beat. — The  nervous  mechanisms  con- 
nected with  the  heart,  which  we  have  just  considered,  only 
deal  with  the  rate  and  force  of  contraction  and  have 
nothing  to  do  with  causing  the  rhythmical  contraction  ; 
the  same  may  be  said  of  the  nervous  ganglia  which  are 
found  in  the  substance  of  the  heart.  The  proof  of  this  is 
very  simple,  for  the  heart  continues  in  cold-blooded 
animals  to  contract  rhythmically  when  all  the  nervous 
connections  are  divided.  Further,  a  strip  of  tissue  may  be 
so  cut  from  the  ventricular  wall  as  to  be  apparently  free  of 
all  ganglionic  structures,  and  such  a  strip  ma}^  under 
suitable  conditions,  be  made  to  contract  automatically  and 
rhythmically. 

Accordingly,  it  is  in  some  peculiarity  of  the  muscle  tissue 
that  the  cause  must  be  sought.  As  the  result  of  many 
observations  it  is  laid  down  as  an  axiom  that  the  heart  is 
automatic,  viz.,  that  the  stimulus  to  activity  arises  from 
within  and  is  not  brought  to  it  from  without.  The  nature 
of  this  inner  stiniidas  cannot  be  regarded  as  solved,  but  it 
is  probable  that  it  is  to  be  sought  largely  in  the  composi- 
tion of  the  blood  or  lymph  circulating  through  the  heart 
tissue,  and  with  special  reference  to  the  inorganic  salts 
these  fluids  contain. 

The  automatic  rhythmic  action  of  the  heart  is  most  highly 
developed  at  the  venous  end  and  least  so  at  the  apex ;  it 
begins  at  the  veins,  courses  over  the  auricles,  and  runs 
down  the  ventricles.  Whatever  may  be  the  rhythm,  the 
venous  end  of  the  heart  sets  the  pace,  as  it  is  expressed, 
for  the  whole  organ.  The  w'ave  of  contraction  passes  from 
chamber  to  chamber  through  the  muscle  substance,  but 
the  muscular  ring  between  auricles  and  ventricles  has  a 
lower  rate  of  conduction  than  the  general  substance  of  the 
heart  wall.  This  fact  has  been  utilized  to  explain  the  short 
pause  between  the  auricular  and  ventricular  contraction. 
Normally,  the  rhythm  of  the  ventricles  follows  that  set  it 
by  the  auricles,  but  this  may  be  destroyed  or  altered  by 
compressing  the  connections  between  auricle  and  ventricle, 
either  by  specially  arranged  clamps  or  by  ligature.     By 

■1—2 


52     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

varying  the  compression  complete  or  partial  blocking  of  the 
normal  rhythm  occurs,  so  that  the  contractions  of  the  ven- 
tricles become  slower  than  those  of  the  auricles. 

The  nature  of  the  inner  stimulus  is,  as  already  sug- 
gested, intimately  connected  with  certain  inorganic  salts 
of  sodium,  calcium  and  potassium.  With  suitable  arrange- 
ments for  keeping  the  heart  of  the  frog  '  fed '  with  a  fluid 
containing  in  solution  chlorides  of  the  above  metals,  the 
heart  will  continue  beating  for  days,  and  even  the 
mammalian  heart  may  be  thus  kept  alive,  provided  it 
be  placed  in  an  atmosphere  of  oxygen  while  being  fed. 
On  this  diet  of  salts  the  heart  finds  the  material  for  its 
inner  stimulus.  It  would  appear  that  not  only  will  no  other 
metals  take  the  place  of  those  named,  but  that  each  has  a 
distinct  role  in  the  function  of  nutrition,  calcium  promot- 
ing contraction,  sodium  and  potassium  bringing  about 
relaxation  of  the  heart. 

Heart  muscle  does  not  behave  in  its  physiological  pro- 
perties the  same  as  skeletal  muscle,  but  possesses  certain 
features  peculiar  to  itself.  If  a  piece  of  ordinary  skeletal 
muscle  be  stimulated  electrically  it  responds  to  a  powerful 
stimulus  with  a  big  contraction,  and  to  a  weak  stimulus 
with  a  small  contraction.  But  the  heart  muscle  when 
stimulated,  if  it  responds  at  all,  gives  as  big  a  contraction 
with  a  weak  stimulus  as  with  a  strong  one.  The  heart's 
motto,  it  has  been  said,  is  expressed  by  *  all  or  nothing.' 
Another  peculiarity  of  heart  muscle  is  that  it  is  only 
capable  of  response  to  stimulation  during  the  phase  of 
diastole ;  if  stimulated  during  systole  no  eflect  follows ; 
this  is  known  as  the  refractory  period  of  the  heart  beat. 
The  stimulus  during  diastole  produces  an  extra  contrac- 
tion, but  this  is  followed  by  a  longer  pause  than  usual,  so 
that  the  extra  beat  is  exactly  counterbalanced. 

The  condition  of  distension  of  the  heart  cavities  is  an 
important  factor  in  the  beat.  Within  reasonable  limits  a  full 
heart  contracts  more  vigorously  than  one  less  full,  though, 
if  too  long  continued,  dilatation  and  damage  of  the  heart 
wall   follow.      We   have   learnt    the   provision    made    for 


THE  HEART  53 

correcting  this,  viz.,  the  depressor  mechanism,  and  the 
cardio-inhibitory  centre. 

Coronary  Circulation. — The  nutrition  of  the  heart  muscle 
is  brought  about  by  the  blood  supplied  to  it  through  the 
coronary  arteries.  Unlike  any  other  arteries  in  the  body, 
the  coronaries  are  filled  during  ventricular  diastole.  During 
systole  the  muscular  pressure  on  the  arteries  becomes 
higher  than  the  pressure  of  the  blood  in  the  vessels,  and  in 
consequence  the  vessels  are  emptied,  while  during  diastole 
they  are  filled.  Cutting  off  the  coronary  circulation  rapidly 
produces  fatal  results. 

Action  of  Drugs  on  the  Heart. — If  aconitin,  miiscarin,  phy- 
sosti<imi)),  a.nd  pilocarpui  be  applied  to  the  heart,  they  cause 
a  gradual  slowing  of  the  heart  beat,  and  finally  stop  it  in 
diastole  as  in  vagus  stimulation.  This  result  is  attributable 
to  the  stimulating  effect  these  drugs  exert  on  the  endings 
of  the  inhibitory  (vagus)  nerves  in  the  wall  of  the  heart. 

Atropin  and  nicotln  increase  the  frequency  of  the  heart 
beats,  behaving  very  much  as  if  the  vagus  were  divided. 
In  fact,  if  stimulation  of  the  vagus  be  made  after  the 
application  of  atropin  no  inhibition  follows,  the  nerve  end- 
ings of  the  vagus  in  the  heart  being  supposedly  paralysed. 
If  atropin  be  injected  into  the  circulation  the  same  results 
are  obtained,  including  a  dilatation  of  the  bloodvessels. 
Atropin  is  able  to  remove  the  inhibitory  action  of  physos- 
tigmin  and  muscarin. 

Adrenalin  applied  to  the  heart  augments  and  strengthens 
its  beat,  while  if  injected  into  the  circulation  it  causes  con- 
striction of  the  vessels  and  a  rise  in  blood  pressure. 

DigitaUn  reduces  the  frequency  of  the  heart  beat,  and 
later  excites  the  cardiac  muscle  to  a  stronger  and  prolonged 
systole.     It  is  described  as  a  heart  tonic. 

Pathological. 

Disease  of  the  heart  of  the  lower  animals  is  uncommon.  It  might 
have  been  thought  that  the  horse  would  be  exposed  to  this  class  of 
trouble,  bearing  in  mind  the  enormous  strain  placed  on  his  heart 
during  labour,  and  the  utter  want  of  consideration  shown  by  the  vast 
majority  of  those  who  ride  and  drive  horses.     But  it  is  not  so.     The 


54     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

hearts  of  horses  exposed  to  the  greatest  strain  seldom  show  any 
pathological  change ;  probablj'  the  most  uncommon  lesions  fomid  on 
post-mortem  examination  are  those  affecting  the  heart.  The  heart 
may  dilate  under  strain,  but  such  dilatation  when  accompanied  by 
hypertrophy  is  compensated,  and  no  indication  of  trouble  exists  during 
life. 

As  evidence  of  the  gross  strain  to  which  horses  are  exposed,  ruptures 
of  the  heart  are  by  no  means  uncommon.  It  is  strange  they  are  not 
more  frequent.  They  probably  would  be  but  for  the  saving  clause  that 
degenerations  of  the  heart  substance  are  rare.  When  the  heart 
ruptures  it  gives  way  in  the  auricle,  where  the  wall  is  thinnest ;  so 
thin,  indeed,  that  in  certain  parts  of  the  auricle  daylight  may  easily  be 
seen  through  the  tissue.  It  is  the  right  and  not  the  left  auricle  which 
suffers,  showing  how  great  is  the  resistance  offered  by  the  pulmonary 
vessels  as  the  result  of  engorgement. 

Valvular  disease  is  not  unknown,  but  so  rare  that  probably  there  is 
no  practitioner  with  a  large  experience  in  the  examination  of  horses  for 
soundness  who  ever  thinks  of  examining  the  heart !  On  the  other 
hand,  irregularities  in  the  heart's  action  are  verj'  common,  frequently 
purely  functional  in  character,  unassociated  with  organic  change,  and 
do  not  interfere  with  the  usefulness  of  the  animal.  A  horse  condemned 
for  heart  disease  on  the  strength  of  an  intermittent  pulse  may  remain 
a  living  reproach  to  the  practitioner. 

In  severe  inflammatory  chest  invasions  of  the  horse,  the  heart,  but 
especially  its  sac,  may  become  acutely  affected.  There  are  few  attacks 
of  severe  pleurisy  in  the  horse  which  are  not  associated  with  pericarditis, 
followed  not  only  by  a  great  thickening  of  the  heart  sac,  but  of  more  or 
less  extensive  effusion  into  it.  The  heart  then  becomes  enveloped  in  a 
water  jacket,  which  greatly  adds  to  the  gravity  of  the  case.  In  the 
above  acute  cases  the  heart  muscle  suffers,  and  haemorrhages  into  it 
are  common  and  widespread. 

In  the  dog  the  heart's  action  is  naturally  intermittent. 

Foreign  bodies  in  the  heart  of  cattle,  especially  cows,  are  well 
known,  and  give  rise  to  a  peculiar  train  of  symptoms.  Vegetations  on 
the  valves  of  both  the  dog  and  pig  are  recognised  in  connection  with 
certain  infectious  diseases. 


CHAPTER  III 

THE  BLOODVESSELS 

The  use  of  the  bloodvessels  is  to  distribute  the  blood  over 
the  body,  to  bring  it  in  contact  with  the  tissues,  and  return 
it  to  the  heart.  To  accomplish  this  purpose  there  are 
arteries,  capillaries,  and  veins. 

The  Arteries  arise  from  one  common  trunk,  the  aorta, 
which  by  the  process  of  dividing  and  subdividing  like  the 
branches  of  a  tree  form  the  arterial  system.  This  system, 
measured  by  its  total  cross  section,  is  very  much  larger 
than  the  parent  trunk,  in  fact  its  sectional  area,  and  hence 
its  cubic  capacity,  has  been  estimated  as  several  hundred 
times  greater. 

The  large  arteries  differ  somewhat  in  construction  from 
the  small  ones.  The  microscope  shows  that  while  the 
large  vessels  are  principally  elastic  the  small  ones  are 
mainly  muscular.  This  latter  fact  does  not  preclude  the 
small  vessels  from  exhibiting  the  elasticity  possessed  by 
the  large  ones,  for  muscular  tissue  is  itself  highly  elastic. 

This  elastic  property  of  arteries  is  an  essential  feature 
in  their  construction ;  it  admits  of  a  vessel  stretching  both 
in  its  width  and  length,  and  at  the  same  time  ensures 
its  recovery  to  its  original  dimensions  after  the  stretching 
force  ceases  to  act.  When  we  remember  the  intermittent 
force  exercised  by  the  left  ventricle  on  the  arteries,  we  have 
no  difficulty  in  understanding  the  necessity  for  this  elastic 
property.  The  arteries  are  always  full,  every  contraction 
of  the  left  ventricle,  for  example,  in  the  horse  during  rest, 
throws  into  them  one  and  a  half  pints  of  blood  which  must 
be  accommodated,  and  this  is  provided  for  by  the  distension 
of  their  walls.     For  every  one  and  a  half  pints  of  blood 


56     A  MANUAL  OF  YETERINAEY  PHYSIOLOGY 

entering  the  aorta,  an  equal  amount  must  pass  out  at  the 
periphery,  and  the  reduction  in  the  diameter  of  the  vessels 
brought  about  by  the  exit  of  this  fluid  is  due  to  the  elastic 
recoil  of  the  arterial  wall. 

We  shall  study  presently  a  further  use  of  the  elastic 
arterial  wall,  when  we  come  to  describe  the  flow  of  fluid 
through  tubes. 

Another  essential  feature  possessed  by  arteries  is  their 
power  of  contractility.  Just  as  we  saw  the  larger  arteries 
were  principally  elastic,  so  the  smaller  ones  are  principally 
contractile.  This  contractility  or  power  of  reducing  their 
diameter  is  produced  by  the  muscular  coat  previously 
spoken  of.  Though  the  smaller  vessels  possess  this  mus- 
cular coat,  it  by  no  means  follows  that  they  are  always 
fully  contracted ;  in  fact  special  nerves  exist  for  the 
purpose  of  supplying  the  iieedful  impulses  to  the  muscular 
tissue  which  controls  or  regulates  the  diameter  of  the 
vessels.  In  this  way  the  muscular  artery  may  be  con- 
tracted or  relaxed  dependently  upon  the  set  of  nerves 
brought  into  operation  :  and  this  movement  of  the  smaller 
muscular  vessels  acts  as  a  tap  and  regulates  the  blood 
supply  to  any  given  part  of  the  body. 

Capillaries. — The  minute  arteries  terminate  in  the  capil- 
laries. It  is  in  these  vessels  that  the  interchange  between 
the  blood  on  the  one  hand  and  the  tissues  on  the  other 
takes  place,  and  this  is  rendered  easy  by  the  fact  that  the 
wall  of  the  capillary  consists  simply  of  a  very  thin  mem- 
brane composed  of  cells  known  as  endothelial  plates.  It  is 
through  this  wall  that  the  exchange  of  material  with  the 
tissues  occurs.  The  capillary  is  capable  of  expanding  and 
contracting  and  so  accommodating  more  or  less  blood  as 
the  case  may  be  ;  this  is  brought  about  by  the  elastic 
nature  of  the  membrane  composing  the  capillary  wall  (for 
there  are  no  nerves  supplied  to  them),  under  the  influence 
of  the  varying  internal  fluid  pressure. 

The  size  of  the  capillaries  varies ;  in  places  such  as  the 
lungs  they  are  relatively  large,  in  other  parts  such  as  the  skin 
they  are  very  small.     Their  size  depends  upon  the  amount 


THE  BLOODVESSELS  57 

of  blood  which  has  to  pass  through  them  ;  in  consequence 
they  are  larger  during  active  exercise  than  during  rest. 
If  they  be  observed  microscopically  in  the  living  animal, 
the  capillaries  may  be  seen  as  a  network  enclosing  small 
islands  of  tissue.  These  are  the  areas  where  the  inter- 
change between  the  blood  and  tissues  occurs. 

The  Veins  receive  their  blood  from  the  capillaries.  They 
are  thinner-walled  than  the  arteries,  and  their  walls  collapse 
when  empty.  Though  some  variation  exists  in  their  struc- 
ture, yet,  speaking  generally,  they  contain  less  elastic  and 
muscular  material  than  arteries,  and  more  fibrous  tissue. 
In  certain  veins,  such  as  the  venas  cavre  and  those  of  the 
pregnant  uterus,  there  is  a  considerable  development  of 
muscular  tissue  in  their  walls. 

The  venous  system  is  larger  than  the  arterial,  and  its 
capacity  is  therefore  greater.  The  abdominal  veins  are 
capable  of  holding  the  whole  blood  of  the  body,  as  we  see 
for  instance  at  post-mortem  examinations.  The  veins  as 
they  pass  from  the  capillaries  towards  the  heart  become 
reduced  in  number  and  increased  in  size,  and  they 
terminate  in  the  right  auricle  of  the  heart  by  means  of 
two  trunks,  the  united  areas  of  which  greatly  exceed  the 
aorta. 

In  the  veins  valves  are  found.  These  are  well-marked 
in  the  veins  of  the  head,  neck,  and  extremities.  The 
valves  look  towards  the  heart  and  supply  a  simple  and 
essential  means  of  ensuring  the  return  flow  of  the  blood 
along  the  veins  to  the  heart.  In  certain  places  such  as  the 
bones,  intestines,  foot,  and  brain,  the  veins  have  no  valves. 

Veins  are  normally  pulseless,  but  an  exception  must  be 
made  to  this  statement  in  the  case  of  the  lower  extremity 
of  the  jugulars,  just  where  they  enter  the  chest.  It  is  quite 
common  in  the  horse  to  observe  pulsations  in  these  vessels 
for  an  inch  or  so  along  the  neck,  due  no  doubt  to  the  contrac- 
tion of  the  right  auricle.  It  is,  however,  distinctly  abnormal 
for  these  venous  pulsations  to  extend  a  great  distance  up  the 
neck.  The  presence  of  a  marked  jugular  pulse  is  frequently 
associated  with  heart  trouble. 


58     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Mechanics  of  the  Circulation. — At  each  systole  of  the 
ventricle  a  certain  amount  of  blood  is  forced  under  great 
pressure  into  an  already  full  aorta,  and  imprisoned  there 
by  the  closure  of  the  aortic  valves.  The  aorta  dilates  to 
receive  this  extra  blood,  because,  owing  to  the  friction  in 
the  smaller  vessels,  or,  as  we  shall  speak  of  it,  the  peripheral 
resistance,  it  is  impossible  for  the  amount  pumped  into  the 
aorta  at  each  systole  to  pass  out  at  once  at  the  periphery ; 
in  this  way  high  blood  pressure  is  produced  in  the  arteries. 
The  increase  in  the  size  of  the  aorta  to  accommodate  this 
extra  blood  commences  near  the  heart,  and  runs  as  a  wave 
to  the  periphery  ;  this  wave  is  the  jndse. 

The  two  important  points  in  the  circulation  which  we 
have  now  to  consider  are  blood  pressure  and  pulse,  and  to 
understand  these  it  is  necessary  that  we  should  study 
briefly  the  laws  which  govern  the  flow  of  fluids  through 
tubes.  If  water  be  pumped  through  a  rigid  tube  or  pipe, 
at  every  stroke  of  the  pump  as  much  fluid  passes  out  at 
the  farther  end  of  the  tube  as  enters  it  at  the  other. 
Between  the  strokes  of  the  pump  no  fluid  issues  from  the 
pipe,  the  jet  is  only  produced  at  the  moment  the  pump  is 
in  action.  No  more  water  can  enter  this  rigid  tube  from 
the  pump  end  than  can  leave  it  at  the  outlet.  If  now 
water  be  pumped  through  a  short  elastic  tube,  the  outlet 
of  which  is  in  no  way  obstructed,  the  current  of  water 
through  it  behaves  just  as  if  it  were  a  rigid  tube,  viz.,  a 
stream  of  water  issues  from  the  outlet  during  the  action  of 
the  pump,  and  nothing  more  happens  until  the  next  stroke. 
An  important  alteration  can,  however,  be  made  to  the 
current  through  the  elastic  tube,  by  offering  an  obstruction 
at  the  outlet  to  the  free  passage  of  the  water.  The  effect 
of  this  obstruction  is  that  the  elastic  tube  expands  to 
accommodate  the  contents,  while  a  stream  pours  from  the 
partly  obstructed  outlet  which  no  longer  corresponds  to  the 
stroke  of  the  pump,  but  is  a  continuous  stream  which  issues 
so  long  as  the  pumping  is  continued.  This  continuous 
stream  is  produced  by  the  elastic  recoil  of  the  tube  keeping 
up  the  pressure  which  the  pump  imparted  to  the  fluid,  and 


THE  BLOODVESSELS  59 

the  reason  why  the  elastic  recoil  of  the  tube  is  now  brought 
into  play  is  owing  to  the  partly  obstructed  outlet  or,  as  we 
have  already  termed  it,  the  peripheral  resistance.  If  the 
elastic  tube  be  of  sufficient  length,  a  continuous  stream 
will  issue  in  spite  of  the  absence  of  a  clamp  ;  this  is 
brought  about  by  the  internal  fluid  friction  against  the 
walls  of  the  tube,  which  of  course  causes  a  peripheral 
resistance.  In  elastic  tubes,  therefore,  the  recoil  of  the 
tube  converts  an  intermittent  into  a  continuous  flow,  and 
the  distension  of  the  tube  which  produces  the  recoil  is 
caused  by  the  peripheral  resistance. 

The  whole  mechanics  of  the  circulation  can  be  worked 
out  on  a  model  consisting  of  a  syringe  to  represent  the 
heart,  elastic  tubes  to  represent  the  bloodvessels,  and  a  few 
clamps  to  offer  the  needful  peripheral  resistance.  With 
such  a  model,  if  water  be  forced  into  the  arterial  tubes, 
the  clamps  being  open  and  the  peripheral  resistance  there- 
fore very  small,  it  is  found,  by  means  of  a  manometer,  that 
the  pressure  in  the  arterial  tube  rises  with  each  stroke 
of  the  syringe,  and  falls  with  the  free  pouring  of  the 
contents  into  the  tubes  representing  the  veins.  As  the 
peripheral  resistance  is  small  the  pulsation  set  up  in  the 
fluid  readily  passes  into  the  veins,  and  a  manometer  will 
here  register  nearly  the  same  rise  and  fall  as  was  met  with 
in  the  arteries. 

If,  how^ever,  the  vessels  be  clamped  so  as  to  produce  a 
resistance,  the  first  stroke  of  the  pump  causes  the  arteries 
to  become  distended,  they  then  recoil,  and  while  under- 
going this  they  receive  another  stroke  from  the  pump  and 
become  still  more  distended.  Once  more  they  recoil  on  their 
contents  and  are  once  more  distended  by  the  action  of  the 
pump,  and  so  on.  If  all  this  time  the  arterial  manometer 
be  watched,  it  will  be  observed  that  the  mercury  or  water 
rises  with  each  stroke  of  the  pump,  but  instead  of  falling 
at  once  to  zero  as  it  did  in  the  undamped  tube,  it  only  has 
time  to  fall  a  short  distance  before  a  second  stroke  of  the 
pump  sends  it  still  higher  than  before ;  this  is  repeated  at 
every  stroke  of  the  pump  until  the  water  or  mercury  refuses 


GO    A  MANITAL  OF  VETERINARY  PHYSIOLOGY 

to  rise  any  higher  in  the  tube,  contenting  itself  by  rising  to 
a  certain  height  at  each  stroke  of  the  pump,  and  falling 
to  a  certain  level  during  the  interval  between  one  stroke 
and  another.  In  other  words,  a  mean  pressure  has  been 
established  in  the  tubes  representing  the  arteries,  which 
has  been  brought  about  by  the  peripheral  resistance,  the 
elastic  recoil  of  the  tube,  and  the  pumping  of  the  syringe. 
So  long  as  these  factors  remain  the  same  the  mean  pressure 
will  not  vary.  If,  however,  the  clamped  vessels  be  released, 
so  as  to  allow  fluid  to  flow  more  easily  into  the  tubes 
representing  the  veins,  at  once  the  manometer  shows  a 
fall  in  the  mean  pressure  owing  to  the  removal  of  a  certain 
amount  of  resistance,  and  by  removing  this  resistance 
completely  the  mean  pressure  falls  to  zero.  The  mean 
pressure,  then,  represents  the  force  which  is  necessary  to 
cause  as  much  fluid  to  pass  through  the  periphery  as  is 
being  pumped  into  the  system  of  tubes  by  the  syringe ;  if 
the  peripheral  resistance  is  high  the  pressure  is  high,  and 
vice  versa. 

A  careful  study  of  this  experiment  places  us  in  complete 
possession  of  the  main  facts  of  the  circulation,  but  even  up 
to  this  point  we  have  not  learned  all  the  lessons  it  is 
capable  of  teaching. 

If  a  manometer  be  placed  on  the  venous  side  of  the 
model,  it  will  show  a  very  low  pressure  at  the  time  when 
the  arterial  pressure  is  high.  If  the  arterial  tubes  be  felt 
it  will  be  observed  that  at  each  stroke  of  the  pump  they 
expand,  producing  what  is  known  in  living  tubes  as  the 
jmlse  ;  this  expansion  of  the  tube  is  greatest  nearest  to  the 
syringe,  dying  out  entirely  at  the  peripheral  resistance.  It 
is  evident  that  if  we  loosen  the  clamps,  and  so  reduce  the 
resistance  and  lower  the  mean  pressure,  that  pulsatile  waves 
will  pass  over  to  the  venous  side  of  the  model,  and  these 
can  again  be  obliterated  by  screwing  up  the  clamp.  Lastly, 
our  model  if  working  at  mean  pressure  will  show  the  effect 
of  injury  to  the  arterial  tubes ;  if  these  be  pricked,  a  con- 
tinuous jet  of  water  shoots  out,  the  strength  of  the  jet 
var^'ing  with  each  stroke  of  the  syringe,  whilst  an  injury 


THE  BLOODVESSELS  61 

to  the  venous  side  produces  no  jet  of  water  but  only  a 
trickling  flow. 

Practically  this  embraces  our  knowledge  of  the  main 
facts  of  the  circulation,  for  all  we  have  found  true  of 
syringe,  elastic  tubes,  and  clamps,  will  be  found  true  of 
heart,  bloodvessels,  and  peripheral  resistance.  The  heart 
has  to  keep  the  arteries  full ;  the  innumerable  smaller 
arteries  with  their  muscular  coat  supply  the  peripheral 
resistance.  Under  the  influence  of  this  and  the  contrac- 
tion of  the  left  ventricle,  the  pressure  in  the  arteries  rises 
so  high,  and  their  distension  is  so  great,  that  as  much 
blood  passes  through  the  periphery  during  the  contraction 
of  the  heart  and  the  ensuing  pause,  as  enters  the  aorta 
during  the  contraction  of  the  left  ventricle.  The  elastic 
system  of  arteries  ensures  that  an  intermittent  is  converted 
into  a  continuous  flow,  and  thus  a  perpetual  pressure  is 
kept  up  on  the  mass  of  blood  during  the  heart's  pause. 
By  a  contraction  of  the  arterioles  the  peripheral  resistance 
is  increased  and  the  l)lood  pressure  raised ;  by  a  relaxation 
of  the  arterioles  the  peripheral  resistance  is  reduced  and 
the  blood  pressure  falls.  We  have  stated  that  a  contraction 
of  the  arterioles  by  increasing  the  resistance  raises  arterial 
pressure  and  as  a  rule  lowers  that  in  the  veins.  This  holds 
equally  true  for  the  pressure  conditions  in  the  vessels  of 
any  locally  circumscribed  area  of  the  body  as  for  the 
vascular  system  generally.  It  must  not,  however,  be  for- 
gotten that  local  efiects  may  and  do  produce  general  effects. 
If,  for  instance,  one  artery  alone  contracts  this  must  lead 
to  an  increase  of  arterial  pressure,  which  produces  an 
increased  flow  of  blood  through  all  the  simultaneously  un- 
contracted  arteries  on  into  the  veins.  When  the  contracted 
artery  is  small,  so  that  the  area  it  supplies  is  limited,  the 
local  effects  are  more  marked  than  the  general  effects.  If, 
on  the  other  hand,  the  local  area  affected  is  at  all  large, 
the  influence  of  changes  in  the  arteries  of  this  area  on  the 
general  blood  pressure  may  be  very  obvious.  We  shall 
meet  with  a  striking  instance  of  this  when  dealing  with  the 
action  of  the  depressor  nerve  on  blood  pressure,  through 


62     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


the  medium  of  alterations  in  the  arteries  which  supply  the 
splanchnic  area,  by  means  of  the  splanchnic  nerve. 

Blood  Pressure. — From  what  has  been  said,  it  is  hardly 
necessary  to  define  blood  pressure  as  the  pressure  exercised 
upon  the  blood  in  the  elastic  vessels,  resulting  from  the 
action  of  the  heart  and  the  peripheral  resistance. 

If  the  peripheral  resistance  is  great  through  a  contraction 
of  the  arterioles,  the  amount  of  fluid  passing  into  the  veins 
is  reduced  in  quantity ;  a  larger  bulk  of  fluid  will  in  conse- 
quence exist  between  the  pump  and  its  outlets,  and  the 
blood  pressure  rises.     If,  on  the  other  hand,  the  blood  is 


Fig.  17. — Diagram  of  Blood  Pressure  (Foster). 
A,  Arteries.      P,  Peripheral  Region  (minute  arteries,  capillaries,  and 
veins).     V,   Veins.     The    ordinates    represent   the    difference    in 
blood  pressure  in  the  several  regions  of  the  vascular  system. 

passing  freely  through  the  dilated  arterioles  the  blood 
pressure  falls.  When  the  heart  is  more  active,  or  when 
the  arterioles  contract,  the  blood  pressure  rises ;  when  the 
heart  is  less  active  or  the  arterioles  dilated  the  blood  pressure 
falls.  The  mean  pressure  in  the  arteries  is  highest  close  to 
the  aorta  and  lowest  in  the  region  of  the  periphery ;  the  fall 
in  pressure  from  the  aorta  to  the  periphery  is  gradual.  At 
the  minute  arterioles  the  fall  in  pressure  is  sudden,  and  in 
the  veins  gradual  and  very  slow  ;  in  fact,  owing  to  causes 
to  be  dealt  with  in  the  chapter  on  Respiration,  a  negative 
pressure  may  exist  in  the  great  veins  near  the  heart. 
Fig.  17  exhibits  in  a  graphic  form  the  fall  in  blood  pressure 
in  the  different  regions  of  the  vascular  system. 


THE  BLOODVESSELS  63 

In  the  carotid  of  the  horse  the  pressure  may  be  from 
8i  inches  to  12|  inches  of  mercury  (215  to  325  mm.),  or  equal 
to  a  column  of  blood  from  9J  feet  to  13f  feet  in  height ;  in  the 
dog  4  inches  to  6|  inches  of  mercury  (100  to  170  mm.),  or 
4^  feet  to  7^  feet  of  blood,  and  even  in  the  rabbit  a  pressure 
of  3  feet  of  blood  may  be  obtained.  The  first  blood  pressure 
experiment  made  was  on  a  horse  ;  the  tube  was  placed  in 
the  femoral  artery,  and  the  blood  rose  8  feet  3  inches  in 
height. 

In  the  following  table  is  given,  according  to  Yolkmann, 
the  mean  pressure  in  the  aorta  of  different  animals  : 

Horse  -  12i^  inches  (321  mm.)  to  6  inches  (150  mm.). 
Dog  -  -  6-^  inches  (172  mm.)  to  4|  inches  (104  mm.). 
Sheep         -     8    inches  (206  mm.)  to  6|,  inches  (156  mm.). 


\i.r/\...rf^\...N^ 


Fig.  18.— Tracing  of  Arterial  Pressure  with  a  Mercury  Mano- 
meter (Foster). 

The  smaller  curves  P,  P  are  the  pulse  curves  due  to  the  heart-beat. 
The  space  from  r  to  /•  embraces  a  respiratory  undulation.  The 
tracing  is  taken  from  a  dog,  and  the  irregularities  visible  in  it  are 
those  frequentl}'  met  with  in  this  animal. 


The  arterial  pressure  varies,  as  we  have  said,  with  each 
systole  of  the  ventricle,  but  besides  this  there  are  also 
certain  larger  and  longer  undulations  obtainable  in  graphic 
records  of  blood  pressure,  which  are  not  due  to  the  heart-beat 
but  are  caused  by  the  respiratory  movements  (Fig.  18). 
Thus  at  every  inspiration  the  blood  pressure  rises  and  at 
every  expiration  it  falls.  Speaking  generally  this  is  true, 
though  the  tracing  (Fig.  19)  shows  that  the  pressure  does 
not  rise  immediately  inspiration  commences,  nor  does  it  fall 
as  soon  as  expiration  begins.  The  cause  of  this  will  be 
explained  when  dealing  with  respiration. 


64     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  blood  pressure  in  the  capillaries  is  very  difficult  to 
ascertain.  It  is  probably  ',  to  1  of  that  in  the  large  arteries 
or  lies  between  20  to  40  mm.  of  mercury. 

Blood  pressure  in  the  veins  is  ^V  o^^  V.-;  o^  ^^^^.t  in  the  large 
arteries.  The  greater  the  distance  the  veins  are  from  the 
heart  the  greater  the  pressure,  so  that  the  highest  pressure 
is  in  the  peripheral  veins  and  the  lowest  in  the  jugular.  In 
a  sheep  the  following  values  were  obtained  : 

Jugular  vein  -  -  -  -    '""  inch  (0"2  min.). 

Facial  vein  _  -  -  -1,    inch  (3  mm.). 

Brachial  vein  -  -  -  -     ,",;   inch  (12  mm.). 

Crural  vein  -  -  -  -     i    inch  (14  mm.). 


Fig.  19. — Rabbit.     Influence  of  IiEspiratory  Movements  upon 
Artekial  Blood  Pressure  (Waller). 

The  blood  pressure  is  the  upper  tracing,  the  respiratory  movement  is 
the  lower  tracing.     I  is  inspiration,  E  expiration. 

In  the  large  veins  just  as  they  enter  the  heart  the 
pressure  is  very  low,  and  here  the  manometer  may  show 
even  a  negative  pressure  at  intervals  ;  in  the  anterior  vena 
cava  of  the  dog  a  negative  pressure  of  -}  inch  of  mercury 
(3  mm.)  may  be  registered.  This  is  due  to  inspiration, 
which  by  producing  a  negative  pressure  in  the  thorax  assists 
the  blood  to  reach  the  right  auricle  ;  it  is  this  negative 
pressure  which  in  the  human  subject  renders  operations  at 
the  root  of  the  neck  dangerous,  air  being  aspirated  into 


THE  BLOODVESSELS  65 

ihe  heart  should  the  vessels  be  wounded.  From  observa- 
tions on  the  horse  the  risk  of  air  entering  during  operation 
may  be  neglected.  Blowing  air  into  the  veins  causes  no 
discomfort  until  a  considerable  amount  has  been  introduced. 
Even  then  only  sighing  respirations  are  produced. 

The  amount  of  blood  which  may  be  removed  from 
the  body  without  lowering  the  blood  pressure  is  surprising. 
This  is  explained  by  the  fact  that  the  vessels  adjust  them- 
selves to  the  reduced  bulk  of  fluid  in  circulation  ;  this 
adjustment  is  effected  by  means  of  a  nervous  apparatus 
to  be  dealt  with  presently,  and  in  this  way  the  blood  pres- 
sure is  kept  up.  Experiments  show  that  it  is  not  until  two- 
fifths  of  the  blood  in  the  body  have  been  removed  that  the 
blood  pressure  begins  to  fall ;  after  cessation  of  haemorrhage 
the  pressure  again  rises,  unless  the  loss  of  blood  amounts 
to  3  per  cent,  of  the  body  weight,  in  which  case  the  low 
pressure  becomes  dangerously  permanent. 

It  is  astonishing  how  rapidly  a  deficiency  in  the  circu- 
lating fluid  is  made  good.  The  fact  is  that  the  tissues  give 
up  their  fluid  in  an  endeavour  to  replace  the  loss  of  blood, 
quite  apart  from  the  repair  which  is  being  effected  through 
the  thoracic  duct.  It  is  the  loss  of  fluid  by  the  tissues 
which  causes  the  thirst  of  hemorrhage. 

Circulation  in  the  Living  Tissues. — The  circulation  in  the 
living  animal  may  be  readily  seen  in  the  web  of  a  frog's  foot, 
or  in  the  mesentery  of  a  mammal,  and  in  this  way  we  learn 
exactly  how  the  corpuscles  behave  within  the  vessels. 

In  all  capillary  vessels  of  small  size  the  corpuscles  pass 
through  singly,  sometimes  revolving  in  the  plasma,  travers- 
ing certain  sections  very  rapidly,  others  very  slowly.  In 
the  vessels  larger  than  the  capillaries,  such  as  the  com- 
mencement of  the  small  veins,  the  stream  of  blood  behaves 
somewhat  differently ;  in  these  the  centre  of  the  vessel  is 
occupied  by  a  column  of  red  cells,  whilst  between  them  and 
the  coats  of  the  vessel  is  a  clear  layer  or  zone  in  which  may 
be  seen  the  white  corpuscles  strolling  lazily  along  the  sides, 
occasional!}'  stopping,  then  moving  forward  once  more. 
This  difference  in  the  behaviour  of  the  corpuscles  is  due 

5 


66     A  MANUAL  OF  VETEEINARy  PHYSIOLOGY 

to  the  physical  fact  that  the  friction  against  the  sides  of 
the  vessel  is  greater  than  in  the  centre  ;  but  apart  from 
that,  there  appears  to  be  an  attraction  exerted  on  the  white 
corpuscles  by  the  endothelium,  so  that  they  may,  as  pre- 
viously pointed  out,  pass  completely  through  the  wall 
of  the  vessel  into  the  surrounding  tissue.  This  is  especi- 
ally well  marked  in  inflammation,  where  the  slowly  moving 
white  corpuscles  become  attached,  as  it  were,  to  the  lining 
of  the  vessel  and  collect  in  masses  ;  with  them  also  may 
be  seen  the  blood  platelets,  which  under  the  normal  con- 
dition of  circulation  occupy  the  central  zone  of  the  vessel 
with  the  red  blood  cells.  Under  inflammatory  action  the 
white  cells  pass  completely  through  the  vessel  wall  in 
large  numbers,  aided,  as  previously  pointed  out,  by  their 
amoeboid  movements  and  the  spaces  existing  between  the 
endothelial  plates  of  the  vessel.  This  is  known  as  the 
migration  of  white  corpuscles.  The  essential  changes 
taking  place  in  inflammation  occur  in  the  wall  of  the 
vessel,  and  the  passage  of  corpuscles  through  this  is  not 
limited  solely  to  the  white,  but  the  red  may  also  pass  out. 
That  inflammatory  changes  are  essentially  due  to  the  con- 
dition of  the  vessel-wall  and  not  to  that  of  the  blood,  is 
proved  by  the  fact  that  an  artificial  corpuscular  fluid  in- 
troduced into  the  inflamed  part  behaves  exactly  as  does  the 
blood. 

The  Pulse. — It  is  a  fact  of  common  observation  that  the 
arteries  throb  or  pulsate  whilst  the  veins  do  not,  and  we 
now  have  to  inquire  what  really  produces  this  pulsation, 
and  why  it  stops  at  the  arterioles. 

When  the  left  ventricle  contracts  and  drives  its  blood  into 
the  aorta,  the  arteries  distend  to  accommodate  it  and  then 
recoil  owing  to  their  elasticity ;  each  expansion  of  the 
arterial  wall  coincides  with  a  contraction  of  the  ventricle, 
and  so  each  beat  or  throb  of  the  pulse  corresponds  with  a 
contraction  of  the  heart.  This  intermittent  expansion  of  the 
arteries  gradually  becomes  less  marked  at  a  distance  from 
the  aorta,  and  dies  out  at  the  arterioles.  We  have  pre- 
viously (p.  58  and  p.  61)  drawn  attention  to  the  fact  that 


THE  BLOODVESSELS  67 

the  elastic  properties  of  the  arterial  wall,  together  with  the 
jyeripheral  resistance  in  the  smallest  bloodvessels,  convert 
the  intermittent  flow  started  by  the  heart  into  the  con- 
tinuous stream  in  the  capillaries  and  veins.  In  seeking 
for  the  cause  of  the  disappearance  of  the  pulse  we  find  it 
similarly  in  the  elastic  property  of  the  arterial  walls.  In 
virtue  of  this  property  each  inch  of  the  arteries  is  engaged, 
by  means  of  its  sudden  distension  after  each  heart-beat 
and  its  more  gradual  elastic  recoil  before  the  next,  in 
sheltering  the  capillaries  from  the  effect  of  that  beat.  The 
oscillations  of  pressure  which  give  rise  to  the  pulse  are,  so 
to  say,  *  damped '  by  the  elastic  arterial  walls,  or  in  other 
words  converted  into  a  steady  pressure,  a  fraction  of  the 
pulse  being  thus  actually  destroyed  by  each  inch  of  the 
arteries.  When  all  the  fractions  thus  destroyed  are  added 
together  we  can  readily  understand  why  the  initial  'Jerk,' 
to  which  the  pulse  is  due,  has  entirely  disappeared  just 
before  it  would  otherwise  have  reached  the  capillaries.  If 
the  arterioles  dilate  considerably,  when,  in  fact,  less  elastic 
recoil  of  their  walls  is  called  into  play  by  the  lessened 
peripheral  resistance,  it  may  be  possible  for  the  '  throb ' 
to  pass  not  only  through  the  arterioles  but  also  the 
capillaries,  and  appear  in  the  veins  ;  in  this  way  a  venous 
pulse  may  be  produced.  An  example  of  this  will  be  given 
when  dealing  with  the  influence  of  certain  nerves  on  the 
vessels  of  the  submaxillary  gland  of  the  dog,  which  cause 
dilatation  of  the  arterioles  and  throbbing  in  the  veins. 

This  intermittent  expansion  of  the  arteries,  called  the 
pulse,  produces  a  wave  in  the  arterial  system  which  is 
spoken  of  as  the  pulse- wave.  From  what  we  have  said 
it  is  evident  that  the  height  of  this  wave  is  greatest 
nearest  the  heart,  and  falls  to  zero  at  the  capillaries. 
The  wave  travels  with  considerable  velocity  ;  from  15 
to  30  feet  per  second.  This  may  easily  be  determined  by 
noting  the  interval  between  the  commencing  successive 
rises  of  two  levers,  resting  consecutively  on  the  wall  of  an 
artery,  at  a  measured  distance  apart.  The  length  of  the 
pulse-wave  is  also  considerable — viz.,  about  18  feet.     This 

5—2 


68     A  MANUAL  OF  VETEEINAEY  PHYSIOLOGY 

is  arrived  at  by  noting  the  time  each  single  pulsation, 
travelling  with  the  previously  determined  velocity,  takes  to 
pass  completely  under  any  one  lever.  Putting  these  data 
together  it  is  evident  that  the  beginning  of  each  pulse- wave 
is  lost  in  the  arterioles  before  its  end  has  left  the  aorta. 

No  mental  confusion  should  exist  as  to  the  difference, 
and  the  causes  of  that  difference,  between  the  rate  of  trans- 
mission of  the  pulse-wave  and  the  velocity  of  the  onward 
flow  of  the  blood.  The  factors  which  give  rise  to  them  are 
quite  distinct.  The  pulse-wave  runs  along  the  surface  of 
the  blood-stream ;  the  blood-current  runs,  as  it  were,  within 


Fig.  20. — Normal  Sphygmogram  modified  from  Dudgeon  ;  Pressure 
2  oz.  (Hamilton). 

v.e,  The  period  of  ventricular  systole  ;  v.d,  the  period  of  ventricular 
diastole  ;  r,  the  period  of  rest ;  a,  b,  c,  primary  or  percussion 
wave  ;  d,  first  tidal  or  predicrotic  wave  ;  e,  aortic  notch  ;  /,  dicrotic 
wave ;  g,  second  tidal  wave. 


the  pulse-wave ;  the  former  travels  at  a  high  speed,  the 
latter  comparatively  slowly,  at  most  some  15  inches  per 
second.  The  case  is  similar  to  that  of  a  wave  seen  moving 
rapidly  over  the  surface  of  a  slowly  flowing  stream. 

The  pulse-wave  can  be  studied  by  means  of  the  graphic 
method  ;  it  is  obvious  that  a  lever  placed  on  a  pulsating 
vessel  will  be  moved  up  and  down,  and  may  be  made  to 
trace  a  curve  which  will  record  the  passage  of  the  pulse- 
wave  under  the  lever  at  that  particular  spot.  A  tracing 
thus  obtained,  known  as  a  sjyhygmogram,  simply  registers 
the  expansion  and  recoil  of  the  artery  while  the  wave  is 
passing ;  it  will  not  give  a  tracing  of  the  pulse-wave  itself, 
which,  as  we  have  seen,  is  18  feet  in  length.  But  we  may  at 
once  say  that  unless  the  proper  degree  of  pressure  is  kept 


THE  BLOODVESSELS  69 

on  the  vessel,  great  irregularity  in  the  sphygmograms  will 
be  produced ;  it  is  held  by  some  that  certain  of  the  tracings 
obtained  are  due  to  instrumental  errors,  and  not  to  the 
true  pulse-wave. 

The  simplest  description  of  a  sphygmogram  (Fig.  20)  is 
that  it  consists  of  a  nearly  vertical  unbroken  upstroke  (the 
anacrotic  limb),  and  an  oblique  downstroke  (the  catacrotic 
limb),  which  is  broken  by  two  or  three  waves  known  as 
catacrotic  waves.  Of  these  two  or  three  waves /(Fig.  20)  is 
one  of  the  few  which  occurs  with  any  regularity,  and  is  known 
as  the  dicrotic  wave.  The  notch  e  is  described  as  the  aortic 
notch,  and  is  caused  by  the  closure  of  the  aortic  valves. 


B 

Fig.  21. — Tracing  from  the  Faci.ax  Artery  of  the  Horse 
(Hajiiltoxu 

A  before,  B  after  destruction  of  the  aortic  valves. 

The  dicrotic  wave  is  produced  by  a  recoil  of  blood,  the 
result  of  closure  of  the  aortic  valves  ;  this  reflected  wave 
passes  from  the  centre  over  the  whole  arterial  system.  The 
smaller  waves  in  the  catacrotic  limb  are  either  vibrations  of 
the  arterial  wall,  or  reflections  of  the  pulse-wave  from  the 
periphery  towards  the  heart.  That  the  dicrotic  wave  is  a 
reflection  from  the  aortic  valves,  is  shown  by  the  tracing 
in  Fig.  21,  taken  from  the  facial  artery  of  the  horse, 
A  before,  and  B  after  destruction  of  the  valves.  In  B  the 
dicrotic  wave  has  disappeared.  A  well-marked  dicrotic 
pulse  gives  a  double  beat  of  the  pulse  for  each  single  con- 
traction of  the  heart. 

In  connection  with  pulses  the  term  tension  has  been 
employed  by  pathologists  ;  thus  pulses  of  high  and  of  low 
tension  have  been  described,  and   an   attempt   has   been 


70     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

made  to  distinguish  between  the  pathologist's  tension  and 
the  physiologist's  pressure.  If  tension  be  defined  as  the 
elastic  force  exerted  by  the  artery  on  the  blood  within,  it  is 
evident  that  this  bears  some  distinct  relation  to  the  force 
distending  the  artery,  viz.,  the  blood  pressure  ;  a  high  blood 
pressure  and  high  arterial  tension  describe  the  same  condi- 
tions. In  an  artery  giving  a  high  tension  the  dicrotic  wave 
is  nearly  extinguished,  the  vessels  in  fact  are  so  full  that 
the  recoil  wave  makes  very  little  impression  on  the  tense 


Fig.  22.—  Sphygmograms  ok  Low  Tension,  Normal,  and  High 
Tension  Pulses  (Waller). 


arterial  wall ;  when  blood  pressure  is  low  and  the  amount 
of  movement  in  the  artery  great,  the  recoil  or  dicrotic  wave 
is  very  marked  (Fig.  22). 

The  pulse  varies  in  character,  depending  upon  age, 
condition,  and  state  of  the  system ;  it  also  differs  according 
to  the  class  of  animal.  The  following  table  shows  the 
pulse-rate  in  different  animals  : 


Elephant 


25  to    28  beats  per  minute. 


Camel     - 

-     28  „     32 

Horse     - 

-     36  „     40 

Ox 

-     45  „     50 

Sheep     - 

-     70  „     80 

Pig 

-     70  „     80 

Dos        - 

-     90  „  100 

THE  BLOODVESSELS  71 

■  Certain  variations  occur  in  the  pulse  rate.  It  is  always 
much  quicker  in  the  young  animal  than  in  the  adult ;  the 
heart  of  a  foal  at  birth  beats  100  to  120  per  minute,  and 
that  of  a  calf  90  to  130  per  minute.  As  the  animal  in- 
creases in  age  the  pulse  rate  drops,  and  in  old  age  the 
pulsations  are  not  only  reduced  in  number  but  weaker. 

The  condition  of  the  arterial  wall  alters  the  shape  and 
nature  of  the  pulse  tracing  in  old  age. 

Between  size  of  body  and  pulse  rate  there  certainly  appears 
to  be  some  connection,  and  in  the  human  family  tall  men 
have  a  slower  pulse  rate  than  short  men  of  the  same  age. 

The  heart  rate  is  rapidly  responsive  to  all  outside  influence 
such  as  excitement  or  fear.  A  harsh  word,  fear,  or  timidity, 
will  cause  the  pulse  of  a  nervous  animal  to  register  nearly 
double  the  number  of  beats  of  the  heart.  To  sickness  or 
injury  the  pulse  is  instantly  responsive,  and  is  one  of  the 
cardinal  aids  both  in  diagnosis  and  prognosis.  Variations 
of  pulse  rate  follow  as  the  result  of  work,  so  that  a  marked 
increase  in  the  number  of  beats  occurs  ;  this  means  a  larger 
amount  of  blood  in  circulation  through  tissues  in  a  state  of 
activity,  and  which  consequently  are  in  urgent  need  both  of 
repair  and  flushing.  In  fact,  there  appears  to  be  little  doubt 
that  it  is  the  substance  flushed  out  of  the  muscles  during 
work  which  stimulates  the  heart  either  directly  or  reflexly, 
though  other  explanations  have  been  offered,  such  as  reflex 
stimulation  of  the  sympathetics, 

A  relationship  exists  l)etween  heart  rate  and  the  con- 
dition of  blood  pressure  ;  when  the  blood  pressure  becomes 
low,  the  heart  rate  increases  as  the  result  of  reflex  stimula- 
tion, by  which  means  the  output  of  blood  is  increased.  If 
the  temperature  of  the  blood  be  raised  the  heart  beat 
increases  in  frequency,  and  there  appears  but  little  doubt 
that  one  cause  of  the  increased  pulse  rate  in  fevers  is  the 
actual  temperature  of  the  circulating  blood.  If  the 
temperature  of  the  l)lood  be  raised  experimentally,  it  is 
found  that  a  point  is  reached  at  which  the  heart  ceases  to 
beat ;  in  the  cat  this  has  been  found  to  be  between  111°  F. 
to  113°  F.  (44°  to  45°  C). 


72    A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

The  Velocity  of  the  Blood  varies  in  the  arteries,  capil- 
laries, and  veins,  being  greatest  in  the  former,  least  in  the 
capillaries,  and  rising  again  in  the  veins. 

The  velocity  of  flow  is  inversely  as  the  sectional  area  of 
the  tubes ;  the  total  sectional  area  of  the  capillaries  is 
greater  than  that  of  the  aorta,  therefore  the  velocity  is 
reduced  ;  from  the  capillaries  to  the  heart  the  area  becomes 
smaller  and  the  velocity  increases.  The  velocity  of  blood- 
flow  depends  on  the  width  of  the  bed  formed  by  the  vessels  ; 
as  the  arterial  system  expands,  the  velocity  diminishes ;  in 
passing  through  the  capillaries,  with  their  immense  network 
the  velocity  is  at  a  minimum  ;  in  passing  towards  the  heart 
the  vessels  are  reduced  in  number,  hence  the  bed  is  smaller 
and  the  velocity  accordingly  increased.  The  cause  of  the 
flow  throughout  the  entire  system  is  the  contraction  of  the 
left  ventricle,  and  the  gradual  fall  in  pressure  which  occurs 
from  the  aorta  to  the  right  auricle. 

The  vascular  system  has  been  compared  to  two  cones 
placed  base  to  base,  the  apex  of  one  being  the  left  ventricle, 
of  the  other  the  right  auricle ;  where  the  bases  of  the  two 
cones  meet  is  the  capillary  network.  The  sectional  area  of 
this  has  been  estimated  by  Volkmann  as  500  times  greater 
than  that  of  the  aorta,  whilst  the  passage  of  blood  through 
it  is  500  times  slower  than  in  the  aorta,  owing  to  the  width 
of  the  bed.  According  to  the  same  authority,  the  velocity  of 
blood  in  the  carotid  of  the  horse  is  from  11'8  inches  to  15*75 
inches  per  second  (30  to  40  cm.)  ;  in  the  metatarsal  artery 
of  the  horse  2*2  inches  (5"5  cm.)  per  second,  and  in  the 
jugular  vein  8*85  inches  (22  cm.)  per  second.  A  horse 
which  gave  a  carotid  velocity  of  12  inches  (30*5  cm.)  per 
second  gave  a  jugular  velocity  of  9  inches  (22*75  cm.)  per 
second.  Chauveau  found  in  the  carotid  artery  of  the  horse 
a  velocity  of  20"47  inches  (52  cm.)  per  second  during  systole, 
8'66  inches  (21*75  cm.)  per  second  at  the  beginning  of 
diastole,  and  5'9  inches  (15  cm.)  per  second  during  the 
pause. 

The  mean  velocity  in  the  carotid  of  the  dog  is  10|  inches 
(2G"5  cm.)   per  second  :  at  the  end  of  diastole  8A  inches 


THE  BLOODVESSELS  73 

(21-5  cm.)  per  second,  and  at  the  end  of  systole  12  inches 
(30'5  cm.)  per  second.  The  velocity  of  the  blood  is  there- 
fore increased  by  each  systole  of  the  ventricle,  decreased 
during  diastole,  and  falls  still  more  during  the  pause.  The 
flow  in  the  arteries  is  assisted  by  expiration,  while  that  in 
the  veins  is  assisted  by  inspiration.  The  velocity  of  the 
blood  is  greater  in  the  pulmonary  than  in  the  systemic 
capillaries,  while  in  the  vense  cavte  it  is  half  of  that  in  the 
aorta. 

Any  attempt  made  to  estimate  the  velocity  of  the  blood 
by  dividing  an  artery,  and  measuring  the  escape  of  blood 
from  its  cut  end  in  a  given  time,  would  lead  to  erroneous  con- 
clusions, for  the  velocity  in  a  closed  artery  and  an  open  one 
are  two  different  things.  In  the  undivided  artery  the  peri- 
pheral resistance  reduces  the  velocity,  in  the  divided  artery 
the  peripheral  resistance  largely  disappears  and  the  velocity 
is  five  or  ten  times  greater,  so  that  the  carotid  of  a  horse 
does  not  bleed  with  a  velocity  of  16  inches  per  second  but 
nearer  160  inches  per  second.  Or  to  put  it  in  a  practical 
way,  if  the  carotid  of  the  horse  has  a  sectional  area  of 
•2  square  inches,  the  amount  of  blood  passing  through  the 
unwounded  vessel  amounts  to  2  oz.  per  second,  while  if  the 
same  vessel  be  divided  the  loss  of  blood  would  be  nearly 
1  pint  per  second. 

The  Duration  of  the  Circulation  depends  upon  the  length 
of  time  it  takes  a  red  corpuscle  to  travel  from  a  given 
point  and  back  to  it  again.*  But  there  are  many  different 
paths  it  can  take.  For  instance,  from  left  heart  through 
coronary  vessels  to  right  heart  and  again  back  to  left  heart 
would  occupy  a  shorter  time  than  a  course  through  the 
liver,  or  through  the  feet  or  tail,  so  that  a  circulation  time 
may  mean  nothing  more  than  that  a  certain  number  of 
corpuscles  have  found  the  shortest  cut  through  the  circula- 
tion, or  on  the  other  hand  have  taken  the  longest.     In  a 

*  The  circulation  time  is  determined  either  by  injecting  an  easily 
distinguishable  salt  into  the  blood,  or  more  precisely  by  increasing  the 
electrical  conductivity  of  the  blood  by  injecting  into  it  a  neutral  salt 
solution. 


74    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

horse  with  a  pulse  frequency  of  42,  the  average  complete 
circuit  is  performed  in  31  "3  seconds  (Hering),  and  is 
equivalent,  according  to  the  latter  observer,  to  about 
28  beats  of  the  heart.  In  the  rabbit  with  a  pulse  frequency 
of  168  per  minute,  the  time  occupied  in  completing  the 
round  of  the  circulation  was  7'79  seconds,  or  again  in 
28  heart  beats  ;  with  the  dog  16*7  seconds  or  in  26*7  heart 
beats. 

Aids  to  the  Circulation. — The  contraction  of  the  left 
ventricle  is  sufficient  to  drive  the  blood  all  over  the  body> 
but  in  the  veins  this  force  is  assisted  by  the  muscles  com- 
pressing the  vessels  from  without,  while  the  presence  of 
valves  within  prevents  regurgitation.  This  is  especially  the 
case  in  the  veins  of  the  limbs  where  the  fluid  has  to  flow 
against  gravity.  The  circulation  in  the  large  veins  near 
the  heart  is  assisted  by  the  process  of  inspiration  and  the 
dilatation  of  the  right  auricle,  both  of  which  have  an 
aspirating  effect  on  the  blood  in  the  larger  veins.  The 
sucking  action  of  the  left  auricle  assists  also  in  drawing  the 
blood  in  the  pulmonary  veins  towards  the  heart. 

Influence  of  the  Nervous  System. — The  bloodvessels  are 
under  the  control  of  the  nervous  system  acting  on  the 
muscular  elements  in  their  walls,  by  which  means  a  reduc- 
tion or  increase  in  their  size  is  produced.  Such  alterations 
are  essential  if  a  mean  blood  pressure  is  to  be  maintained ; 
a  rise  in  pressure  in  one  part  of  the  system  may  be  com- 
pensated by  a  fall  in  another,  and  this  is  entirely  brought 
about  by  an  alteration  in  the  diameter  of  the  small  arteries 
or  arterioles.  The  evidence  that  the  nervous  system  does 
j)ossess  this  power  over  the  bloodvessels,  is  furnished  by  the 
simple  experiment  of  dividing  the  cord  below  the  medulla, 
and  maintaining  life  by  artificial  respiration.  The  imme- 
diate effect  of  division  is  an  enormous  fall  in  blood  pressure ; 
in  the  dog  it  will  drop  two-thirds  below  the  normal,  and 
this  is  due  to  the  vessels  dilating  as  the  result  of  paralysis, 
their  tone  having  been  lost  through  the  injury  to  the  cord. 
From  this  experiment  it  is  quite  certain  that  structures 
above  the  section  are  responsible  for  the  nerve  control  of 


THE  BLOODVESSELS  75 

the  vessels,  and  these  can  be  further  localized  by  making 
a  section  above  the  medulla,  the  influence  of  which  on  the 
blood  pressure  is  nil.  It  may,  in  fact,  be  readily  shown 
that  in  the  medulla,  in  the  region  of  the  fourth  ventricle, 
is  a  small  area  the  function  of  which  is  to  produce  and 
maintain  the  contracted  condition  of  the  bloodvessels 
defined  as  tone,  and  to  this  area  the  name  vaso-motor  centre 
has  been  given.  By  means  of  it  the  calibre  of  the  blood- 
vessels throughout  the  body  is  regulated. 

Experiment  shows  that  in  addition  to  the  head  centre, 
the  medulla,  there  are  subcentres  for  vaso-motor  action 
in  the  cord.  If,  for  example,  the  cord  be  divided  in  the 
lumbar  region,  the  vessels  of  the  hind  limbs  dilate,  and  the 
blood  pressure  falls ;  but  if  the  animal  be  kept  alive 
the  blood  pressure  gradually  returns  to  normal,  the  sub- 
centre  in  the  cord  carrying  out  the  work.  This  pressure 
is  again  at  once  lost  by  destroying  the  already  divided  cord. 

Other,  and  perhaps  more  obvious,  evidence  of  the  influence 
of  the  nervous  system  over  the  bloodvessels  is  furnished  by 
the  ear  of  the  rabbit,  for  if  the  sympathetic  be  divided  in 
the  neck,  the  ear  on  that  side  suddenly  becomes  flushed 
with  blood,  hot,  and  congested,  and  vessels  not  previously 
visible  to  the  naked  eye  now  become  very  apparent ;  and  if 
the  upper  end  of  the  nerve  be  stimulated,  so  as  to  imitate 
roughly  the  impulses  passing  along  it  in  an  intact  condition, 
the  vessels  at  once  contract,  the  flushed  appearance  disap- 
pears, and  the  ear  becomes  cooler. 

Since,  in  the  above  experiments,  mere  severance  of  the 
nerves  which  connect  the  bloodvessels  with  the  central 
nervous  system  leads  to  a  dilation  of  the  arterioles,  it  is 
evident  that  impulses  are,  under  normal  conditions,  being 
continually  sent  out  along  the  nerves  from  the  vaso-motor 
centres.  These  impulses  keep  the  arterioles  normally  in  a 
state  of  medium  or  partial  constriction,  and  this  condition 
is,  as  we  have  already  said,  known  as  arterial  *  tone.''  Now, 
inasmuch  as  the  function  of  the  vaso-motor  nerves  is  to 
regulate  the  blood-supply  to  any  given  area  of  the  body  in 
exact   accordance  with    the   varying   needs   of   that   area, 


76     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

*  tone '  becomes  a  factor  of  the  utmost  importance  in  this 
regulative  mechanism.  Without  it  all  the  arteries  of  the 
body  would,  in  the  ordinary  passive  condition  of  rest,  be 
dilated  to  their  full  extent ;  hence  no  increased  supply  of 
blood  could  be  provided  except  by  an  augmented  activity 
of  the  heart,  which  would,  of  course,  affect  the  body  as  a 
whole,  and  not  any  one  limited  part  of  it.  '  Tone  '  ensures 
that  an  arteriole  may  both  dilate  and  contract,  according 
as  it  receives  less  or  more  of  the  continuous  constricting 
impulses,  and  thus  the  regulation  of  a  varying  blood-supply 
is  made  extremely  perfect. 

Hitherto  we  have  only  spoken  of  the  constrictor  influence 
over  the  bloodvessels,  but  the  nervous  system  likewise 
exercises  a  dilator  effect.  In  contrast  to  the  constrictor 
influence,  the  dilator  is  not  tonic  in  its  action.  It  might  be 
supposed  that  a  dilator  effect  would  naturally  follow  as  the 
result  of  removing  a  constrictor  influence  from  a  vessel, 
without  the  intervention  of  a  separate  or  antagonistic  nerve 
supply ;  and  this  is  exactly  what  does  happen  in  most 
cases.  But  it  is  equally  certain  that  special  vaso-dilator 
nerves  exist,  of  which  perhaps  the  chorda  tympani  is  the 
best  example.  This  nerve  supplies  the  bloodvessels  of  the 
submaxillary  gland  with  dilator  fibres  ;  if  the  nerve  be  cut 
no  evident  change  in  the  bloodvessels  occurs,  but  if  the 
end  in  connection  with  the  gland  be  stimulated  the  vessels 
dilate,  the  arteries  throb,  and  the  blood  passes  through  the 
gland  with  such  rapidity  that  the  venous  blood  becomes 
arterial  in  appearance.  Much  the  same  phenomenon 
occurs  when  the  dilator  nerves  to  the  vessels  of  the  penis 
are  brought  into  activity.  It  is  by  no  means  certain  that 
all  vessels  have  both  a  constrictor  and  dilator  nerve  supply, 
in  fact  there  are  vessels  in  the  body  where  no  vaso-motor 
nerves  of  any  kind  can  be  demonstrated,  such  for  example 
as  the  brain,  heart,  and  lungs,  while  it  is  pretty  certain 
that  muscles  do  not  contain  any  vaso-constrictor  nerves. 

It  is  here  convenient  to  notice  that  the  vaso  -  motor 
supply  to  muscles,  which  we  have  said  is  essentially 
dilator  in  eft'ect,  is  brought  into  action  reflexly  when  the 


THE  BLOODVESSELS  77 

muscle  contracts.  Li  this  way  an  extra  blood  supply  is 
furnished  during  the  period  of  functional  activity  only. 

No  special  centre  has  been  demonstrated  in  connection 
with  the  vaso-dilator  service,  though  several  subcentres  in 
the  cord  and  medulla  exist. 

A  consideration  of  the  origin  and  distribution  of  the 
nerves  governing  the  bloodvessels  may  now  be  undertaken. 

The  vaso-constrictor  fibres  for  the  whole  body  leave  the 
spinal  cord  by  the  inferior  roots  from  the  first  dorsal  to 
the  third  or  fourth  lumbar.  They  do  not  at  once  pass  to 
their  destination,  but  through  the  medium  of  the  white 
rami  communicantes  they  enter  the  sympathetic  nervous 
system,  by  linking  up  with  the  ganglia  of  that  chain  lying 
on  either  side  of  the  spine.  Up  to  the  time  of  entering  the 
ganglia  the  constrictor  nerves  are  medullated,  but  after 
passing  through  it  they  lose  their  medulla,  and  a  fresh  lot  of 
fibres,  now  non-medullated,  arise  and  proceed  to  the  blood- 
vessels. 

The  fibres  for  the  head  and  neck  are  derived  from  the 
first  four  thoracic  roots  of  the  spinal  cord,  and  having 
passed  through  the  vertebral  sympathetic  ganglia  they 
proceed  to  the  inferior  cervical  ganglion,  and  by  means  of 
the  cervical  sympathetic  pass  to  the  superior  cervical 
ganglion.  From  this  ganglion  fibres  are  sent  out  to  supply 
the  carotid  artery  and  its  branches. 

The  constrictors  for  the  fore  Umh  leave  the  cord  by  the 
fourth  to  the  tenth  thoracic  roots,  they  pass  into  the 
stellate  ganglion,  from  which,  as  grey  rami,  they  emerge 
and  join  the  cervical  nerves  which  contribute  to  the 
brachial  plexus,  and  through  this  supply  the  vessels  of  the 
fore  limbs. 

Those  for  the  liind  limb  arise  from  the  spinal  roots  from 
the  eleventh  dorsal  to  the  third  lumbar,  and  by  white  rami 
join  the  lumbar  and  sacral  ganglia  of  the  sympathetic 
chain ;  issuing  from  these  as  grey  fibres,  they  pass  into  the 
sacral  plexus,  and  supply  all  the  vessels  of  the  hind  limbs. 

The  abdominal  constrictor  nerves  are  the  splanchnics, 
greater  and  lesser,  the  former  from  the  last  seven  dorsal, 


78     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  latter  from  the  first  two  or  thee  lumbar  roots.  The 
fibres  pass  to  the  semilunar  ganglion  of  the  solar  plexus, 
and  from  this  they  issue  as  non-meduUated  fibres,  supply- 
ing all  the  bloodvessels  of  the  abdominal  organs.  The 
splanchnics  are  the  chief  constrictor  nerves  of  the  body  ; 
section  causes  a  dilatation  of  the  vessels  they  supply  and 
a  considerable  fall  in  blood  pressure,  especially  in  those 
animals  where  the  alimentary  canal  is  largely  developed  as 
in  herbivora. 

It  will  be  observed  that  the  essential  feature  in  the 
distribution  of  these  constrictor  nerves  is  that  they  pass 
through  the  sympathetic  system  before  going  to  the  blood- 
vessels ;  and  from  being  medullated  spinal  nerves  they 
become  non-medullated  sympathetic. 

The  dilator  nerves  in  their  distribution  behave  very 
differently  to  the  constrictor ;  they  leave  the  brain  or  cord 
by  any  cerebro-spinal  nerve,  they  may  or  may  not  pass 
into  a  sympathetic  ganglion  before  distribution,  and  in 
contrast  to  the  constrictor  fibres  they  pass  direct  to  their 
destination,  instead  of  taking  a  roundabout  course ;  finally 
they  do  not  lose  their  medulla  until  near  their  termination. 
Examples  of  typical  dilator  nerves  have  previously  been 
given,  viz.,  the  chorda  tympani  to  the  submaxillary  gland, 
and  the  nervi  erigentes,  stimulation  of  which  causes 
erection  of  the  penis  ;  the  former  passes  in  company  with 
a  cranial  nerve,  and  the  latter  with  a  spinal  nerve  to  its 
destination. 

Some  spinal  nerves  contain  both  constrictor  and  dilator 
fibres,  for  example  the  brachial  and  sciatic  nerves.  On 
section  of  these  the  loss  of  constrictor  influence  is  at  first 
the  most  prominent  feature,  as  shown  by  the  hot  flushed 
limb,  but  later,  as  the  constrictor  fibres  degenerate,  the 
dilator  fibres  become  apparent,  as  on  stimulation  the  con- 
strictor nerves  fail  to  react  while  the  dilator  nerves  respond. 

From  what  has  been  said,  it  is  evident  the  nerve  supply 
to  the  bloodvessels  is  elaborate  and  complex. 

The  vaso-motor  centre  both  in  the  medulla  and  cord  are 
extremely  sensitive  to  the  varying   amounts  of   carbonic 


THE  BLOODVESSELS  79 

acid  in  the  blood  ;  an  increased  venous  condition  of  blood 
leads  to  a  constriction  of  the  arterioles  and  a  raising  of  the 
blood  pressure.  Li  asphyxia  the  arterioles  remain  con- 
stricted under  the  influence  of  the  now  intensely  venous 
blood,  as  it  stimulates  the  vaso-motor  centre  to  unwonted 
activity,  and  though  the  initially  high  blood  pressure 
subsequently  falls  to  zero,  it  does  not  do  so  because  the 
arterioles  have  relaxed,  but  because  the  heart  has  failed. 

As  previously  mentioned,  there  is  no  single  definitely 
located  centre  presiding  over  vaso-dilatation,  though  such 
may  possibly  exist. 


Fig.   23.— Tracing,  showing  the  Effect   on   Blood  Pressure   of 

STIMULATING   THE    CENTRAL    EnD     OF    THE     DEPRESSOR    NeRVE    IN 

THE  Rabbit  (Foster). 

The  time  marker  below  marks  seconds.     At  x  an  interrupted  current 
is  thrown  into  the  nerve,  and  the  blood  pressure  gradually  falls. 

The  vaso-constrictor  centre  may  be  influenced  reflexly 
by  two  distinct  kinds  of  impulse,  which  pass  from  the 
periphery  to  the  centre,  the  effect  of  which  is  either  to 
stimulate  or  depress  the  centre ;  these  are  known  as 
pressor  and  dejn-essor  eftects,  and  the  fibres  which  convey 
them  are  known  as  pressor  and  depressor  nerves. 

Pressor  fibres  are  found  in  all  sensory  nerves,  and 
stimulation  of  them  produces  an  impulse  which  passes 
to  the  vaso-motor  centre,  from  which  constrictor  impulses 
pass  to  the  splanchnic  area,  causing  contraction  of  the 
vessels  and  a  rise  in  blood  pressure. 


80     A  MANUAL  OF  YETEKINAliY  PHYSIOLOGY 

The  deiiressor  nerve  of  the  heart  (see  p.  50)  is  the  best 
example  of  its  class  ;  by  means  of  it  impulses  are  conveyed 
to  the  medulla,  and  from  there  transmitted  through  the 
spinal  cord  and  sympathetic  system  to  the  splanchnic  area. 
The  effect  is  to  depress  or  lower  the  blood  pressure  by 
causing  the  abdominal  arteries  to  dilate  (Fig,  23). 

Depressor  fibres  also  exist  which  cause  similar  reflex 
vaso-dilator  effects  which  are,  however,  too  local  to  pro- 
duce any  general  fall  of  arterial  pressure,  such  as  results 


Fig.   24. — Blood   Pressure   Curve   of   a   Rabbit,  recorded   on   a 

SLOWLY     moving     SURFACE,     TO     SHOW     TrAUBE-HeRING     CuRVES 

(Foster). 

The  heart-beats  are  the  closely  situated  up  and  down  strokes,  readily 
seen  by  means  of  a  lens.  The  next  curves  are  those  of  respira- 
tion ;  the  large  bold  undulations  being  Traube-Hering  curves.  In 
each  TraubeHering  curve  there  are  about  nine  respiratory  curves, 
and  in  each  respiratory  curve  about  nine  heart-beats. 

from  stimulation  of  the  depressor  nerve.  Such  a  mechanism 
is  in  operation  in  erectile  and  other  tissues. 

Under  certain  conditions,  such  as  asphyxia  and  haemor- 
rhage, the  vaso  -  motor  centre  transmits  to  the  vessels 
rhythmic  constrictor  impulses,  which  result  in  the  appear- 
ance, on  a  simultaneous  record  of  blood  pressure,  of 
undulations,  known  as  Traube-Hering  curves  (Fig.  24). 
Such,  of  course,  can  only  be  detected  by  taking  a  tracing 
of  the  blood  pressure.  The  existence  of  these  waves  is 
indicative  of  abnormal  excitation  of  the  vaso-motor  centre. 

"When  vessels  are  robbed  of  all  nervous  connections,  they 
do  not  necessarily  lose  their  powers  of  contraction  ;  the 
muscular  tissue  of  the  arterial  wall  responds  to  the  exciting 


THE  BLOODVESSELS  81 

influence  of  tension,  so  that  increased  blood  pressure 
causes  contraction  and  a  reduced  pressure  relaxation  of 
the  vessel  wall. 

A  very  close  parallelism  exists  between  the  nerve  fibres 
which  constrict  the  vessels  and  those  which  cause  a  more 
forcible  contraction  of  the  heart ;  in  both  cases  they  are  of 
the  non-medullated  variety,  in  each  they  excite  muscular 
action,  and  by  so  doing  increase  the  wear  and  tear  of  the 
tissues  involved.  In  the  same  way  a  close  similarity 
exists  between  those  fibres  which  dilate  the  bloodvessels 
and  those  which  slow  the  heart ;  both  are  medullated  and 
muscle-restraining,  and  in  consequence  both  excite  pro- 
cesses of  repair  rather  than  of  disintegration. 

Peculiarities  in  the  circulation  through  various  tissues 
occur  as  the  result  of  their  special  function  ;  they  are 
observed  in  the  brain,  erectile  tissues,  etc.  The  great 
vascularity  of  the  brain  necessitates  that  the  blood  should 
pass  to  it  with  a  degree  of  uniformity  which  will  ensure  the 
carrying  out  of  its  functions.  We  see  this  provided  for  in 
the  frequent  arterial  anastomoses,  for  example,  the  Circle 
of  Willis  and  the  Rete  Mirabile  of  ruminants,  which  ensures 
that  not  only  does  the  blood  enter  with  diminished  velocity, 
but  that  if  a  temporary  obstruction  occurs  in  one  vessel  its 
work  is  readily  performed  by  the  others.  The  rete  mirabile 
alluded  to,  which  forms  the  arterial  plexus  on  the  base  of 
the  brain  of  ruminants,  is  considered  by  some  to  regulate 
the  flow  of  blood  to  the  brain  when  the  head  is  depressed 
during  grazing,  and,  it  is  said,  accounts  for  the  absence 
of  cerebral  haemorrhage  in  these  animals.  It  is  probable 
that  this  may  be  one  of  its  functions,  but  the  horse 
possesses  no  rete,  and  his  head  is  depressed  during  grazing 
for  more  hours  out  of  the  twenty-four  than  is  the  case  with 
ruminants.  It  has  probably,  therefore,  some  other  function 
to  perform. 

The  pulsations  observed  in  the  exposed  brain  are  not  due 
to  the  pulse  in  the  arteries,  but  arise  from  the  respiratory 
movements ;  expiration  causes  the  brain  to  rise  by  hinder- 

6 


82     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

ing  the  letiirn  of  blood,  whilst  inspiration  causes  it  to  fall 
by  facilitating  the  tlovv. 

The  venous  arrangement  of  the  brain  is  very  remarkable  ; 
the  walls  of  the  veins  are  composed  of  layers  of  the  dura 
mater,  and  even  portions  of  the  cranial  bones  may  enter 
into  their  formation.  The  veins  or  sinuses  of  the  brain 
are  large  cavities,  which  from  their  arrangement  are  most 
unlikely  to  suffer  from  compression,  and  from  the  rigidity 
of  their  walls  are  not  capable  of  bulging  as  most  veins  do 
when  obstructed  ;  in  this  way  the  easy  return  of  the  venous 
blood  is  provided  for. 

The  cerebral  circulation  is  considerably  assisted  by  the 
presence  of  fluid  within  the  ventricles  of  the  brain.  This 
fluid  readily  passes  from  ventricle  to  ventricle,  and  from 
ventricle  to  spinal  cord ;  in  this  way,  as  the  external 
pressure  becomes  greater  the  internal  becomes  less,  and  so 
compression  of  the  brain  substance  is  avoided.  It  will  be 
remembered  that  no  vaso -motor  nerves  have  been  satis- 
factorily demonstrated  in  the  brain. 

The  singular  arrangement  of  the  venous  plexuses  of  the 
corpus  cavernosum  penis,  admits  of  this  organ  attaining  a 
vast  increase  in  size,  a  condition  which  in  the  brain  every 
measure  is  adopted  to  prevent.  The  considerable  si/e 
of  the  venous  plexuses  of  the  penis,  their  frequent  inter- 
communication, the  muscular  pressure  to  which  the  veins 
leading  from  the  sinuses  are  exposed,  produce  under  the 
direction  of  the  vaso-motor  nervous  system  a  considerable 
increase  in  the  volume  of  the  part. 

In  some  other  organs  the  distribution  of  the  bloodvessels 
is  also  peculiar.  It  is  not  known  why  the  spermatic  artery 
and  plexus  of  veins  should  take  such  a  remarkably  tortuous 
course  ;  possibly,  in  some  way  or  other,  it  may  be  con- 
cerned with  the  secretion  of  the  glands,  but  its  use  is  far 
from  clear.  On  the  other  hand,  tortuous  vessels  in  the 
walls  of  hollow  viscera,  such  as  the  stomach  and  intestines, 
perform  a  very  evident  function.  We  have  only  to  think  of 
the  size  of  a  collapsed  and  full  stomach  in  the  horse,  to 
recognise  the  necessity  for  some  arrangement  existing  to 


THE  BLOODVESSELS  83 

prevent  stretching  of  the  vessels  or  interference  with  the 
blood  supply. 

The  vast  venous  and  arterial  plexuses  of  the  foot  of  the 
horse  are  a  peculiarity  in  the  circulation  dealt  with  in  the 
chapter  devoted  to  the  Foot. 

Pathological. 

It  is  a  remarkable  fact  that  very  little  of  the  hard  life  of  a  horse  falls 
on  his  arteries  ;  with  age  the  vessels  become  more  rigid,  but  no  sudden 
strain  produces  aneurisms,  such  as  might  be  expected  from  the  class  of 
work  performed.  There  is,  however,  one  kind  of  strain  which  arises 
in  the  hunting  field,  or  under  similar  circumstances,  in  which  the  walls 
of  the  external  and  internal  iliac  arteries  suffer ;  in  consequence  of  this 
a  thrombus  forms  in  the  vessels,  which  become  partly  or  completely 
obliterated.  Collateral  circulation  sufiices  in  a  state  of  repose,  during 
which  not  a  sign  of  any  circulatory  trouble  is  evident,  but  as  soon  as 
the  animal  gets  to  woi'k,  sudden  and  painful  muscular  cramps  occur, 
and  finally  temporary  paralysis  follows.  These  symptoms  completely 
pass  away  with  rest  and  return  with  work. 

Parasitic  trouble  of  the  vessels  is  very  common,  the  main  seat  being 
the  anterior  mesenteric  artery,  which  is  rendered  rigid  and  aneurismal, 
and  has  its  lumen  obliterated  by  Strongyliis  armatus.  It  is  remark- 
able how  very  little  interference  with  the  intestinal  circulation  occurs 
in  consequence  of  this  parasitic  invasion,  and  it  is  equally  astonishing 
how  few  horses  are  free  from  this  infection.  It  is  probably  the  most 
widely  spread  equine  parasite. 

Pulse. — The  older  physicians  studied  the  pulse  with  care,  at  the 
present  day  it  does  not  receive  the  same  amount  of  attention.  It  is 
not  sufficient  to  know  the  number  of  pulsations ;  the  important  point 
is  the  character  of  the  wave. 

A  pulse  may  be  quick  or  sloio.  Either  of  these  may  be  strong, 
weak,  hard,  or  soft.  Strong  and  weak  refer  to  the  force  of  the 
ventricular  contraction,  hard  and  soft  refer  to  the  tension  as  judged 
by  the  finger — viz.,  the  amount  of  pressure  required  to  obliterate  the 
pulsations.  A  further  division  of  pulses  is  into  large  and  small;  this 
group  refers  to  the  volume  of  the  artery.  There  is  no  pulse  specially 
indicative  of  any  given  affection,  but  the  character  of  the  pulse  in  the 
prognosis  of  disease  is  of  the  utmost  clinical  value. 


6—2 


CHAPTER  IV 
RESPIRATION 

Section    1. 
The  Lungs. 

The  lungs  occupy  the  whole  cavity  of  the  thorax ;  during 
life  no  space  exists  between  the  pulmonary  and  costal 
pleura,  so  that  the  case  is  an  air-tight  one.  So  long  as 
this  air-tight  condition  is  maintained,  any  movement  which 
tends  to  increase  the  size  of  the  case,  such  as  the  retreat  of 
the  diaphragm  and  the  advance  of  the  ribs,  causes  a  disten- 
sion of  the  sacs  and  the  air  rushes  in  ;  by  a  reversed  process 
it  is  pressed  out,  viz.,  by  a  collapse  of  the  chest  wall.  If, 
however,  the  cavity  of  the  chest  be  opened  to  the  external 
atmosphere  the  lungs  collapse  owing  to  their  elastic  recoil, 
and  the  fact  that  the  atmospheric  pressure  within  and 
without  them  is  now  the  same.  Such  a  condition  would 
lead  in  the  horse  to  asphyxia,  as  the  pleural  cavities  com- 
municate, but  in  those  animals  where  the  right  and  left 
pleural  sacs  are  distinct,  the  lung  on  the  wounded  side  only 
collapses. 

The  process  by  which  the  chest  is  filled  with  air,  known 
as  Inspiration,  is  a  purely  muscular  act.  The  diaphragm 
as  the  chief  muscle  of  inspiration  contracts,  and  thereby 
recedes ;  the  ribs  are  rotated,  being  drawn  forwards  and 
outwards,  their  posterior  edges  everted,  and  the  intercostal 
space  widened.  By  this  means  the  capacity  of  the  chest  is 
increased  and  the  lungs  tend  to  fill  the  space  thus  created. 
By  doing  so  they  rarefy  the  air  already  within  them,  so 
that  a  difference  in  pressure  occurs  between  the  air  in  the 

84 


KESPIRATION  85 

lungs  and  that  outside  the  body,  and  air  rushes  in  to  restore 
equiHbrium  ;  this  .inrush  is  inspiration. 

The  increase  in  the  size  of  the  chest  which  occurs  during 
quiet  inspiration  in  the  horse  is  stated  by  Colin  to  be  as 
follows :  the  antero-posterior  or  longitudinal  diameter  of 
the  chest  is  lengthened  between  4  and  5  inches  (10  to 
12"5  cm.),  whilst  the  transverse  diameter  between  the 
eleventh  and  twelfth  ribs  is  increased  by  1|  inches  (4  cm.). 

Only  the  last  twelve  or  thirteen  pairs  of  ribs,  under 
ordinary  circumstances,  take  any  important  share  in 
respiration  ;  this  is  due  to  the  true  ribs  being  more  or  less 
covered  by  the  scapula  and  its  attached  muscles.  When, 
however,  a  difficulty  occurs  in  the  breathing,  the  elbows 
are  turned  out  which  brings  other  muscles  into  play  as 
auxiliaries  in  respiration,  and  a  certain  number  of  the 
true  ribs  now  assist  in  increasing  the  capacity  of  the 
chest. 

The  Movements  of  the  Diaphragm  are  interesting  and 
peculiar ;  this  large  flat  muscle  with  its  thin  tendon 
centrally  placed  works  to  and  fro.  In  the  body  it  is 
placed  obliquely  forwards,  extending  from  the  loins  to  the 
sternum,  and  the  main  to  and  fro  movement  occurs  in  its 
large  upper  half  rather  than  in  the  narrower  portion  below. 
Through  the  centre  of  the  diaphragm  the  posterior  vena 
cava  and  oesophagus  pass  ;  it  is  obvious  that  any  free 
movement  of  the  diaphragm  of  this  part  would  cause  a 
*  pull '  on  these  structures.  This  is  prevented  by  move- 
ment occurring  principally  in  the  upper  half  and  sides  of 
the  muscle,  while  the  lower  part  and  centre  take  very  little 
share.  When  the  diaphragm  is  receding,  it  carries  back 
with  it  all  the  structures  on  the  abdominal  side  which  are 
adjacent  to  it ;  thus  the  liver,  stomach,  and  spleen  are 
especially  affected  b}^  this  movement. 

The  diaphragm  is  curved  forward.  This  curve  is  pro- 
duced by  the  pull  exerted  on  the  muscle  by  the  air-tight 
thorax  supplemented  by  the  pressure  from  behind.  The 
diaphragm  never  becomes  flat,  even  under  pathological 
conditions  when  the  cbest  cavity  contains  gallons  of  fluid. 


86    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

There  is  no  flattening  so  long  as  the  thorax  is  air-tight ; 
as  soon  as  air  enters  it  would  flatten  if  it  were  not  for  the 
abdominal  viscera  pressing  it  forward  from  behind. 

In  Fig.  25  is  a  diagrammatic  horizontal  section  of  the 
chest,  looked  at  from  above,  showing  the  position  of  the 
diaphragm  during  inspiration  and  expiration  and  the  dis- 
placement of  the  abdominal  viscera.     Observe  the  extent 


Fig.  25. — Horizontal  Section  of  the  Horse's  Chest,  looked  at 
from  above,  illustrating  the  movements  of  the  diaphragm 

(Sussdorf). 

a,  right  lung ;  6,  left  lung.  1.  Position  of  the  diaphragm  during  deep 
expiration  ;  c,  liver  during  deep  expiration  ;  d,  stomach  during 
deep  expiration ;  e,  spleen  during  deep  expiration.  2.  Position  of 
diaphragm  during  deep  inspiration ;  c',  position  of  liver ;  d',  of 
stomach ;  e',  of  spleen  during  deep  inspiration  ;  /,  posterior  vena 
cava  as  it  passes  through  the  diaphragm. 


to  which  the  sides  of  the  diaphragm  move  as  compared 
with  the  centre.  The  squeezing  to  which  the  liver  is 
exposed  must  be  an  important  help  to  its  circulation, 
while  the  movements  of  the  diaphragm  must  materially 
assist  the  flow  of  blood  in  the  phrenic  veins  and  posterior 
vena  cava. 

Fig.  26  gives  a  side  view  of  the  horse's  chest,  the  dia- 


RESPIKATION 


87 


phragm  is  attached  around  the  margm  AFE.  The  dotted 
line  AE  indicates  the  convexity  of  the  muscle  and  the 
extent  to  which  it  bulges  into  the  chest.  The  effect  of 
this  bulging  is  that  the  lungs  rest  on  or  wrap  around  the 
diaphragm,  and,  as  it  were,  envelop  it.  The  lungs  do  not 
reach  as  low  as  the  cartilage  of  the  false  ribs,  but  about  the 


Fig.  26. — Diagram  of  the  Extent  of  the  Chest  in  the  Horse, 
AND  Position  of  the  Diaphragm. 

The  area  BCDE  is  under  the  scapula  and  its  muscles,  and  practically 
not  available  for  auscultation :  the  surface  ABEF  is  the  available 
area  of  the  chest  wall.  The  lung  reaches  to  within  a  hand's- 
breadth  of  the  false  ribs.  AF  represents  the  last  rib  ;  BE  runs 
parallel  to  the  posterior  edge  of  the  triceps.  CD  corresponds  to 
the  position  of  the  first  rib. 

The  diaphragm  bulges  into  the  chest  centrally,  thus  separating  the  two 
lungs  ;  the  curved  dotted  line  falling  from  A  to  E  represents  the 
central  line  of  the  diaphragn),  and  shows  the  extent  to  which  it 
encroaches  on  the  chest. 


breadth  of  a  hand  above  them.  The  cut  edges  of  the  ribs 
in  the  above  figure  from  F  downwards  indicate  the  lower 
and  posterior  margin  of  the  lung  ;  the  sections  of  the  ribs 
from  A  to  C  indicate  the  upper  border  of  the  lung.  It  is 
only  the  circumference  of  the  diaphragm  which  is  muscular. 


88     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

and  this  muscular  margin  is  widest  at  the  sides  and  runs 
up  to  less  than  half  the  width  above,  where  it  is  attached 
to  the  last  ribs.  The  central  portion  of  the  diaphragm  is 
a  felt  work  of  tendinous  fibres,  running  from  the  muscle  to 
the  centre  and  in  other  directions.  Filling  in  the  upper 
and  central  portions  of  the  diaphragm  are  large  muscular 
pillars,  attached  to  the  spine,  which  support  the  con- 
siderable weight  hung  on  the  diaphragm,  viz.,  the  liver, 
with  the  stomach  and  contents.  The  diaphragm  extends 
several  inches  behind  its  suspending  pillars.  Neither  the 
muscle  of  the  diaphragm  nor  the  supporting  pillars  are 
markedly  responsive  to  electrical  stimulation  as  compared 
with  voluntary  muscle. 

Expiration. — The  chest  having  been  filled  with  air,  the 
next  process  is  its  expulsion,  and  the  mechanism  here  con- 
cerned is  not  fully  agreed  upon  by  physiologists.  Whilst 
some  hold  that  it  is  a  purely  non-muscular  act,  others  con- 
tend that  certain  muscles  share  in  the  process.  All  are 
agreed  that  the  elastic  recoil  of  the  lungs  is  the  important 
factor ;  there  is  also  the  elasticity  of  the  cartilages  of  the 
ribs,  which  are  seeking  a  return  to  their  position  of  repose  ; 
and  further,  the  elastic  pressure  of  the  displaced  abdominal 
organs  acting  on  the  diaphragm  ;  added  to  which  is  the 
contraction  of  the  abdominal  muscles,  which  presses  the 
viscera  still  more  firmly  against  the  diaphragm.  The 
factors  named  are  sufificient  in  themselves  to  ensure  that 
air  is  expelled  from  the  lungs,  though  certain  muscles 
attached  to  the  ribs  may  facilitate  their  return  to  the 
position  of  repose. 

The  Foetal  Lung  contains  no  air  and  therefore  sinks  in 
water.  The  first  few  inspiratory  gasps  at  birth  distend  the 
alveoli,  but  for  some  time  the  conditions  present  in  the 
adult,  viz.,  the  negative  pressure  in  the  pleural  cavity,  and 
the  collapse  of  the  lungs  on  opening  the  chest,  are  not 
found  in  the  very  young  animal.  Such  only  occur  when 
the  cavity  of  the  thorax  is  larger  than  the  lung  in  a  state 
of  collapse.  In  the  foetus  the  lungs  exactly  fill  the  chest  in 
the  condition  of  expiration,  and  it  is  not  until  the  chest 


EESPIRA.TION  89 

cavity  grows,  as  it  were,  too  large  for  the  lungs  that  a 
negative  pressure  in  the  thorax  is  produced.  Later  on 
(p.  112)  the  cause  of  the  first  act  of  breathing  will  be  ex- 
plained. Thoracic  development  in  young  animals  is  very 
rapid  ;  a  foal  will  increase  1^^  inches  (4  cm.)  in  circum- 
ference within  the  first  few  hours  after  birth  ;  when  this 
absolute  increase  in  chest  capacity  is  established,  a  nega- 
tive pressure  in  the  pleural  cavity  is  obtained. 

Muscles  of  Respiration. — The  action  of  the  muscles  of  the 
chest  during  respiration  has  been  much  disputed.  The 
external  intercostals  doubtless,  from  the  direction  taken 
by  their  fibres,  pull  the  ribs  forward,  and  by  so  doing 
increase  the  transverse  diameter  of  the  chest ;  in  this 
respect  they  are  regarded  as  inspiratory  muscles.  The  in- 
ternal intercostals,  the  fibres  of  which  run  in  an  opposite 
direction  to  the  external,  draw  the  ribs  backwards  and  act 
as  muscles  of  expiration  ;  and  speaking  generally,  we  may 
say  that  those  muscles  which  draw  the  ribs  forward  are 
inspiratory,  whilst  those  which  draw  them  back  are  ex- 
piratory. The  following  table  shows  the  inspiratory  and 
expiratory  muscles  of  the  chest  : 

Muscles  of  Inspiration.  Muscles  of  Expiration. 

Diaphragm.  Abdominal  muscles. 

External  intercostaJs.  Internal  intercostals. 

Serratus  anticus.  Transversalis  costarum. 

Levatores  costarum.  Serratus  posticus. 

Serratus  magnus  (during  diffi-         Triangularis  sterni 

cult  respiration). 

Latissimus  dorsi         ,;  ,, 

Scaleni  ,,  ,, 

In  some  animals  the  ribs  do  very  little  work  and  the 
diaphragm  becomes  the  chief  respiratory  muscle.  In  most 
quadrupeds  the  sternum  is  fixed  to  the  ribs  and  undergoes 
little  or  no  movement ;  even  the  most  powerful  respiratory 
movements  in  the  horse  give  rise  to  no  sternal  movement. 
On  the  other  hand,  there  is  a  moderate  amount  of  move- 
ment between  the  sternal  ribs  and  the  cartilages.  During 
laboured    respiration   any   muscle   which    can    assist    in 


yo    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

advancing  the  ribs  directly  or  indirectly  is  brought  into 
play.     This  is  well  marked  in  dyspnoea. 

After  the  expiratory  act  there  is  a  pause  before  the  next 
inspiration.  In  the  horse  at  rest  the  period  of  expiration 
is  as  a  rule  longer  than  that  of  inspiration,  though  the 
proportion  between  the  two  is  not  invariable.  During 
work  the  value  of  the  inspiratory  and  expiratory  acts  is 
about  equal. 

During  inspiration  a  slight  negative  pressure  exists  in  the 
trachea,  and  during  expiration  a  slight  positive  pressure. 
In  the  pleural  cavity  a  negative  pressure  is  always  present, 
due  to  the  tendency  of  the  elastic  lungs  to  collapse.  The 
value  of  this  pull  of  the  lungs  on  the  chest  wall  has  been 
ascertained  for  the  sheep  to  be  about  I  inch  (3  mm.)  of 
mercury,  and  during  dyspnoea  |  inch.  In  the  dog  during 
inspiration  the  negative  pressure  in  the  pleural  sac  is 
I  inch  (6  mm.)  of  mercury,  whilst  during  expiration  i  inch 
(3  mm.)  has  been  observed.  In  the  horse  ^  inch  (6  mm.) 
has  been  registered  during  a  powerful  expiration,  and 
1^  inches  (28  mm.)  during  a  powerful  inspiration.  The 
negative  pressure  can  be  recognised  post-mortem  by  the 
rush  of  air  immediately  the  chest  is  punctured. 

The  number  of  respirations  varies  with  the  class  of 
animal ;  as  a  rule,  the  larger  the  animal  the  slower  the 
respirations  : 


Horse     - 

8  to  10  per  minute, 

Ox 

-     12  „  15     „ 

Sheep  and  Goat 

-     12  „  20     „ 

Dog       - 

-     15  „  20     „ 

Pig 

-     10  „  15     „         „ 

Rumination  increases  the  frequency  of  respiration,  and 
muscular  exertion  in  all  animals  at  once  causes  it  to  rise. 
In  experiments  on  respiration  this  is  most  marked  ;  walk- 
ing a  horse  will  nearly  treble  the  number  of  respirations, 
but  the  breathing  begins  to  fall  immediately  the  horse 
stops,  though  it  does  not  reach  the  normal  for  a  few 
minutes. 


EESPIRATION  91 

The  ratio  of  heart-beats  to  respiration  has  been  placed  at 
1  :  4  or  1  :  5. 

The  Effect  of  Respiration  on  Circulation. — We  have  pre- 
viously alluded  to  the  influence  of  respiration  on  the  circu- 
lation, and  the  assistance  this  renders  in  aspirating  the 
blood  into  both  sides  of  the  heart ;  further  we  have  drawn 
attention  to  the  value  of  the  negative  pressure  in  the  chest 
in  connection  with  the  diastole  of  the  heart. 

The  whole  of  the  negative  pressure  found  in  the  heart 
(p.  37)  is  not  due  to  the  diastole  alone,  but  to  the  diastole 
plus  the  aspiratory  movement,  for  if  the  chest  be  opened 
a  smaller  amount  of  negative  intra-cardiac  pressure  is 
registered. 

In  dealing  with  blood  pressure  (p.  63)  attention  was 
drawn  to  certain  undulations  of  respiratory  origin.  These 
are  produced  by  the  decrease  of  pressure  on  the  vessels  of 
the  thorax  during  inspiration ;  this  reduction  of  pressure  is 
small,  but  it  produces  sufficient  suction  to  afifect  sensibly 
the  thin-walled  veins  opening  into  the  right  auricle,  the 
blood  pressure  in  which  is  very  low.  By  this  suction  more 
blood  is  aspirated  at  every  inspiration  into  the  right  auricle, 
and  consequently  more  blood  is  ejected  from  the  left 
ventricle.  In  this  way  we  have  the  arterial  pressure  raised 
during  inspiration,  followed  by  a  fall  during  expiration 
(see  Fig.  18,  p.  63).  But  the  rise  does  not  take  place 
immediately  inspiration  begins,  nor  does  the  fall  occur 
immediately  expiration  starts,  but  shortly  after  in  both 
cases,  as  may  be  seen  in  Fig.  19,  p.  64.  The  explanation 
of  this  is  that  the  pulmonary  vessels  have  a  greater 
capacity  during  inspiration  than  during  expiration,  and 
the  increased  amount  of  blood  entering  the  right  heart  on 
inspiration  is  first  used  to  fill  the  pulmonary  vessels,  and 
this  accomplished,  the  general  blood  pressure  rises  by  the 
excess  being  passed  on  to  the  left  heart ;  similarly  the 
fall  does  not  occur  immediately  expiration  begins,  as  the 
pulmonary  vessels  have  not  yet  returned  to  their  expira- 
tory capacity. 

In  examining  the  blood  pressure  and  respiratory  curves 


92    A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

of  the  dog,  it  is  observed  that  the  pulse  frequency  is 
increased  during  inspiration,  and  reduced  during  expira- 
tion ;  this  reduction  in  the  frequency  of  the  pulse  is  due  to 
the  stimulation  of  the  cardio-inhibitory  centre,  whilst  the 
increase  is  caused  by  a  diminished  activity  of  the  inhibitory 
mechanism  (see  p.  48). 

In  speaking  of  inspiration  raising  blood  pressure  we  must 
not  forget  the  mechanical  advantages  of  a  contraction  of 
the  diaphragm  compressing  the  liver  and  posterior  vena 
cava,  and  so  forcing  more  blood  towards  the  heart ;  this 
no  doubt  is  another  cause  of  the  rise  of  blood  pressure 
during  inspiration. 

The  Nostrils.  —  Before  the  air  reaches  the  lungs  it  is 
warmed  by  passing  through  the  nasal  cavities,  so  that  it 
enters  the  trachea  at  nearly  the  body  temperature.  The 
incoming  air  also  becomes  saturated  with  watery  vapour ; 
this  saturation  likewise  occurs  in  the  nasal  chambers.  In 
the  majority  of  animals  air  may  pass  either  through  the 
nose  or  mouth  to  enter  the  trachea,  but  in  the  horse,  owing 
to  the  length  of  the  soft  palate,  nasal  respiration  alone  is 
possible ;  we  therefore  find  in  this  animal  the  nasal 
chambers  with  their  inlets  and  outlets  well  developed. 
The  opening  into  the  nostrils  of  the  horse  is  large, 
funnel-shaped,  and  capable  of  considerable  dilatation  ;  it 
is  partly  cartilaginous,  and  partly  muscular.  Immediately 
inside  the  nostril  is  a  large  blind  sac,  termed  the  false 
nostril,  and  one  of  its  uses  appears  to  be  to  increase  the 
capacity  of  the  nasal  opening  by  allowing  considerable  and 
rapid  dilatation.  Another  use  is  in  the  production  of  the 
peculiar  snorting  sound  made  by  a  horse  either  when  he  is 
alarmed  or  very  '  fresh.' 

During  forced  inspiration  the  nostril  expands,  especially 
the  outer  segment,  viz.,  that  part  in  communication  with 
the  false  nostril,  and  the  air  is  rapidly  drawn  up  through 
the  nasal  chambers ;  during  expiration  the  outer  segment 
of  the  nostril  collapses,  but  the  inner  segment,  composed 
principally  of  the  cartilaginous  ala,  dilates.  Thus  the 
movement  of  the  outer  and  upper  part  of  the  nostril  is 


RESPIRATION  93 

principally  inspiratory,  of  the  lower  and  inner  part  mainly 
expiratory,  producing  a  peculiar  double  motion  of  the 
nostrils  well  seen  after  a  gallop  or  in  acute  pneumonia 
(Fig.  27).  The  dilatation  of  the  inner  segment  of  the 
nostril  is  brought  about  by  muscular  contraction  and  by 
the  rush  of  expired  air ;  striking  the  cartilaginous  wing 
of  the  nostril  the  current  is  directed  outwards  at  an 
obtuse  angle  to  its  course  down  the  nostrils,  as  may  be 
well  seen  on  a  frosty  morning  when  a  horse  is  respiring 
rapidly. 


.>^._.Ex. 


Fig.  27.— Nostril  of  Horse. 
In,  The  inspiratory  portion  ;  Ex,  the  expiratory  portion. 

The  nasal  chambers  are  remarkable  for  their  great  depth 
and  narrowness ;  the  cavities  are  partly  filled  by  the 
turbinated  bones,  which  nearly  touch  the  septum  on  each 
side,  so  that  a  deep  but  thin  column  of  air  passes  through 
the  chambers  ;  the  result  of  this  arrangement  ensures  that 
the  air  is  saturated  with  vapour  and  raised  to  the  proper 
temperature. 

The  nasal  chamber  is  divided  into  two  parts,  the  lower  or 
respiratory  and  the  upper  or  olfactory.  The  latter  will  be 
dealt  with  under  the  Senses.  It  comprises  the  upper  portion 
of  the  superior  turbinated  bone,  ethmoid  cells,  and  a  portion 
of  the  middle  meatus ;  the  respiratory  channel  on  the  other 
hand  lies  on  the  inferior  part  of  the  nasal  chamber  and 
comprises  the  inferior  meatus,  inferior  turbinated  bone, 
part  of  the  superior  and  part  of  the  middle  meatus. 


94    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  Glottis. — The  air  having  been  warmed  by  passing 
over  the  septum  and  turbinated  bones,  enters  the  glottis, 
the  arytenoid  cartilages  being  separated  to  a  greater  or 
less  extent  to  enlarge  the  opening.  In  quiet  respiration 
this  enlargement  of  the  glottis  ib  not  very  marked,  but 
during  work  the  cartilages  are  powerfully  drawn  upwards 
and  backwards,  and  the  V-shaped  glottis  fully  opened 
(Figs.  32  and  33,  p.  124).  It  is  a  remarkable  fact  that 
the  laryngeal  opening  should  be  so  relatively  small,  con- 
sidering the  diameter  of  the  trachea  and  the  size  of  the 
nasal  openings. 

During  inspiration  the  larynx  and  trachea  slightly 
descend,  while  they  ascend  during  expiration.  This  is 
particularly  well  seen  in  horses  during  the  hurried  respira- 
tions of  disease,  producing  a  well-marked  rhythmical 
movement  of  the  laryngeal  region  and  base  of  the 
tongue. 

The  Facial  Sinuses  are  cavities  in  the  face  communicating 
with  the  nasal  chambers  ;  they  are  of  considerable  size, 
occupy  nearly  the  entire  facial  region,  and  they  give  the 
needful  bulk  to  the  head  without  adding  to  its  weight ;  they 
are  lined  by  a  membrane  which  is  continuous  with  that  of 
the  nose.  These  sinuses  are  filled  with  air  which  enters 
them  through  a  foramen  at  the  posterior  part  of  the  middle 
meatus ;  during  every  act  of  respiration  air  is  passing  in  or 
out  of  them.  At  first  sight  it  would  appear  that  air  ought 
to  enter  the  sinuses  during  inspiration,  but  the  reverse  is 
the  case  ;  it  is  only  during  expiration  that  they  are  filled, 
whilst  during  inspiration  air  is  sucked  out.  Considering 
the  position  of  the  common  inlet  to  these  sinuses,  it  is 
difUcult  to  understand  why  they  should  fill  during  expira- 
tion, though  the  advantage  of  hot  instead  of  cold  air  enter- 
ing is  evident. 

Respiratory  Changes  in  the  Air  and  Blood. — The  changes 
which  the  air  undergoes  on  passing  into  the  lungs  must 
now  be  considered. 


EESPIEATION  95 

Atmospheric  Air  contains  in  100  Parts  : 

By  Volume.  Bij  Weight. 
Oxygen          -             -     2096  23015 

Nitrogen        -  -     79-01  76-985 

Carbonic  Acid  -         '03  

The  above  gases  are  the  essential  constituents  of  the 
atmosphere ;  the  new  elements  argon  and  krypton  have, 
so  far  as  is  known,  no  physiological  significance,  and  in 
the  table  are  included  in  the  nitrogen. 

The  proportion  of  carbonic  acid  is  small ;  it  is  a  natural 
impurity  in  the  air,  though  essential  to  plant  life.  The 
atmosphere  also  contains  moisture  the  amount  of  which 
depends  upon  the  temperature ;  the  higher  the  temperature 
the  greater  the  amount  of  water  which  the  air  can  contain  as 
vapour,  and  the  lower  the  temperature  the  less  the  amount. 
Air  may  be  dry  or  saturated,  the  latter  term  implying 
that  it  contains  as  much  vapour  as  it  can  hold  at  the 
observed  temperature ;  it  generally  contains  about  one  per 
cent,  of  moisture,  and  is  spoken  of  as  dry  if  it  contains 
one-quarter  per  cent.  The  air  which  passes  from  the  lungs 
is  always  saturated  with  moisture. 

When  air  is  taken  into  the  lungs  it  alters  in  composition  : 
it  loses  a  proportion  of  its  oxygen  and  gains  in  carbonic 
acid,  as  may  be  seen  in  the  following  table : 

T,T.  .,  Carbonic 

Nitrogen.     Oxygen.  ^^^^_ 

Composition  of  inspired  aii-        -      79-01  20-96  0-03 

expired  air         -      79-01  16-02  4-38 

-4-94  +4-36 

The  volume  of  oxygen  absorbed  is  slightly  greater  than 
that  of  the  carbonic  acid  which  takes  its  place,  so  that  if 
dried  and  reduced  to  standard  barometric  pressure  and 
temperature,  the  volume  of  dry  air  expired  is  slightly  less 
than  that  of  the  air  inspired.  But  since  expired  air  is 
usually  warmer  than  inspired  (not  always  in  the  Tropics) 
and  is  saturated  with  aqueous  vapour,  the  volume  expired 
is  actually  greater. 


96    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  proportion  which  the  volume  of  oxygen  absorbed 

bears  to  the  vokime  of  carbonic  acid  given  off  is  termed  the 

COo. 
respiratory  quotient,  and  is  expressed  as  —  ""     The    quotient 

varies  with  different  animals,  and  depends  upon  the  nature 
of  the  diet.  On  a  carbo-hydrate  diet  less  oxygen  is 
required,  for  the  reason  that  the  oxygen  and  hydrogen  in 
the  molecule  exist  in  the  proportion  to  form  water,  so  that 
oxygen  is  required  for  the  carbon  only.  On  a  fat  diet 
oxygen  is  required  for  both  the  hydrogen  and  carbon  in  the 
molecule. 

In  herbivora  the  respiratory  (juotient  is  "9    to  1"0 
In  carnivora    ,,  „  „  "75  ,,     -8 

In  omnivora    ,,  „  ,,  "87 

That  is  to  say,  for  every  1  part  of  oxygen  absorbed  by 
herbivora  there  is  produced  "9  to  1  part  of  carbonic  acid, 
and  for  every  1  part  of  oxygen  absorbed  by  carnivora 
•75  to  '8  part  of  COo  is  produced.  In  carnivora  it  will  be 
observed  that  the  amount  of  COo  produced  is  considerably 
less  than  the  amount  of  oxygen  absorbed,  for  the  reason 
that  the  latter  instead  of  being  devoted  to  the  oxidation  of 
carbon  and  reappearing  as  COo,  is  employed  in  the  oxida- 
tion of  hydrogen  and  returned  as  water.  The  value  of  the 
respiratory  quotient  lies  in  its  being  a  measure  of  the  com- 
bustions occurring  in  the  body  as  a  whole  ;  as  a  rule  the 
amount  of  carbon  dioxide  formed  is  less  than  the  oxygen 
absorbed,  but  there  are  exceptions,  and  a  respiratory 
quotient  may  be  above  unity  as  in  hibernating  animals  in 
storing  up  fat  for  the  winter,  and  it  is  not  unknown  among 
other  animals  after  a  diet  rich  in  carbo-hydrates.  In  such 
cases  the  CO.,  discharged  is  in  excess  of  the  oxygen 
absorbed,  the  necessary  oxygen  contained  in  the  COo  being 
obtained  from  the  intra-molecular  store,  of  which  we  shall 
hear  presently.  These  high  quotients  are  only  observed 
where  the  conversion  of  carbo-hydrates  into  fats  is  occurring, 
as  in  the  fattening  of  animals. 

There  are  other  gases  returned  from  the  lungs  besides 


RESPIRATION  97 

carbonic  acid  and  oxygen,  but  very  little  is  known  about 
them.  According  to  Reiset,  both  hydrogen  and  marsh  gas 
are  given  oft'  in  the  expired  air  of  ruminants,  in  fact,  he 
places  the  latter  at  183  cubic  inches  in  '24  hours.  Both  are 
supposed  to  be  derived  from  the  intestinal  canal,  being 
absorbed  into  the  blood  by  the  vessels  of  the  intestinal  wall. 
In  our  observations  on  the  gases  of  respiration  of  horses, 
it  was  found  after  deducting  the  oxygen,  carbonic  acid,  and 
nitrogen,  that  a  balance  remained  the  nature  of  which  was 
unfortunately  not  ascertained  ;  possibly  it  was  a  mixture  of 
hydrogen  and  marsh  gas,  but  it  did  not  amount  to  any- 
thing like  the  quantity  found  by  Reiset. 

The  nitrogen  of  the  air  is  returned  unaltered. 

We  have  previously  learnt  the  changes  occurring  in  the 
blood  during  its  passage  through  the  lungs ;  we  have  now 
to  study  the  way  in  which  the  interchange  of  gases  between 
this  fluid  and  the  air  is  brought  about. 

Absorption  of  Gases  in  Liquids. — The  law  regulating  the 
absorption  of  gases  by  fluids  is  very  clear.  Every  fluid  in 
which  a  gas  is  soluble  absorbs  the  same  volume  of  gas,  no 
matter  what  the  pressure  may  be ;  but  as  the  number  of 
molecules  in  a  gas  depends  upon  the  pressure,  it  is  evident 
that  the  tceight  of  the  absorbed  gas  rises  and  falls  in 
proportion  to  the  pressure ;  this  is  known  as  the  law  of 
Dalton  and  Henry. 

The  volume  of  gas  absorbed  by  a  fluid  depends  upon  the 
nature  of  the  gas ;  for  instance,  1  volume  of  water  will 
absorb  1180  volumes  of  ammonia  gas,  whilst  the  same 
volume  of  water  will  only  absorb  "00193  volume  of 
hydrogen.  The  temperature  of  the  water  is  also  an  im- 
portant factor,  for  the  higher  the  temperature  the  less  the 
gas  absorbed. 

If,  now,  instead  of  taking  a  single  gas  to  be  absorbed  by 
a  fluid  we  take  a  mixture  of  gases,  it  is  found  that  the 
volume  of  each  gas  forming  the  mixture  is  absorbed  as 
perfectly  as  if  it  were  the  only  gas  present ;  no  more  and 
no  less  is  absorbed,  whether  the  gas  be  by  itself  or  whether 
it   form  only   a   proportion   of   the  mixed   gases   present. 

7 


9S    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

This  is  explained  as  resulting  from  the  fact  that  one  gas 
does  not  exercise  any  pressure  upon  the  other  gases  with 
which  it  forms  a  mixture.  The  term  used  by  Bunsen  to 
define  the  pressure  exerted  by  one  gas  in  a  mixture  of 
gases  is  '  imrtial  pressure.'  For  example,  100  volumes  of 
air  contain  at  freezing-point  and  standard  barometric 
pressure  (30  inches)  21  volumes  of  oxygen  and  79  volumes 
of  nitrogen  ;  what  is  the  partial  pressure  exercised  by  each 
gas  in  this  mixture  ? 

30x21  63   inches   of    mercury,   which   is   the   partial 

100  pressure  of  the  oxygen  ; 
and 

30  X  79  23 '7  inches  of   mercury,  which   is   the  partial 

100  pressure  of  the  nitrogen. 

The  term  '  partial  pressure '  occurs  so  constantly  in  the 
following  pages,  that  the  above  may  help  to  make  the 
matter  clear. 

If  a  mixture  of  gases,  say  the  atmosphere,  be  exposed 
over  a  fluid  already  containing  some  of  these  gases  dissolved 
in  it,  it  is  found  that  if  the  proportion  of  dissolved  gases  in 
the  fluid  is  less  than  their  proportion  in  the  atmosphere, 
the  latter  pass  into  the  fluid  until  the  amounts  of  gases 
in  the  fluid  and  that  in  the  air  are  equal.  On  the  other 
hand,  if  the  fluid  contains  more  dissolved  gas  than  the 
atmosphere  above  it,  gas  will  pass  from  the  fluid  to  the 
atmosphere  until  the  amounts  in  each  are  equal.  This 
is  really  a  process  of  diffusion,  and  plays  a  most  im- 
portant physical  part  in  respiration  ;  it  is  one  of  the  means 
by  which  the  carbonic  acid  passes  from  the  blood  into 
the  air-cells,  and  the  oxygen  from  the  air-cells  into  the 
blood. 

If  two  different  gases  be  placed  in  a  jar,  in  a  short  time  a 
complete  mixture  will  have  occurred,  as  both  gases  pass 
each  into  the  other  until  a  thorough  and  equal  mixture 
has  taken  place.  This  is  termed  the  process  of  diffusion, 
and  is  the  chief  means  by  which  the  air  in  the  deeper 
part  of  the  lungs  mixes  with  the  fresh  air  introduced  by 
breathing. 


KESPIRATION  99 

Such  are  the  physical  laws  which  it  is  necessary  to 
understand  before  the  processes  involved  in  respiration  can 
be  fully  comprehended. 

The  Respiratory  Exchange  in  the  Lungs  and  Tissues. — 
The  respiratory  exchange  in  animals  is  of  two  kinds :  the 
external  respiration,  which  takes  place  between  the  air  and 
the  blood  through  the  medium  of  the  lungs,  and  the  internal 
respiration,  which  occurs  between  the  tissues  on  the  one 
hand  and  the  blood  and  lymph  on  the  other.  As  we  shall 
see,  both  are  complex  questions  which  are  far  from 
settled. 

The  blood  having  been  robbed  of  about  35  per  cent,  or 
more  of  its  oxygen  in  the  tissues,  the  haemoglobin  makes  its 
way  back  to  the  lungs  in  a  partly  reduced  condition  ;  here 
it  circulates  through  the  vast  capillary  system  spread  over 
the  alveoli  of  these  organs,  and  is  brought  as  closely  as 
possible  into  contact  with  the  air  (alveolar)  in  the  ultimate 
air-passages.  Between  it  and  the  air  we  have  only  the 
membrane  of  the  air-sac  and  the  wall  of  the  capillary,  both 
of  which  are  bathed  in  tiuid ;  through  this  wet  membrane 
the  oxygen  instantaneously  passes,  being  greedily  absorbed 
by  the  hemoglobin  of  the  red  cells ;  of  necessity  the  gas 
must  first  pass  into  the  blood  plasma,  and  from  there  it  is 
taken  up  by  the  red  corpuscles.  The  oxygen  is  not  simply 
absorbed  by  the  red  cells,  but  forms  with  the  hsemoglobin 
a  weak  chemical  compound.  Experiment  has  clearly  shown 
that  the  union  of  hsemoglobin  with  oxygen  is  largely  inde- 
pendent of  pressure,  and  therefore  does  not  obey  the  law 
of  Dalton  and  Henry,  which  it  certainly  would  do  if  simply 
absorbed. 

We  have  yet  to  learn  how  it  is  that  the  oxygen  in  the  air- 
vesicles  passes  into  the  capillaries  to  form  this  chemical 
union  with  htemoglobin.  Here  we  have  one  of  the  physical 
laws  brought  into  play  which  we  have  previously  described. 
When  the  venous  blood  arrives  in  the  lungs  it  has  lost 
much  of  its  oxygen,  the  partial  pressure  of  the  oxygen  is 
low,  whereas  the  partial  pressure  of  the  oxygen  in  the 
atmosphere  of  the  air-cells  is  relatively  high  ;  the  result  of 

7—2 


100    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

this  is  that  practically  instantaneous  diffusion  occurs 
through  the  moist  membrane  separating  the  gas  and  the 
fluid.  The  oxygen  entering  the  blood  plasma  unites  at 
once  with  haemoglobin  ;  the  latter  takes  up  all  or  nearly  all 
the  oxygen  it  is  capable  of  holding  (an  amount  which  is 
much  greater  than  if  simple  absorption  of  oxygen  by 
haemoglobin  occurred),  and  distributes  it  to  the  tissues 
through  the  medium  of  the  arterial  circulation. 

The  tissues  are  greedy  for  oxygen  ;  their  oxygen  pressure 
is  practically  )iil;  once  more  diffusion  occurs.  The  high 
partial  pressure  of  the  oxygen  in  the  arterial  blood  becomes 
(through  loss  of  oxygen  to  the  tissues)  low  partial  pressure 
in  venous  blood,  and  the  partly  reduced  haemoglobin  is 
carried  to  the  lungs,  where  the  process  just  described  is 
repeated.  But  the  loss  of  oxygen  to  the  tissues  is  not  the 
only  change  the  blood  undergoes,  for  not  only  is  its  haemo- 
globin partly  reduced,  but  as  the  outcome  of  tissue  activity 
increased  quantities  of  another  gas  are  added  to  it.  The 
gas  alluded  to  is  carbonic  acid ;  this  is  largely  taken 
up  by  the  venous  blood  and  conveyed  to  the  lungs,  and 
the  method  by  which  it  is  got  rid  of  will  be  explained 
presently. 

The  fate  of  the  oxygen  in  the  tissues  is  quite  unknown. 
It  is  supposed  to  be  stored  up  in  some  way  or  other  until 
required,  but  in  connection  with  this  subject  it  is  necessary 
that  we  should  glance  at  internal  respiration — viz.,  the 
respiration  which  takes  place  in  the  tissues. 

The  most  remarkable  feature  in  the  respiration  of 
muscle  (and  we  select  this  tissue  to  elucidate  the  point 
under  consideration)  is  that  although  the  working  of  a 
muscle  cannot  occur  without  oxidation  processes  taking 
place,  yet  no  free  oxygen  can  be  obtained  from  it ;  the 
partial  pressure  of  oxygen  in  muscle  is  practically  nil.  Yet 
oxidation  processes  are  occurring  in  muscle,  and  under 
such  conditions  that  no  free  oxygen  can  reach  it,  as,  for 
example,  when  the  muscle  preparation  of  a  frog  is  placed 
in  an  atmosphere  of  hydrogen.  Such  a  muscle  preparation 
may  be  made  to  contract  in  an  atmosphere  of  hydrogen 


RESPIRATION  101 

and  produce  carbonic  acid,  without  there  being  a  trace  of 
free  oxygen  either  in  the  atmosphere  surrounding  it,  or  in 
the  muscle  itself,  and  this  process  may  be  continued  until 
the  muscle  is  exhausted. 

The  question  is,  therefore,  How  does  the  oxygen-free 
muscle  obtain  oxygen  for  the  production  of  COo  ?  In  other 
words,  what  becomes  of  the  oxygen  taken  to  muscles  ? 
Few  things  in  the  whole  range  of  physiology  are  more 
difficult  to  understand  :  oxygen  goes  to  the  muscle,  it  uses 
oxygen,  yet  no  free  oxygen  is  found  in  it ! 

It  is  supposed  that  when  the  oxygen  reaches  the  muscle 
it  is  stored  up  in  its  substance  amongst  the  muscle  mole- 
cules, hence  it  has  been  termed  intra-molecular  oxygen  ; 
it  there  forms  a  complex  substance  which  readily  yields 
carbonic  acid  and  other  matters  on  decomposition,  and  this 
passes  into  the  bloodvessels  of  the  muscle  and  is  carried 
away  to  be  got  rid  of  at  the  lungs. 

It  has  been  suggested  that  the  storing  up  of  oxygen  in 
the  tissues  may  be  closely  allied  to  the  storing  up  of  oxygen 
by  haemoglobin,  though  with  this  difference,  that  the  com- 
pound which  holds  the  oxygen  in  the  tissues  is  more  stable 
than  the  oxygen-holding  substance  in  the  blood.  All  we 
know  of  the  fate  of  the  oxygen  is  that  it  eventually  deter- 
mines the  production  of  certain  changes  in  the  tissues, 
which  lead  to  the  'formation  of  carbonic  acid  and  other 
substances ;  but  the  changes  which  the  oxygen  undergoes 
from  the  time  it  leaves  the  blood  and  passes  into  the 
muscle  substance,  to  the  moment  it  issues  from  the  tissues 
united  with  carbon  as  carbonic  acid,  are  completely  un- 
known. 

The  oxidations  taking  place  in  muscle  and  in  every  other 
tissue  in  the  body  occur  in  the  substance  of  the  tissue  and 
not  in  the  blood  or  lymph  surrounding  it.  Experiments 
made  to  determine  whether  oxidations  occur  in  the  blood 
have  failed,  although  readily  oxidizable  substances  have 
been  employed  for  the  purpose.  We  have  said  the  tissues 
are  greedy  for  oxygen  and  use  it  up  or  store  it  away  as 
quickly  as  it  arrives ;  here  is  a  very  good  example  of  their 


102    A  MANUAL  OF  VETEKINARY  PHYSIOLOGY 

action  in  this  respect.  If  a  comparatively  stable  oxygen- 
holding  substance  such  as  methylene  blue  be  injected  into 
the  circulation  and  the  animal  destroyed,  it  is  found  that 
although  the  blood  is  dark  blue  in  colour,  yet  the  tissues 
are  normal  in  appearance  until  they  are  exposed  to  the 
air,  when  they  turn  a  vivid  blue.  The  explanation  is  that 
the  tissues  have  robbed  the  methylene  blue  of  oxygen  and 
formed  a  colourless  reduction  product,  which  on  exposure 
to  the  air  takes  up  oxygen  and  again  forms  methylene 
blue. 

Fate  of  the  Carbonic  Acid. — In  the  systemic  capillaries 
the  partial  pressure  of  the  carbonic  acid  is  lower  than  the 
partial  pressure  of  this  gas  in  the  tissues,  the  result  of 
which  is  that  it  is  hurried  into  the  blood  by  the  process  of 
diffusion  ;  but  here,  as  with  oxygen,  simple  absorption  of 
the  gas  by  the  plasma  would  not  be  sufficient  for  the 
purpose  of  carrying  off  the  whole  of  the  CO2  resulting  from 
tissue  activity,  so  that  there  must  be  some  substance  in 
the  blood  capable  of  fixing  CO..  until  the  lungs  are 
reached. 

If  the  serum  of  blood  be  exposed  to  the  vacuum  of  an  air- 
pump,  it  is  found  to  yield  little  oxygen  but  a  quantity  of 
CO.2 ;  it  yields  little  oxygen  because,  as  we  have  already 
learned,  this  is  combined  in  the  red  cells  ;  but  the  fact  that 
it  yields  large  quantities  of  C0._>  points  to  the  blood  plasma 
as  the  chief  means  by  which  this  substance  is  carried. 
It  has  been  determined  experimentally  that  blood  plasma 
will  absorb  more  CO2  than  the  same  quantity  of  water,  and 
it  is  evident,  therefore,  that  there  is  something  in  the 
plasma  which  assists  in  carrying  it.  What  this  'something' 
may  be  is  doubtful,  but  it  is  generally  believed  that  the 
sodium  carbonate  of  the  blood  unites  with  a  portion  of  the 
carbonic  acid,  though  other  substances  may  assist.  Between 
the  amount  absorbed  by  the  plasma,  and  that  held  in 
chemical  combination  by  certain  salts  of  the  plasma,  the 
total  amount  is  carried  along  in  the  venous  blood-stream, 
the  partial  pressure  of  the  COo  in  this  fluid  being  high.  On 
arriving  at  the  lungs  the  venous  blood  circulates  through  the 


EESPIEATION  103 

capillary  network  spread  over  the  walls  of  the  alveoli,  the 
same  wet  membrane  intervening  between  it  and  the  external 
air  as  was  described  in  speaking  of  the  oxygen.  The  partial 
pressure  of  the  COo  in  the  air  of  the  air-sacs  being  lower 
than  that  of  the  blood,  diffusion  occurs  between  the  blood 
and  the  air,  the  COo  passing  out  until  equilibrium  is 
established.  The  air  now  in  the  alveoli  of  the  lungs 
having  lost  some  of  its  oxygen,  and  considerably  gained  in 
its  carbonic  acid — in  other  words,  having  the  partial 
pressure  of  its  gases  altered — diffusion  between  the  air  in 
the  ultimate  air-cells  and  bronchial  tubes  rapidly  occurs 
until  the  balance  is  restored,  thus  rendering  the  air  in  the 
alveoli  fit  for  further  blood-reviving  processes. 

We  have  dealt  with  the  COo  in  the  blood  as  if  it  were 
entirely  carried  by  the  sodium  carbonate,  but  doubt  is  cast 
on  this  view,  and  Bohr  states  that  CO.,  and  haemoglobin 
form  a  loose  chemical  compound,  the  CO2  uniting  with  the 
proteid  portion  of  the  molecule,  thus  leaving  unaffected  the 
iron  moiety  with  which  the  oxygen  is  united.  The  carbon 
dioxide  haemoglobin  in  this  way  carries  one-third  of  the  CO., 
from  the  tissues  to  the  lungs,  and  under  the  influence  of 
the  oxygen  in  the  air  of  the  alveoli  the  CO.^  is  expelled 
from  the  corpuscles  and  discharged  into  the  alveoli.  The 
manner  in  which  the  combined  oxygen  is  liberated  in  the 
tissues,  and  the  combined  CO.,  liberated  in  the  lungs,  is 
explained  by  saying  that  certain  gases  have  a  tendency 
to  leave  the  substances  with  which  they  are  united,  when 
the  pressure  in  the  surrounding  medium  becomes  reduced  ; 
this  process  is  termed  '  dissociation.'  Dissociation  liberates 
the  oxygen  in  the  tissues  where  the  oxygen  pressure  is  nil, 
and  assists  in  liberating  the  COo  in  the  lungs,  where  the 
CO.,  pressure  is  low,  from  the  substances  with  which  these 
are  chemically  combined,  viz.,  haemoglobin  and  sodium 
carbonate. 

In  treating  of  the  exchange  of  gases  between  the  alveolar 
air  and  the  blood,  diffusion  has  been  represented  as  the 
only  factor  at  work  ;  but  it  is  urged  by  some  physiologists, 
that  the  cells  lining  the  vessels  and  alveolar  walls  cannot 


104    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

be  mere  passive  witnesses  of  these  remarkable  changes,  but 
like  the  cells  in  other  parts  of  the  body  are  capable  of 
taking  an  active  share  in  local  matters.  In  other  words, 
there  is  a  vital  aspect  to  this  question,  as  well  as  a  physical 
and  chemical  one.  An  experiment  of  Haldane's  with  carbon 
monoxide  is  very  suggestive  in  this  respect.  He  found  that 
though  he  could,  outside  the  body,  get  31  per  cent,  of 
haemoglobin  to  combine  with  the  gas,  yet  when  air,  con- 
taining the  same  percentage  of  CO  as  that  to  which  the 
haemoglobin  had  been  exposed,  was  inhaled  for  even  three 
or  four  hours,  no  more  than  26  per  cent,  of  the  haemo- 
globin of  the  blood  combined  with  it.  In  other  words,  the 
cells  of  the  pulmonary  alveoli  would  appear  to  possess 
that  same  selective  power  which  may  be  seen  elsewhere, 
as,  for  example,  in  the  kidney,  and  preferred  to  allow  oxygen 
rather  than  carbon  monoxide  to  pass.* 

The  respiratory  exchange  is  influenced  by  age,  being 
more  energetic  in  young  than  in  adult  animals ;  this  may 
be  due  not  only  to  actually  increased  metabolism,  but  also 
to  size.  It  is  a  well-known  fact  that  the  metabolism  in  a 
mouse  is  relatively  much  greater  than  in  a  horse  ;  it  is  a 
question  of  weight  and  surface.  The  larger  an  animal  the 
smaller  the  proportion  bertween  its  weight  and  the  extent  of 
its  surface.  In  other  words,  body  weight  and  surface  do 
not  vary  in  proportion  with  each  other,  and  a  mouse  in 
relation  to  its  weight  has  a  larger  body  surface  than  a 
horse,  and  therefore  loses,  and  has  to  make  more  heat. 

Muscular  work  has  an  important  influence  upon  the 
respiratory  exchange,  and  this  will  be  considered  in  the 
chapters  on  Nutrition  and  the  Muscular  System.  Broadly, 
it  increases  both  the  COo  discharged  and  oxygen  absorbed, 

*  The  question  of  the  absorption  of  oxygen  and  the  discharge  of 
carbon  dioxide  is  by  no  means  so  simple  as  might  appear  from  these 
pages.  Physiologists  are  not  agreed  as  to  whether  the  process  is  one 
of  diffusion,  or  a  secretion  into  the  blood  and  an  excretion  from  it. 
So  difficult,  indeed,  is  the  problem,  that  those  who  have  devoted  years 
to  its  study  declare  it  is  beyond  explanation  in  the  present  state  of 
physical  and  chemical  knowledge. 


RESPIEATION  105 

though  in  experiments  on  the  horse  (Zuntz  and  Lehmann) 
it  did  not  influence  the  respiratory  quotient. 

The  influence  of  food  on  the  respiratory  exchange  is  very 
marked  ;  during  starvation  it  at  first  undergoes  a  marked 
decrease,  and  then  remains  constant.  With  food  the 
exchange  rises,  the  absorption  of  oxygen  increases,  and 
the  output  of  COo  rises.  Proteid  food  is  much  more 
energetic  in  this  respect  on  a  fasting  animal  than  is  a  diet 
of  fat. 

Temperature  has  a  marked  influence  on  respiratory  ex- 
change, and  this  will  be  found  dealt  with  in  the  chapter  on 
Animal  Heat. 

Deficiency  in  Oxygen. — When  an  animal  is  compelled  to 
breathe  the  same  air  over  and  over  again,  there  is  a 
gradual  loss  of  oxygen  and  an  increase  in  carbonic  acid, 
and  though  death  will  ultimately  ensue  unless  the  air  be 
renewed,  it  is  remarkable  that  before  this  occurs  nearly  the 
whole  of  the  oxygen  will  have  been  consumed  from  the 
atmosphere.  This  is  further  evidence,  if  any  be  needed, 
that  the  oxygen  is  not  simply  absorbed  by  the  blood,  and 
that  its  absorption  does  not  obey  the  ordinary  laws  of  pres- 
sure. Experimental  inquiry  has  proved  that  animals  may 
live  in  an  atmosphere  containing  only  14  per  cent,  of  oxygen, 
but  that  distress  appears  at  11  per  cent.,  and  rapid  asphyxia 
follows  when  the  oxygen  falls  to  3  per  cent. 

In  poisoning  by  carbon  monoxide  the  latter  gas  turns  the 
oxygen  out  of  the  blood-cells,  yet  although  the  whole  of 
the  red-cells  are  converted  into  carriers  of  carbon  monoxide, 
the  animal  may  still  be  kept  alive  in  an  atmosphere  of  pure 
oxygen  under  pressure,  the  amount  of  oxygen  dissolved  by 
the  plasma  at  an  oxygen  pressure  of  two  atmospheres 
being  sufficient  to  carry  on  the  functions. 

HyperpncEa  is  the  term  applied  to  the  slightly  increased 
amplitude  and  frequency  of  the  respiratory  movements, 
such  as  occurs  in  gentle  exercise,  as  the  immediate  result 
of  any  commencing  defective  oxygenation  of  the  blood,  or 
other  cause  which  acts  as  a  stimulus  to  the  respiratory 
centre    (see   p.  108).      When   the    stimulus   is    strong   or 


106    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

continued,  a  further  increase  in  the  force  and  frequency 
of  the  respiratory  movements  takes  place,  and  this  condi- 
tion is  known  as  dyspncea.  The  later  stage  of  dyspnoea  is 
characterized  by  the  respiratory  movements  becoming 
'  convulsive  '  in  their  activity,  and  this  finale  to  dyspnoea 
marks  the  onset  of  true  asphyxia. 

If  the  air  supply  be  entirely  cut  off,  asphyxia  and  death 
rapidly  ensue.  Asphyxia  has  been  divided  into  three 
stages.  In  the  first  the  attempts  at  breathing  are  laboured 
and  painful,  deep  and  frequent,  and  all  the  respiratory 
muscles,  including  the  supplemental  ones,  are  brought 
into  play ;  convulsions  occur,  and  the  blood  pressure  rises. 
In  the  second  stage  the  inspiratory  muscles  are  less  active, 
the  expiratory  still  powerful,  and  the  convulsions  cease. 
In  the  third  stage  the  animal  lies  unconscious,  occasional 
violent  inspiratory  gaspings  occur,  the  mouth  is  open  (even 
in  the  horse),  the  pupils  dilated,  the  pulse  barely 
perceptible  or  absent ;  during  this  stage  the  blood  pressure 
rapidly  falls.  Death  occurs  in  from  five  to  six  minutes 
from  the  commencement  of  the  first  stage.  Young  animals 
are  less  easily  asphyxiated  than  adults  for  the  reason 
that  their  tissue  respiration  is  much  less ;  the  length  of 
time  necessary  to  drown  puppies  and  kittens  is  evidence 
of  this,  and  they  may  recover  even  after  prolonged 
immersion. 

Excess  of  Oxygen. — When  the  excess  of  oxygen  is  con- 
siderable, viz.,  equal  to  a  pressure  of  five  atmospheres, 
warm-blooded  animals  die  with  convulsions.  By  increasing 
the  amount  of  oxygen  above  that  contained  normally  in 
air,  the  blood  cannot  be  made  to  take  up  much  more 
oxygen  than  if  the  normal  amount  only  were  present ;  a 
pressure  of  ten  atmospheres  only  causes  an  increase  of 
3-4  per  cent,  absorbed,  so  that  the  blood  contains  23*4 
per  cent,  of  oxygen  instead  of  20  per  cent.  The  practical 
application  of  this  fact  in  the  treatment  of  certain  diseases 
by  the  inhalation  of  oxygen  is  interesting.  If  we  double 
the  amount  of  oxygen  in  the  air,  less  than  1  per  cent,  of 
the  extra  addition  is  absorbed.     Either  the  small  amount 


RESPIRATION  107 

of  extra  oxygen  thus  absorbed  must  be  very  valuable,  or 
we  must  find  some  other  explanation  of  the  undoubted 
advantage  of  oxygen  inhalation  in  disease. 

The  physiology  of  the  matter  is,  in  effect,  this :  The  air 
contains  20  per  cent,  of  oxygen  which  is  more  than  enough 
for  the  needs  of  the  body ;  even  the  venous  blood  is  return- 
ing to  the  lungs  with  from  ten  to  twelve  volumes  of  oxygen 
per  cent,  unused,  while  if  the  oxygen  in  the  air  be  doubled 
less  than  1  per  cent,  of  the  extra  is  absorbed.  It  may, 
however,  be  that  the  excess  of  oxygen  in  the  alveolar 
air  (see  p.  99)  of  the  lungs  during  oxygen  inhalation, 
enables  the  tissues  to  obtain  their  normal  amount  more 
easily. 

By  apncea  is  understood  a  standstill  of  respiration,  no 
movement  whether  inspiratory  or  expiratory  being  made. 
Apnoeic  pauses  may  be  produced  experimentally  by 
blowing  air  into  and  sucking  it  out  of  the  lungs  at  a 
more  rapid  and  forcible  rate  than  the  ordinary  respiratory 
rhythm  of  the  animal. 

Something  similar  in  appearance  is  witnessed  in  chloro- 
form poisoning,  and  the  term  apnoea  is  frequently  used 
clinically  as  synonymous  with  asphyxia.  The  physiologist 
uses  it  in  another  sense ;  apnceic  pauses  may  be  produced 
under  conditions  absolutelj^  the  reverse  of  asphyxia,  as  by 
rapidly  and  forcibly  blowing  air  into  the  lungs.  Under 
these  conditions  some  observers  suppose  that  a  diminished 
irritability  of  the  respiratory  centre  is  produced  as  the 
result  of  '  hyper-oxygenation  of  the  blood,  though  it  is 
difficult  to  see  how  this  condition  is  brought  about.  There 
is,  on  the  other  hand,  good  reason  to  think  that  repeated 
expansion  of  the  lungs  causes  stimulation  of  the  inhibitory 
fibres  of  the  vagus,  and  so  acts  on  the  centre,  and  this  view 
is  supported  by  the  fact  that  apnoea  is  difficult  to  produce 
after  the  vagi  have  been  divided.  A  final  view  of  the 
cause  of  apnoea  assumes  the  normal  stimulus  to  the 
respiratory  centre  to  be  COo  (see  p.  112),  in  which  case 
rapid  inflation  of  the  lungs  would  result  in  a  more 
efficient   removal    of   this    gas,   and   thus  to  a  diminution 


108  A  MANUAL  OF  YETEEINARY  PHYSIOLOGY 

in  the  stimulus  to  inspiration.  Possibly  both  this  view 
and  that  of  the  inhibition  of  the  centre  by  stimulation 
of  the  vagus  fibres  is  correct,  the  two  working  con- 
currently. 

The  Nervous  Mechanism  governing  Respiration  is  presided 
over  by  a  centre  in  the  medulla,  the  position  of  which  in 
certain  animals  is  very  accurately  defined,  but  which  in 
general  terms  may  be  spoken  of  as  being  situated  close  to 
the  deep-seated  origin  of  the  vagus  and  in  front  of  the  vaso- 
motor centre.  The  respiratory  centre  was  at  one  time  con- 
sidered to  consist  of  an  inspiratory  and  expiratory  portion, 
but  the  present  view  is  to  regard  the  expiratory  centre  as 
doubtful ;  at  any  rate  it  cannot  be  localized,  and  though 
there  are  certain  facts  which  suggest  its  existence,  such  as 
the  act  of  straining  in  parturition,  micturition,  or  defeca- 
tion, still  as  compared  with  the  inspiratory  centre  it  occupies 
a  very  subordinate  position.  Hence  it  has  been  proposed 
to  call  the  respiratory  centre  the  'inspiratory  centre'; 
inspiration  can  only  be  carried  out  by  rhythmical  impulses 
from  this  region  of  the  medulla,  while  there  seems  no 
doubt  that  expiration  may  be  a  purely  passive  act.  It  is 
believed  that  the  respiratory  centre  is  connected  with  every 
sensory  nerve  in  the  body,  for  the  centre  may  be  readily 
stimulated  reflexly  through  sensory  nerves,  as  an  example 
of  which  may  be  quoted  the  sudden  inspiratory  gasp  given 
when  cold  water  is  dashed  on  the  skin. 

But  besides  these  there  are  special  motor  nerves  wholly 
or  almost  wholly  concerned  in  respiration  with  which  this 
centre  is  closely  in  touch,  for  instance,  the  facial  supplying 
the  nostrils,  the  recurrent  of  the  vagus  which  dilates  the 
glottis,  the  phrenics  which  stimulate  the  diaphragm,  the 
dorso-lumbar  nerves  which  supply  the  intercostal  and 
abdominal  muscles.  All  these  are  interested  in  the  pro- 
duction of  that  perfect  and  orderly  sequence  of  events 
which  beginning  at  the  nostrils  pass  to  the  flank,  and  are 
so  intimately  concerned  in  the  production  of  the  respiratory 
rhythm. 

The  respiratory  centre  is  automatic,  that  is  to  say,  it  is 


EESPIRATION 


109 


within  itself  that  the  discharges  are  generated  which  issue 
forth  as  inspiratory  impulses  ;  it  is,  indeed,  as  automatic 
as  the  heart,  for  if  every  nerve  leading  to  it  were  divided 
the  respiratory  centre  would  still  go  on  working.  This 
view  is  not  accepted  by  all  physiologists,  many  of  whom 
regard  the  stimulus  to  respiration  as  being  a  refler  one — 
viz.,  derived  from  without  the  centre.  The  centre  consists 
of  two  halves,  right  and  left,  both  of  which  work  together 
3'et  may  be  shown  experimentally  to  be  capable  of  inde- 
pendent action.  Section  of  the  cord  between  the  medulla 
and  the  phrenics  leads  to  immediate  cessation  of  all  re- 


FiG.  28. — Diagram  to  illustrate  the  Chief  Nervous  Connections 
OF  THE  Respiratory  Centre.     (After  Waller.) 


spiratory  movements,  excepting  those  of  the  mouth  and 
nostril,  which  are  anterior  to  the  section,  and  hence  to 
death  from  asphyxia. 

The  respiratory  centre  may  be  stimulated  reflexly 
through  any  sensory  nerve  of  which  an  example  has 
previously  been  given.  It  is  by  no  means  necessary  that 
the  sensory  nerves  carrying  impulses  to  the  centre  should 
be  devoted  exclusively  to  this  function  ;  probably  the  centre 
is  linked  up  with  all  the  cranial  and  spinal  nerves. 

The  nature  of  the  impulses  issuing  from  the  centre 
depend  upon  the  character  of  the  impulses  which  stimu- 
lated their  production ;  thus  the  breathing  may  be  con- 
trolled or  even  entirely  stopped   for  a  few  seconds,  or  it 


110    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

may  be  liastened  or  slowed  down,  or  quickened  in  rhythm, 
decreased  in  depth,  or  both  rhythm  and  depth  increased. 
From  the  brain  impulses  may  pass  to  the  centre  which 
may  cause  an  animal  to  increase  its  respirations  as  in 
sniffing,  or  to  suppress  them  entirely,  as  when  its  head  is 
under  water.  In  the  diagram  Fig.  28  we  have  adopted 
Waller's  symbols  to  signify  an  increase  or  decrease  of 
respiration,  and  it  will  be  seen  that  the  cortex  can  supply 
either  plus  or  minus  influences.  Another  reflex  path  to 
the  respiratory  centre  is  that  furnished  by  the  nostrils 
through  the  medium  of  the  nasal  branch  of  the  fifth 
nerve ;  through  this  channel  principally  minus  influences 
are  transmitted,  viz.,  respiration  is  diminished.  From  the 
skin  plus  or  minus  influences  may  pass  to  the  respiratory 
centre.  A  bucket  of  water  dashed  against  a  horse  when 
the  breathing  is  failing  in  chloroform  narcosis  will  start 
an  inspiration,  and  painful  sensory  impressions,  as  in 
'  firing,'  greatly  increase  the  respiratory  movements.  From 
the  larynx  important  impulses  pass  to  the  respiratory 
centre  through  the  suijerior  laryngeal  nerve.  If  this 
nerve  be  divided  and  the  end  in  connection  with  the 
brain  stimulated  respiration  is  inhibited,  in  fact,  if  the 
stimulation  be  severe  inspiration  becomes  weaker  and 
weaker,  and  finally  the  breathing  stops  in  expiration. 
This  points  to  the  superior  laryngeal  as  stimulating  expira- 
tion and  inhibiting  inspiration.  The  same  result  occurs 
with  the  sensory  fibres  of  the  glosso-pharyngeal  supply- 
ing the  pharynx.  Here  it  is  intimately  connected  with 
the  act  of  swallowing,  producing  an  inhibition  of  respira- 
tion the  moment  the  epiglottis  is  pressed  against  the 
larynx. 

Influence  of  the  Vagus  on  Rcsjnration. — The  vagus  is  the 
most  important  afferent  or  ingoing  channel  to  the  re- 
spiratory centre  ;  it  covers  the  area  from  the  glottis  to  the 
alveoli  of  the  lungs. 

If  both  nerves  are  cut  the  respirations  become  slower 
and  deeper ;  if  one  nerve  only  is  divided,  this  efl'ect  does 
not  follow.     Evidently,  therefore,  there  are  tonic  impulses 


RESPIEATION  111 

passing  up  the  vagus  which  maintain  the  normal  re- 
spiratory rhythm,  which  is  lost  when  the  nerves  are 
divided.  If  the  cut  vagus  be  stimulated,  using  that 
portion  still  in  connection  with  the  brain,  the  respiration 
may  be  affected  in  at  least  two  different  ways  ;  either  all 
respiratory  movements  may  partly  or  completely  cease, 
or  the  rate  of  inspiration  may  be  increased,  and  if  the 
stimulation  be  powerful  the  respiration  may  stop  in  in- 
spiratory tetanus.  The  interpretation  of  the  results  of 
these  experiments  is  that  two  kinds  of  sensory  fibres 
exist  in  the  vagus  which  act  on  the  respiratory  centre  ; 
in  what  way  they  act  is  not  fully  agreed  upon  by 
physiologists. 

Stating  the  case  broadly  the  two  kinds  of  fibres  are 
regarded  as  insj^iratory  and  expiratory,  viz.,  fibres  stimu- 
lating the  inspiratory  and  ex2:»iratory  portions  of  the  centre 
respectively,  and  this  view  is  necessary  if  it  be  held,  as 
some  authorities  do,  that  respiration  is  a  reflex  act  and 
not,  as  we  have  so  far  assumed,  an  automatic  activity. 
Both  sets  of  fibres  are  in  alternate  activity,  and  the 
question  is,  What  is  their  normal  stimulus'?  This  has 
been  determined  to  be  due  to  the  alternate  distension  and 
collapse  of  the  air-vesicles,  for  experiment  shows  that  if  air 
be  pumped  into  the  lungs  expiration  is  excited,  and  if  it  be 
sucked  out  inspiration  is  excited.  Accordingly  distension 
of  the  air-vesicles  by  the  normal  process  of  inspiration 
excites  expiratioq,  and  contraction  of  the  air-vessels  in 
expiration  excites  inspiration.  If,  however,  the  respiratory 
centre  is  regarded  as  primarily  automatic,  the  inspiratory 
set  of  fibres  may  be  considered  to  increase  the  rate  of 
respiration,  the  expiratory  fibres  as  inhibiting  or  con- 
trolling inspiration,  and  thus  producing  expiration.  If 
this  view  be  adopted  the  act  of  inspiration  proceeds  from 
the  automatic  centre  which  requires  no  other  stimulus 
than  that  which  it  generates  within  itself,  while  expiration 
proceeds  from  the  stimulation  caused  by  distension  of  the 
air-vesicles. 

The  nature  of  the  internal  stimulus  to  the  respiratory 


112    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

centre  has  led  to  much  discussion  ;  it  appears  now  to  be 
generally  accepted  that  the  most  important  stimulus  to  its 
automatic  action  is  the  gases  in  the  blood.  Venous  blood 
circulating  through  the  centre  causes  the  respiration  to 
increase  in  force  and  rate,  while  blood  containing  a  full 
amount  of  oxygen  lowers  the  excitability ;  the  respirations 
slow  down,  or  even  become  suspended.  Carbonic  acid 
may  be  accepted  as  the  chief  stimulus  to  the  respiratory 
centre. 

Cause  of  First  Inspiration. -^Ki  this  point  it  is  convenient 
to  consider  a  question  previously  deferred  (p.  89) — viz.. 
What  is  the  cause  of  the  first  act  of  inspiration  in  the 
foetus?  When  the  placental  circulation  is  cut  off,  the 
respiratory  centre  of  the  ftetus  becomes  stimulated  through 
the  increased  venous  character  of  the  blood  now  circulating 
through  it.  As  a  result  of  this,  inspiration  is  automatically 
produced,  but  it  is  also  assisted  by  reflex  impulses  carried 
from  the  surface  of  the  skin  due  to  handling  and  drying. 
Handling  the  skin  of  the  fcetus  while  still  in  iitero  with  the 
placental  circulation  intact  may  provoke  respirations,  and 
in  all  animals  the  very  first  act  of  the  mother  is  to  dry 
the  foetus  and  stimulate  the  skin  by  licking. 

Dirision  of  the  Phrenic  Nerves.— ^Ne  have  referred  to  the 
cutting  off  of  the  respiratory  centre  by  dividing  the  cord 
above  the  phrenic.  If  the  cord  be  divided  below  the  point 
of  exit  of  the  phrenics,  the  channel  between  the  re- 
spiratory centre  and  lungs  via  the  spinal  cord  is  not 
interfered  with,  but  the  resulting  paralysis  of  the  abdominal 
and  intercostal  muscles  necessitates  that  the  action  of  the 
diaphragm  shouM  be  more  powerful.  If  one  phrenic  nerve 
be  divided  half  the  diaphragm  is  paralyzed,  if  both  be 
divided  the  whole  diaphragm  is  paralyzed  and  eventually 
undergoes  fatty  degeneration.  Sussdorf  states  that  division 
of  the  phrenic  nerves  in  the  horse  leads  to  difficulty  in 
breathing,  increased  heart  action,  and  the  collection  of 
faeces  in  the  rectum.  In  about  twenty-four  hours  these 
symptoms  pass  away,  and  if  the  animal  be  worked  no 
appreciable  difficulty  in  breathing  is  subsequently  observed. 


EESPIKATION  113 

Division  of  Seventh  Pair. — Colin  has  shown  that  if  the 
seventh  pair  of  nerves  be  divided  in  the  horse  and  the 
animal  worked  asphyxia  results.  This  nerve  dilates  the 
nostrils ;  when  divided  the  paralj^zed  flaccid  nostrils  are 
drawn  inward  at  each  inspiration  and  so  close  the 
opening. 

The  Amount  of  Air  Required. — Numerous  respiration 
experiments  have  been  made  on  all  animals,  to  determine 
the  amount  of  air  they  require  and  the  gases  of  respiration. 
The  horse  is  of  all  others  the  one  to  which  perhaps  the 
greatest  practical  interest  attaches  in  this  respect,  though  a 
knowledge  of  it  in  connection  with  other  animals  is  of 
value. 

The  lungs  of  a  horse  will  contain  about  Ih  cubic  feet 
(42'5  litres)  of  air  at  the  end  of  a  deep  inspiration ;  during 
ordinary  repose  he  draws  into  them  between  80  and 
90  cubic  feet  (2,265  to  2,548  litres)  of  air  in  the  hour, 
though  considerable  variation  may  be  found  even  in  the 
same  animal. 

An  average  inspiration  in  the  horse  during  repose 
amounts  to  about  250  cubic  inches  (4"1  litres),  and  the 
amount  of  air  which  flows  in  and  out  during  ordinary  quiet 
respiration  is  known  as  the  tidal  air.  Speaking  roughly  it 
is  only  one-tenth  of  what  the  lungs  can  contain ;  the 
remaining  nine-tenths  are  made  up  of  complemental,  reserve, 
and  residual  air.  The  complemental  air  is  that  over 
and  above  the  tidal  which  can  be  taken  in  by  a  forced 
inspiration,  while  the  reserve  is  a  somewhat  similar  amount 
which  can  be  expelled  by  a  forced  expiration.  The  most 
powerful  expiratory  eftbrt  is  unable  to  remove  from  the 
lungs  all  the  air  they  contain,  and  this  amount  is  known 
as  the  residual  air.  The  great  variations  which  have  been 
observed  in  the  amount  of  air  taken  in  by  the  same  animals, 
under  apparently  identical  conditions,  cannot  be  adequately 
explained ;  the  slightest  disturbing  influence  alters  both 
the  rhythm  and  depth  of  the  respirations.  Under  the 
influence  of  work  the  amount  of  air  required  is  greater, 
and  as  a  rule  the  faster  the  pace  the  more  air  needed  ; 

8 


114    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

but  many  disturbing  factors  occur  which  render  experiments 
on  this  subject  very  contradictory,  and  productive  of  the 
greatest  variation.  During  severe  work,  such  as  a  gallop, 
a  horse  is  taking  air  into  his  lungs  to  the  extent  of 
850  cubic  feet  (24,067  litres)  per  hour  at  least,  and  probably 
more ;  the  respirations  from  being  9  to  10  per  minute 
during  repose,  may  now  be  anything  between  70  and  100 
per  minute.  The  effect  of  taking  in  all  this  extra  air  is 
that  the  pulmojiari/  ventilation  is  increased  ;  it  is  calculated 
that  in  man  a  deep  inspiration  more  than  doubles  the 
capacity  of  the  alveoli  by  distending  them.  In  such  paces 
as  the  canter,  trot  and  walk,  the  amount  of  air  used  is 
correspondingly  less ;  immediately  the  pace  slackens  or  the 
horse  stops  the  respirations  at  once  fall,  and  the  amount 
of  air  inspired  becomes  reduced.  This  is  one  of  the  great 
difficulties  attending  respiration  experiments  on  horses 
under  natural  conditions. 

A  horse  in  a  state  of  repose,  according  to  Zuntz  and 
Lehmann,  produces  3  cubic  feet  (85  litres)  of  CO.,  per  hour, 
and  absorbs  nearly  3^  cubic  feet  (99  litres)  of  oxygen  ;  the 
expired  air  is  found  to  have  lost  4  per  cent,  of  its  oxygen 
and  gained  3A  per  cent,  of  COo.  This  is  very  much  more 
than  we  found,*  but  it  agrees  pretty  closely  with  the 
observations  made  on  other  animals  and  on  man.  It  may 
be  noted  that  even  in  animals  which,  from  their  small  size 
or  other  causes,  lend  themselves  to  exactitude  in  experi- 
mentation, the  most  divergent  results  have  been  obtained, 
and  the  same  thing  is  observed  in  man. 

There  are  certain  evident  factors  which  considerably 
influence  the  amount  of  COo  produced  and  Oo  absorbed, 
and  of  these  muscular  work  and  the  nature  of  the  diet  are 
the  most  prominent.     As  the  result  of  muscular  activity 

*  '  The  Chemistry  of  Eespiration  in  the  Horse  during  Rest  and 
Work,'  Journal  of  Physiology,  vol.  xi.,  1890.  It  is  now  considered 
that  samples  of  air  are  not  sufficient  to  determine  respiratory  ex. 
changes,  the  CO.,  has  a  tendency  to  accumulate  in  the  tissues,  and  an 
apparatus  which  admits  of  prolonged  observation  is  necessary,  such  as 
was  employed  by  Zuntz  and  Lehmann.  The  apparatus  employed  in 
Qur  observations  is  shown  in  Figs.  29  and  30, 


RESPIRATION 


115 


Fig.  29. — Horse  in  Position  on  Respiration  Apparatus. 


Fig.  30. 

1,  The  face  mask  ;  7,  inlet  tube  to  bag ;  8,  valve  box  through  which 
the  expired  air  passes  to  10,  a  rubber  bag  of  20  cubic  feet  capacity. 
After  an  experiment  the  air  is  pressed  out  of  the  bag,  and,  passing 
through  11,  is  measured  in  the  meter.  4,  A  chamber  containing 
a  tray  of  coke  saturated  with  caustic  potash,  through  which  the 
inspired  air  passes  and  is  robbed  of  its  COg. 

8—2 


116    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  production  of  CO2  is  increased  ;  in  the  same  way  the 
amount  of  oxygen  absorbed  is  greater,  but  experiment  has 
failed  to  prove  a  definitely  immediate  relationship  between 
the  amount  of  oxygen  absorbed  and  the  amount  of  work 
produced.  A  diet  rich  in  starch  (carbo-hydrates)  increases 
the  amount  of  COo  produced,  whilst  fats  have  not  such  a 
marked  effect  in  this  direction.  The  respiratory  quotient 
(p.  96)  approaches  unity  in  animals  fed  on  a  diet  rich  in 
carbo-hydrates,  viz.,  there  is  very  nearly  as  much  CO2 
given  off  as  Oo  absorbed  ;  this  is  not  the  case  with  animals 
living  on  a  flesh  diet,  where  the  respiratory  quotient  may 
fall  as  low  as  '7. 

The  following  table  gives  the  amount  of  air  respired  and 
the  gases  of  respiration  for  several  animals ;  it  is  an  old 
table  by  Boussingault.  Recent  observations  on  the  horse, 
which  we  have  previously  quoted,  give  about  half  the  values 
as  compared  to  those  assigned  by  Boussingault. 


Body  Weight. 

Amount  of  Air 

inspired  in 

24  hours. 

Amount  of 

Oxygen  consumed 

in  24  hours. 

Amount  of  Car- 
bonic Acid  pro- 
duced in  24  hours. 

Horse 

990  lbs. 

Cubic  feet. 

3373 

Cubic  feet. 

150 

Cubic  feet. 
151-0 

Cow 

990    „ 

2782 

122 

122-8 

Ass    

330   „ 

1112 

50 

50-4 

Pig    

165    „ 

1216 

54-7 

55-1 

Sheep 
Dog 

99   „ 
44   „ 

720 
298 

32-4 
14-0 

22-6 
10-3 

Alveolar  Air. — We  have  indicated  that  the  lungs  cannot  be 
completely  emptied  of  air  ;  a  column  of  air  even  after  death 
exists  from  the  larynx  to  the  alveoli  of  the  lungs.  The  air  in 
the  air-sacs  does  all  the  work,  and  is  known  as  the  alveolar  ; 
from  the  air-sacs  to  the  nostrils  is  the  '  dead  space  '  of  the 
respiratory  tract,  and  the  composition  of  the  air  at  any  two 
parts  of  this  space  is  not  the  same,  it  grows  progressively 
poorer  in  oxygen  in  the  direction  of  the  lungs,  and  richer 
in  carbon  dioxide.  During  expiration  the  first  air  to  leave 
is  that  from  the  '  dead  space,'  and  the  alveolar  air  follows 
towards  the  end  of  the  expiration.     Puring  inspiration  the 


RESPIRATION  117 

incoming  air  is  diluted  with  that  ah-eady  in  the  lungs,  and 
its  chemical  composition  at  once  alters.  The  air  which 
distends  the  alveoli  is  therefore  a  mixture  of  air  already  in 
the  alveolus  combined  with  air  from  the  bronchial  passage, 
and  the  air  in  the  bronchial  passage,  in  its  turn,  becomes  a 
mixture  of  bronchial  air  and  air  derived  from  the  higher 
air-passages.  How  far  the  air  of  an  ordinary  inspiration 
travels  is  difficult  to  determine ;  under  the  most  favour- 
able circumstances  some  of  the  axial  stream  of  the 
current  might  reach  the  alveoli,  especially  of  the  anterior 
lobes,  but  the  bulk  of  it  will  get  no  further  than  the 
bronchi,  and  by  so  doing  will  have  displaced  and  mixed 
with  the  air  lately  occupying  the  bronchi,  now  occupying 
the  alveoli.  In  man  it  is  estimated  that  in  this  way 
about  one-eighth  of  the  alveolar  air  is  changed  at  each 
respiration. 

Every  endeavour  has  been  made  to  ascertain  the  com- 
position of  the  air  in  the  alveoli  of  the  lungs ;  if  this 
could  be  put  beyond  doubt  the  vexed  question  of  whether 
the  gaseous  exchange  is  due  to  diffusion  or  not  would  be 
capable  of  settlement.  If,  for  instance,  the  pressure  of 
carbon  dioxide  in  the  pulmonary  capillaries  was  found  to 
be  the  same  as  the  pressure  of  this  gas  in  alveolar  air, 
diffusion  would  account  for  the  exchange.  The  question 
is  a  very  difficult  one,  but  easier  to  settle  from  its  carbon 
dioxide  aspect  than  from  the  oxygen  side.  Haldane  and 
Priestley,  whose  work  we  have  closely  followed,  found  in 
their  experiments  on  alveolar  air  that  under  ordinary 
atmospheric  pressure  it  contains  practically  a  constant 
percentage  of  CO.,,  and  this  is  brought  about  by  the 
influence  of  this  gas  on  the  respiratory  centre.  Carbon 
dioxide  regulates  the  ventilation  in  the  lungs,  and  thus 
provides  the  means  for  getting  rid  of  itself ;  the  more  COo 
in  the  blood  the  greater  the  alveolar  ventilation. 

Influence  of  Work  on  Respirations. — It  is  by  no  means 
clear  why  work  causes  an  increase  in  the  number  and 
depth  of  respirations.  Changes  in  the  composition  of  the 
blood-gas  stimulating  the  respiratory  centre  has  been  urged 


118    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

as  a  reason,  but  does  not  stand  the  test  of  experimental 
inquiry,  for  it  is  found  that  the  blood  leaving  the  heart 
during  work  is  normal  in  composition.  On  the  other  hand, 
the  evidence  that  the  panting  respirations  of  work  are  due 
to  a  *  something '  produced  in  the  muscles  is  very  strong, 
for  if  the  spinal  cord  of  a  dog  be  divided  and  the  hind  legs 
stimulated,  increased  respiratory  movements  are  caused, 
just  as  if  the  animal  had  been  running  some  distance. 
What  the  substance  is  is  unknown  ;  some  observers  have 
regarded  it  as  sarco-lactic  acid  or  acid  phosphates,  but 
nothing  is  definitely  known,  though  it  is  interesting  to 
observe  that  dilute  acids  injected  into  the  blood  give  rise 
to  the  same  condition.  But  hurried  respirations  may  also 
be  produced  through  the  circulatory  system.  In  an  animal 
in  training  the  breathlessness  which  it  is  one  of  the  objects 
of  training  to  get  rid  of,  is  due  to  the  fact  that  more 
blood  is  brought  to  the  lungs  than  can  be  disposed  of.  If 
the  right  heart  pumps  into  the  lungs  more  blood  than  the 
lungs  can  return  to  the  left  heart  breathlessness  follows. 
The  gallop  by  which  an  animal  gets  its  '  wind '  and 
'  staying '  power,  operates  through  the  circulatory  system. 
Fortunately,  the  vessels  of  the  lungs  are  capable  of  con- 
siderable adjustment,  they  hold  more  blood  during 
inspiration  than  expiration,  and  in  this  way  may  be  re- 
garded as  a  safety  valve  to  the  heart.  The  important 
practical  questions  of  work,  '  condition,'  and  fatigue  are 
again  referred  to  in  the  chapter  dealing  with  the  Muscular 
System. 

Air  vitiated  by  Respiration  was  at  one  time  believed  to 
be  poisonous,  either  on  account  of  its  deficiency  in  oxygen, 
its  increase  in  carbon  dioxide,  or  to  the  organic  matter 
mixed  up  with  it.  Some  modern  investigators  attribute 
the  ill-effects  of  vitiated  air  mainly  to  the  absence  of  free 
ventilation  and  the  warm  and  humid  atmosphere,  by  which 
the  respiratory  exchange  and  body  metabolism  are  affected. 
Even  the  number  of  bacteria  in  the  air  is  no  guide  to 
purity ;  there  may  be  fewer  in  respired  air  than  in  the 
same  air  before  respiration,  in  consequence  of  their  being 


RESPIRATION  119 

arrested  in  the  lungs.  On  the  other  hand,  Haldane  and 
Lorrain  Smith  attributed  the  ill-effects  of  respiratory- 
impurity  to  the  excess  of  COo  and  deficiency  of  O2, 
hyperpnoea  beginning  when  the  COj  rises  to  3  or  4  per 
cent. 

The  amount  of  air  required  for  ventilation  purposes  is  a 
question  of  hygiene,  and  reference  should  be  made  to  works 
on  that  subject. 

Respiratory  Murmur. — An  accurate  acquaintance  with  the 
normal  respiratory  murmur  is  essential  to  the  physician. 
The  air-sounds  both  of  inspiration  and  expiration  should 
be  heard  all  over  the  chest,  the  inspiratory  murmur  being 
louder  and  better  marked  than  the  expiratory ;  in  fact,  in 
many  perfectly  healthy  chests  the  expiratory  murmur  can 
scarcely  be  heard.  The  normal  murmur  whether  inspiratory 
or  expiratory  is  soft  in  character  ;  there  is  no  harshness. 
The  sound  is  best  represented  by  the  noise  made  by  the 
stream  of  air  which  issues  from  a  pair  of  hand  bellows 
when  gently  blown. 

The  respiratory  murmur,  also  known  as  the  vesicular 
murmur,  is  caused  by  the  friction  of  the  air  entering  the 
alveoli.  In  those  portions  of  the  lung  lying  close  to  the 
bronchi  and  larger  tubes  there  is,  in  addition  to  the 
vesicular  murmur,  a  sound  produced  by  the  trachea  and 
glottis.  This  is  not  distinct  from  the  vesicular  sound  but 
is  added  to  it,  the  result  being  that  the  respiratory  murmur 
over  the  tubes  is  louder  than  elsewhere.  The  expiratory 
sound  is  weaker  and  shorter  than  the  inspiratory,  that 
is  to  say  the  sound  is  not  continued  to  the  end  of 
expiration  but  dies  away  before  that  is  reached.  The 
expiratory  murmur  immediately  follows  the  inspiratory 
without  a  pause,  but  there  is  a  marked  pause  between  the 
end  of  one  expiration  and  the  beginning  of  the  next  in- 
spiration. 

The  ordinary  murmur  is  best  heard  where  the  chest  wall 
is  thin  ;  if  the  ribs  be  covered  with  fat  or  any  great  thick- 
ness of  muscle  the  sound  may  be  entirely  lost.  It  is  also 
important  to  note   that   there  are  some  chests   perfectly 


120    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

healthy  where,  for   no   apparent   reason,   the   respiratory 
murmur  is  obscure  or  even  undetectable. 


Pathological. 

Pneumonia  and  Pleurisy  in  the  horse  are  very  common  in  early 
life,  and  attended  by  a  high  mortality.  The  lungs  and  pleura,  separately 
or  combined,  may  suffer  a  degree  of  inflammation  varying  from  snaall 
localized  trouble,  to  general  and  extensive  intiammation  of  the  pleura 
and  lungs.  The  whole  of  the  lung  tissue  is  never  affected  ;  even  in 
the  most  severe  cases  of  pneumonia  there  is  some  breathing  area 
available :  the  upper  portion  of  both  lungs  generally  escapes.  Effusion 
of  tiuid  into  the  cavity  of  the  thorax  is  a  common  sequel  to  pleurisy 
n  the  horse. 

Both  the  above  pathological  conditions  and  their  progress  are  deter- 
mined by  auscultation  and  percussion :  there  are  many  departures 
from  the  normal  respiratory  murmur,  all  of  which  have  their  signi- 
ficance. 

Apoplexy  of  the  Lungs  arises  as  the  result  of  overwork,  especially 
in  hot  weather ;  but  it  may  also  occur  in  the  winter.  Horses  ridden 
to  death  in  the  hunting  field,  in  the  name  of  '  sport,'  die  as  a  rule 
from  pulmonary  apoplexy ;  the  lungs  cannot  get  rid  of  their  abnormal 
burden  of  blood  to  the  left  heart. 

Bronchitis  is  probably  rarely  a  disease  distinct  from  pneumonia. 

'  Broken  Wind '  is  one  of  the  most  interesting  of  the  various  chest 
diseases  of  the  horse ;  it  is  a  condition  peculiar  to  this  animal,  liable 
to  occur  suddenly  and  frequently'  traced  to  errors  in  dieting.  To  state 
the  case  shortly  the  lungs  lose  their  power  of  elastic  recoil,  and  do  not 
collapse  even  after  death ;  the  respirations  are  greatly  increased,  the 
expiratory  effort  being  powerful  and  prolonged,  a  chronic  typical 
cough  becomes  established,  and  the  animal  unfit  for  anything  but 
slow  work. 

Roaring  is  a  nervous  affection,  to  which  sutficient  allusion  is  made 
in  the  section  dealing  with  the  larynx. 

Spasm  of  the  Diaphragm  is  another  respiratory  affection  due  to 
disordered  nervous  supply.  The  sound  emitted  is  qaite  unlike  that  in 
the  human ;  it  appears  to  come  from  within  the  chest  or  abdomen,  and 
is  represented  by  a  dull  '  thud '  like  a  magnified  heart  beat,  which  in 
its  frequency  and  regularity  it  closely  resembles,  and  for  which  it 
may  easily  be  mistaken. 

Rupture  of  the  Diaphragm  is  a  common  lesion  frequently  due  to 
disorders  of  the  digestive  canal,  tlie  gas  generated  in  the  intestine 
being  sufficient  to  burst  the  diaphragm.  Falls  are  by  no  means  an 
uncommon  cause ;   for  example,  an  animal  falls  on  to  its  head,  and 


RESPIRATION  121 

tHe  abdominal  viscera  are  propelled  against  the  diaphragm.  The 
diaphragm  rarely  gives  way  below,  nearly  always  above,  and  in  the 
tendinous  substance  rather  than  the  muscular.  This  point  is  of  physio- 
logical interest. 

In  the  nasal  passages  the  only  affection  of  any  moment  is  a  collection 
of  pus  in  the  facial  sinuses. 

Laryngitis  is  frequently  the  result  of  strangles  infection,  or  of 
ordinary  cold.     It  presents  no  physiological  features  of  interest. 

In  the  ox  pneumonia  is  rare,  with  the  exception  of  the  special  highly 
infectious  type,  constituting  one  of  the  animal  plagues.  Practically 
none  of  the  other  diseases  mentioned  above  as  affecting  the  horse  are 
found  in  any  ruminant. 


V 


u 


Fig.  31. — The  Position  of  thk  Musclks  oi'  the  Larynx  in  the 

Horse. 

rt,  Epiglottis  ;  h,  opening  leading  to  the  glottis  ;  c,  portion  of  the  aryte- 
noid cartilage  ;  d,  position  of  the  joint  formed  between  the  cricoid 
and  arytenoid  cartilages  ;  e,  the  trachea. 

The  wing  of  the  thyroid  cartilage  has  been  removed  so  as  to  expose  the 
constrictor  muscles ;  4,  4  repi'esents  its  cut  edge. 

1  and  2,  Thyro-arytenoideus :  1  anterior,  2  posterior  fasciculus.  The 
space  between  these  two  mviscles  indicates  the  position  of  the 
ventricle  of  the  larynx.  3,  Crico-arytenoideus  lateralis.  5,  Crico- 
thyroid muscle,  the  bulk  of  wliich  lies  inside  the  thyroid  cartilage, 
and  cannot,  therefore,  be  seen.  6,  Crico-arytenoideus  posticus. 
7,  Portion  of  cricoid  cartilage  ;  the  shaded  portion  in  front  of  the 
figure  represents  where  it  and  the  thyroid  meet.  8,  Arytenoideus 
muscle. 


122    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Section  2. 
The  Larynx. 

The  larynx  serves  a  twofold  purpose,  viz.,  respiration 
and  phonation  ;  in  animals  the  former  holds  the  more  im- 
portant position,  the  voice-producing  function  being  of  a 
very  subordinate  character. 

The  larynx  may  be  described  as  a  cartilaginous  box 
placed  at  the  summit  of  the  trachea,  the  opening  into  it 
being  capable  of  increasing  or  decreasing  in  size,  and  so 
allowing  a  larger  or  smaller  amount  of  air  to  enter  the 
lungs.  Within  the  larynx  are  two  elastic  cords  arranged 
V-shaped,  the  function  of  which  is  connected  solely  with 
the  production  of  sound  (Fig.  32).  Both  the  respiratory 
and  vocal  functions  require  that  the  several  parts  of  the 
larynx  should  move,  viz.,  that  the  mouth  of  the  organ 
should  be  widened  or  narrowed,  or  that  the  cords  should 
be  approximated,  drawn  apart,  tightened  or  slackened. 
These  movements  are  brought  about  by  certain  groups  of 
muscles,  those  which  approximate  the  walls  of  the  glottis 
being  known  as  the  adductors,  whilst  those  which  widen 
it  are  known  as  the  abductors. 

The  Muscles  of  the  Larynx  may  therefore  be  divided  into 
those  of  respiration  and  phonation  (Fig.  31).  As  the  most 
important  feature  in  respiration  is  the  opening  or  dilating 
of  the  glottis,  the  term  respiratory  muscle  might  be  con- 
fined to  the  dilator  of  the  glottis,  while  the  constrictors 
would  represent  the  vocal  muscles ;  but  the  constrictors 
are  not  entirely  without  a  respiratory  function,  as,  for 
example,  in  coughing,  so  that  in  the  following  table  they 
are  included  under  this  head. 

Respiratory  Muscles. 

Dilator  or  abductor,  Crico-arytenoideus  posticus. 

Constrictors  or  adductors  of   the      Crico-arytenoideus  lateralis,  Ary- 
glottis,  tenoideus,   and    Tliyro  -  aryte- 

noideus. 


RESPIRATION 


123 


The  crico-arytenoideus  lateralis  and  posticus  are  direct 
antagonists;  the  lateralis  depress  the  arytenoid  cartilages 
and  close  the  entrance  into  the  glottis,  the  posticus  swing 
the  arytenoids  upwards  and  outwards  and  enlarge  the 
glottis. 


^•^z 


Fig.  32.  -The  L.a.rynge.\l  Opening   during   Qrdinary 
Eespiration. 

1,  The  epiglottis  ;  2,  margin  of  arytenoids  ;  3,  vocal  cord  ;  4,  pharynx 
laid  open.  The  Y-shaped  slit  is  the  glottis.  Note  how  much 
wider  the  epiglottis  is  than  the  opening  it  has  to  cover. 


Plionatorij  Muscles. 

Muscle  which   relaxes   the  vocal  Thijro-arytenoideus,  anterior  and 

cords,  especially  posterior  fasciculus. 

Muscle  which   renders  the  cords  Crico-tliijroid. 

tense, 

Muscles   which   bring   the    cords  The  respiratory  adductors. 

together, 

Muscle  which    moves   the   cords  The  respiratory  abductor. 

apart, 


124    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  entrance  to  the  larynx  is  formed  hy  the  two  arytenoid 
cartilages,  the  epiglottis,  and  the  aryepiglottic  folds;  beyond 
these  is  the  glottis  proper,  viz.,  the  A^-shaped  opening  formed 
by  the  vocal  cords.  When  the  laryngeal  opening  dilates, 
the  vocal  cords  pass  towards  the  wall  of  the  cavity  and 
render  the  V-shaped  space  wider  ;  when  the  larynx  closes 
the  cords  are  approximated  and  the  space  rendered 
narrower  (Figs.  32  and  33).  During  ordinary  respiration 
there   is   very  little   if   any  alteration   in  the   shape   and 


\ 


Fig.  33, — The  Laryngeal  Opening  during  Hurried  Respiration, 
SEEN  in  a  State  of  Dilation. 

1,  Epiglottis  ;  2,  margin  of  arytenoids ;  3,  vocal  cord  ;  4,  pharynx  laid 
open.  Note  the  size  and  shape  of  the  glottal  opening  as  compared 
with  Fig.  32. 

size  of  the  glottis ;  but  during  exertion  every  inspiratory 
movement  is  accompanied  by  an  increase  in  size,  every 
expiration  by  a  decrease.  At  each  expiration  the  vocal 
cords  pass  towards  the  centre  line,  and  at  each  inspiration 
return  to  the  wall  of  the  larynx.  The  closure  of  the 
larynx,  such  as  during  the  act  of  swallowing,  is  a  powerful 
movement,  and  if  the  finger  at  this  moment  be  introduced 


RESPIRATION  125 

into  the  cavity  and  placed  between  the  arytenoids,  it  ex- 
periences considerable  pressure.  The  closure  of  the  larynx 
is  brought  about  by  the  depression  and  approximation  of 
the  arytenoid  cartilages  and  the  approximation  of  the  vocal 
cords ;  in  addition,  during  the  act  of  swallowing  the  base 
of  the  tongue  presses  the  epiglottis  over  the  arytenoids 
and  renders  the  part  both  air-  and  water-tight. 

The  Epiglottis  is  much  larger  than  the  opening  it  is  in- 
tended to  seal  during  a  condition  of  laryngeal  repose.  It 
is  carried  backwards  by  the  base  of  the  tongue  and  pressed 
over  the  arytenoids ;  the  larynx  at  the  same  moment 
advances,  with  its  arytenoid  cartilages  closely  approximated. 
After  the  act  of  swallowing  the  tongue  advances,  the  larynx 
recedes,  and  the  epiglottis  returns  to  its  position  by  means 
of  its  elastic  recoil.  It  is  not  essential  to  a  food-  or  water- 
tight condition  of  the  larynx  that  the  epiglottis  should  exist ; 
it  has  been  removed  both  by  disease  and  experimentally, 
and  its  place  is  then  taken  by  the  base  of  the  tongue.  Nor 
is  an  arytenoid  cartilage  essential  to  safety  in  swallowing. 

The  Nervous  Mechanism  of  the  Larynx  is  peculiar.  Sensa- 
tion to  the  mucous  lining  membrane  and  motor  power  to 
the  crico- thyroid  muscle  is  supplied  in  the  majority  of 
animals  by  the  superior  laryngeal  branch  of  the  vagus,  this 
nerve  containing  both  sensory  and  motor  fibres.  In  the 
horse  the  motor  fibres  running  in  the  superior  laryngeal  are 
derived  from  the  first  cervical  nerve  and  not  from  the  vagus. 
All  the  other  muscles  both  abductor  and  adductor  are 
supplied  with  motor  power  by  the  inferior  or  recurrent 
laryngeal  branch  of  the  vagus.  It  is  strange  that  both 
abductor  and  adductor  muscles  should  have  the  same 
source  of  nerve  supply,  and  one  naturally  asks  what  it 
is  which  determines  that  only  the  opening  or  only  the 
closing  muscles  shall  act  at  any  given  moment  ?  No  satis- 
factory explanation  of  this  fact  has  been  offered.  All  we 
know  is  that  both  dilator  and  constrictor  fibres  run  in  the 
recurrent  laryngeal  nerve  and  are  quite  distinct,  and  that  in 
some  animals  the  different  bundles  have  been  experimentally 
isolated  and  injured;  injury  to  the  dilator  fibres  producing 


126    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

abductor  paralysis,  and  injury  to  the  fibres  going  to  the 
muscles  which  close  the  larynx  producing  adductor 
paralysis. 

If  the  recurrent  laryngeal  be  cut  and  the  peripheral  end 
strongly  stimulated,  the  glottis  almost  invariably  is  found 
to  close ;  in  other  words  only  the  adductor  fibres  appear  to 
be  acted  upon.  If  a  weak  stimulation  be  applied  the  glottis 
opens,  viz.,  the  abductor  muscles  are  affected. 

Another  curious  fact  in  the  history  of  these  recurrent 
nerves  is  furnished  by  pathology.  In  the  disease  of  horses 
known  as  '  roaring,'  there  is  paralysis  of  the  left  abductor 
muscle  of  the  larynx,  viz.,  the  crico-arytenoideus  posticus, 
the  wasting  and  fatty  degeneration  due  to  paralysis  being 
very  marked.  It  is  not  unusual  to  find  the  adductor 
muscles  normal  in  appearance,  or  presenting  very  little 
sign  of  disease,  and  even  if  pale  and  wasted  the  degree 
of  degeneration  cannot  be  compared  with  that  furnished  by 
the  abductor  muscle.  This  is  a  difficult  fact  to  explain ; 
one  would  think  that  as  both  abductor  and  adductor 
muscles  receive  the  same  nerve  supply,  equal  wasting  would 
occur  in  both  groups.  Again,  it  is  observed  when  the 
recurrent  has  been  divided  experimentally,  that  the  abductor 
muscle  loses  its  irritability  long  before  the  adductors,  and 
the  same  fact  may  be  observed  in  post  mortem  stimulation 
of  the  nerves.  If  the  recurrent  laryngeal  nerves  be  divided 
under  ether,  and  the  peripheral  ends  stimulated,  adduc- 
tion of  the  larynx  is  obtained ;  but  if  the  ether  narcosis 
be  pushed  to  a  dangerous  extent  and  the  nerves  then 
stimulated  the  larynx  dilates,  that  is  abduction  follows. 
These  and  other  observations  have  furnished  a  law  which 
is  of  clinical  significance,  viz.,  that  in  functional  disturbance 
of  the  larynx  the  adductor  muscles  are  first  affected,  but 
that  in  changes  accompanied  by  organic  lesions  the 
abductor  muscles  are  the  first  to  suffer. 

When  one  recurrent  laryngeal  nerve  is  divided  the  vocal 
cord  on  that  side  remains  immovable  and  therefore  cannot 
approach  its  fellow ;  the  healthy  cord  endeavours  to  com- 
pensate  for   the   weakness   of   its   companion   by  passing 


EESPIEATION  127 

beyond  the  middle  line  of  the  larynx,  in  its  attempt  to 
come  into  contact  with  it. 

The  inspiratory  distress  occasioned  in  *  roaring '  is  not 
brought  about,  as  has  been  described,  by  a  paralyzed  vocal 
cord  flapping  about,  for  the  elastic  nature  of  the  cord  and 
other  reasons  negative  this.  The  sound  is  produced  by  the 
paral^^zed  left  arytenoid  being  drawn  into  the  glottis  at  each 
inspiration,  which  is  the  explanation  why  the  noise  which 
accompanies  the  disease  is  always  inspiratory  and  never 
expiratory. 

Phonation. — Voice  is  produced  by  the  approximation  and 
vibration  of  the  vocal  cords,  the  pitch  of  the  voice  being 
produced  by  the  tension  of  the  cords,  whilst  the  quality  is 
due  to  the  shape  of  the  cords,  viz.,  their  thickness  or  thin- 
ness. The  position  of  the  resonant  chambers  such  as  the 
mouth,  pharynx,  posterior  nares,  and  even  nasal  chambers 
also  importantly  affects  the  quality  of  the  voice.  It  is 
obvious  that  the  chief  alterations  in  the  larynx  during 
phonation  refer  to  the  vocal  cords ;  these  are  approximated 
by  the  adductor  muscles,  and  separated  by  the  abductor 
muscles,  whilst  they  are  relaxed  by  the  thyro-arytenoideus 
and  tightened  by  the  crico-thyroid.  The  latter  muscle  has 
a  peculiar  action,  it  lowers  the  thyroid  cartilage  on  the 
cricoid  and  swings  the  wing  of  the  thyroid  outwards,  thus 
rendering  the  cords  tense.  These  changes  in  the  vocal 
cord  produce  changes  in  the  shape  of  the  V-shaped  glottal 
opening;  in  a  high  note  the  glottis  is  reduced  to  a  mere 
slit,  in  deeper  notes  the  cords  are  separated.  If  air  be 
forced  through  the  larynx  of  a  dead  horse  and  the  tension 
of  the  cords  altered,  a  sound  remarkably  like  a  neigh  may 
be  produced.  The  ventricles  of  the  larynx  and  cavities  of 
the  mouth,  nose,  pharynx,  etc.,  act  as  resonators.  Being 
filled  with  air,  they  effect  the  needful  alterations  in  the 
quality  of  the  voice  and  assist  in  giving  it  its  distinctive 
character  ;  thus  the  false  nostrils  furnish  the  *  snort '  of 
the  frightened  or  '  fresh '  horse,  the  nasal  chambers  the 
whinny  and  neigh  of  pleasure,  the  mouth  and  pharynx  the 
neigh  of  impatience,  loneliness,  excitement,  etc.     We  do 


128    A  MANUAL  OF  VETEPJNAEY  PHYSIOLOGY 

not  consider  that  the  guttural  pouches  act  as  resonators, 
and  Colin  obtained  no  alteration  in  the  character  of  the 
neigh  by  opening  them. 

The  voice  of  each  class  of  animal — horse,  ass,  ox,  sheep, 
and  pig — is  so  distinctive  that  we  may  recognise  their 
presence  without  seeing  them ;  yet  though  the  larynx  in 
all  these  animals  differs  more  or  less,  the  difference  is  not 
sufficient  to  offer  any  explanation  as  to  why  the  sounds  it 
emits  are  so  entirely  distinct.  The  voice  of  male  and 
female  animals  differs  in  intensity.  The  wild  neigh  of  the 
stallion  is  very  different  from  the  neigh  of  the  mare,  and 
the  bellowing  of  the  bull  is  distinct  from  the  '  lowing '  of 
the  cow.  The  operation  of  castration  has  a  remarkable 
effect  on  the  voice,  the  neigh  of  the  gelding  resembling 
that  of  the  mare. 

In  the  horse  the  voice  is  used  during  sexual  and  ordinary 
excitement,  also  during  fear  or  especially  loneliness,  during 
pain,  anger,  and  as  a  mark  of  pleasure.  It  is  not  possible 
to  convey  in  words  the  difference  in  the  notes  produced,  but 
they  are  easy  to  recognise.  The  horse  is  essentially  a 
sociable  animal ;  when  accustomed  to  be  in  the  company  of 
others  he  dislikes  separation,  and  shows  it  by  persistent 
neighing,  which  is  perhaps  more  noticeable  amongst  army 
horses  than  any  others.  The  neigh  of  pleasure  is  often 
spoken  of  as  the  '  whinny ' ;  the  word  rather  conveys  an  idea 
of  the  sound  made.  Sounds  which  can  only  be  described 
as  *  screams  '  are  often  evoked  during  '  horse-play '  and 
temper,  or  by  mares  during  oestrum.  It  is  not  a  scream 
as  we  know  it  in  the  human  subject,  but  no  other  word 
conveys  an  idea  of  its  shrillness.  If  a  horse  cries  from 
pain  (which  is  very  rare),  as  during  a  surgical  operation, 
the  cry  is  a  muffled  one  and  short ;  it  is  a  groan  rather 
than  a  cry. 

In  the  cerebral  cortex  voice  is  represented  in  the  pra- 
crucial  and  neighbouring  gyrus  of  the  dog,  and  corre- 
sponding regions  in  other  animals.  Stimulation  of  this 
region  leads  to  hi-lateral  adduction  of  the  cords ;  it  is 
curious  why  stimulation  of  one  side  of  the  brain  should 


EESPIRATION  129 

lead  to  movements  of  both  vocal  cords.  There  is  no  region 
of  the  cortex  of  the  dog  which  leads  to  abduction  of  the 
cords,  though  such  a  region  is  found  in  the  cat.  The 
cortical  centre  communicates  with  a  subordinate  centre  in 
the  medulla  situated  in  the  region  of  the  fourth  ventricle, 
and  stimulation  of  certain  parts  of  this  centre  leads  to 
abduction  and  of  others  to  adduction  of  the  cords. 

Neighing  in  the  horse  is  produced  by  an  expiration, 
partly  through  the  nostrils  and  partly  through  the  mouth  ; 
■braying  in  the  ass  is  both  inspiratory  and  expiratory, 
nostrils  and  mouth  each  taking  a  share  in  it.  The 
ventricles  of  the  larynx  are  large  in  the  horse  and  relatively 
still  larger  in  the  ass  and  mule ;  they  act  as  resonators 
and  allow  of  free  vibration  of  the  vocal  cords.  According 
to  Chauveau  both  ass  and  mule  have  the  subepiglottic 
sinus  provided  with  a  thin  membrane  capable  of  vibrating. 
In  the  ox,  sheep,  and  goat,  the  larynx  is  very  simple,  there 
are  onl}^  rudimentary  vocal  cords  and  no  ventricles.  The 
bellowing  of  the  ox  and  bleating  of  the  sheep  are  exniratory 
efiforts  through  the  mouth.  The  dog  and  cat  have  a  larynx 
something  like  that  of  the  horse,  but  the  ventricles  are 
shallow ;  the  voice  is  produced  almost  entirely  through  the 
mouth,  though  both  growling  and  purring  may  occur 
through  the  nostrils. 

Yawning  is  a  deep  slow  inspiration  followed  by  a  short 
expiration ;  the  air,  even  in  the  horse,  is  taken  in  by  the 
mouth,  which  is  widely  opened  and  the  jaws  crossed. 

Sneezing  and  Coughing  are  expiratory  efforts.  The  former 
occurs  solely  through  the  nose  and,  excepting  in  the  dog 
and  cat,  is  unaccompanied  by  the  peculiar  sound  attending 
this  act  in  the  human  subject.  If  snufl"  be  introduced  into 
the  nostrils  of  the  horse,  a  peculiar  though  well  known 
vibration  of  the  nostrils  occurs  as  if  the  animal  were 
blowing  its  nose,  and  this  is,  in  fact,  what  it  accomplishes. 
It  is  an  entirely  nasal  sound,  the  mouth  takes  no  share  in 
the  act.  Coughing  occurs  through  the  mouth,  the  long 
palate  in  the  horse  being  raised  for  the  purpose.  Before 
coughing  can  occur  the  lungs  must  be  filled  with  air  and 

9 


IBO    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  glottis  closed  ;  a  forcible  expiration  follows,  the  glottis 
opens,  and  the  air  is  expelled  through  the  mouth. 

Hiccough  is  due  to  a  sudden  contraction  of  the  diaphragm. 
While  the  air  is  rushing  into  the  lungs  the  glottis  closes, 
and  the  incoming  air,  striking  the  closed  glottis,  produces 
the  sound.  The  condition  knov/n  as  spasm  of  the 
diaphragm  in  the  horse  is  very  different  from  a  human 
hiccough,  and  has  been  referred  to  more  fully  on  p.  120. 


CHAPTER    V 
DIGESTION 

Section  1. 
Digestion  in  the  Mouth. 

Prehension  of  Food.  —  The  methods  by  which  animals 
convey  food  to  the  mouth  differ  according  to  the  species. 
In  the  horse  the  Hps  play  an  important  part,  for  which 
purpose  they  are  thick,  remarkably  strong,  and  endowed 
with  acute  sensation ;  in  the  ox  they  serve  a  subordinate 
function,  being  rigid  and  wanting  in  mobility  ;  in  the  sheep 
the  upper  lip  is  cleft  in  such  a  manner  as  to  divide  it 
completely  into  two  parts,  each  possessing  independent 
movement ;  in  the  pig  the  lower  lip  is  pointed  and  the 
upper  one  insignificant. 

In  manger  feeding  the  horse  collects  the  food  with  the 
lips,  but  in  grazing  cuts  oft'  the  grass  with  the  incisor 
teeth,  drawing  the  lips  back  in  order  that  they  may  bite 
closer  to  the  ground.  In  the  ox  the  tongue  is  protruded 
and  curled  around  the  grass,  which  is  thus  drawn  into  the 
mouth  and  taken  off  between  the  incisor  teeth  and  the 
dental  pad.  In  the  sheep  the  divided  upper  lip  allows  of 
the  incisors  and  dental  pad  biting  close  to  the  ground,  so 
that  animals  of  the  sheep  and  goat  class  can  live  on  land 
where  others  such  as  the  horse  and  ox  would  starve.  In 
whatever  way  the  food  is  cut  off',  it  is  carried  back  by  the 
movements  of  the  tongue  to  the  molar  teeth,  there  to 
undergo  a  more  or  less  complete  grinding. 

In  the  ox  and  sheep  the  incisor  teeth  move  freely  in  their 

131  9—2 


132     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

sockets,  the  object  of  which  is  to  prevent  injury  to  the 
dental  pad,  for  which  purpose  also  they  are  placed  very 
obliquely  in  the  jaw.  In  the  horse  the  incisor  teeth  in 
early  life  are  very  upright  but  become  oblique  with  age. 
The  molars  in  all  herbivora  are  compound  teeth  ;  in  the 
horse  they  are  very  large,  especially  those  in  the  upper 
jaw.  Being  composed  of  materials  of  different  degrees 
of  hardness  they  wear  with  a  rough  surface,  which  is  very 
essential  to  the  grinding  and  crushing  they  have  to  inflict 
on  grasses  and  grain.  The  teeth  in  herbivora,  both 
incisors  and  molars,  are  constantly,  though  slowly,  being 


Fig.  34. — Schematic  Transversk  Section  of  the  Upper  and  Lower 
Jaws  of  the  Horse  between  the  Third  and  Fourth  Molars, 
SHOWING  the  Position  of  the  Tables  of  the  Teeth  during 
Rest  and  Mastication. 

UJ  upper  jaw,  LJ  lower  jaw,  EM  right  molar,  LM  left  molar,  ELM 
right  lower  molar,  LLM  left  lower  molar.  1,  The  position  of  the 
teeth  during  rest,  the  outside  edge  of  the  lower  row  in  apposition 
with  the  inside  edge  of  the  upper.  2,  The  jaws  fully  crossed 
masticating  from  right  to  left ;  the  tables  of  both  upper  and  lower 
molars  now  rest  on  each  other.  .3,  The  position  half  way  through 
the  act  of  mastication  ;  the  outer  half  of  the  lower  teeth  wearing 
against  the  inner  half  of  the  upper. 


pushed  out  of  the  sockets  which  hold  them ;  in  this  way 
wear  and  tear  is  compensated  for,  whilst  the  fang  of  the 
tooth  becomes  correspondingly  reduced  in  length.  It  is 
owing  to  this  fact  that  the  incisor  teeth  alter  in  shape  and 
direction,  and  so  enable  the  age  to  be  determined.  The 
tables  of  the  molar  teeth  are  not  flat  but  oblique  ;  this  is 


DIGESTION  133 

especially  well  seen  in  the  horse  where  the  cutting  surface 
is  chisel  shaped,  the  upper  teeth  being  longest  on  the  out- 
side, while  those  of  the  lower  row  are  longest  on  the  inside 
(see  Fig.  34).  This  arrangement  produces  sharp  teeth, 
which  are  a  constant  source  of  trouble  and  loss  of  condition 
in  horses. 

The  movements  of  the  tongue  are  important.  In  the  ox 
and  dog  they  are  very  extensive,  the  former  animal  having 
no  difficulty  in  protruding  the  tongue  and  even  introducing 
the  tip  into  the  nostrils.  It  is  not  a  very  common  habit  with 
horses  to  protrude  the  tongue  except  when  yawning,  but  they 
have  considerable  power  in  withdrawing  it  in  the  mouth. 
A  great  difference  exists  between  the  tongue  of  the  horse 
and  that  of  the  ox ;  the  former  is  flabby,  broad  and  flat  at 
the  end,  constricted  opposite  the  frenum,  and  swells  out 
at  the  apex;  it  is  comparatively  smooth  on  its  surface. 
The  tongue  of  the  ox  narrows  from  base  to  apex,  the 
latter  being  pointed  ;  it  is  very  rough,  which  prevents  it 
from  losing  its  hold  on  the  food,  protects  it  from  such 
injury  as  might  be  inflicted  by  coarse  grasses,  and  is  also 
of  value  to  the  animal  in  cleaning  its  body.  The  tongue  is 
supplied  with  motor  power  by  the  hypoglossal  nerve  and 
with  sensation  by  the  lingual  branch  of  the  fifth,  which 
supplies  the  anterior  two  thirds  of  the  mucous  membrane, 
the  posterior  third  being  supplied  by  the  lingual  branch 
of  the  glosso-pharyngeal ;  the  same  nerve  also  supplies 
the  sense  of  taste  to  this  part  of  the  organ,  while  taste 
for  the  anterior  two  thirds  is  supplied  by  the  chorda 
tympani  of  the  seventh  pair. 

The  inside  of  the  mouth  of  the  ox  is  covered  with  long 
papilla?,  which  look  backwards  ;  these  would  appear  to  be  of 
use  in  preventing  the  food  from  falling  out  of  the  mouth. 
In  the  horse  no  such  papillae  exist,  in  fact  the  lining  mem- 
brane of  the  part  is  remarkably  smooth.  The  majority  of 
animals  have  grooves  in  the  palate ;  they  are  well  marked 
in  the  horse,  ox,  sheep,  and  even  in  the  dog.  Their  func- 
tion is  probably  connected  with  assisting  the  tongue  to  pass 
the  food  back  in  the  mouth. 


134    A  MANUAL  OV  VETERINARY  PHYSIOLOGY 

Drinking  is  performed  by  the  animal  drawing  the  tongue 
backwards  and  thus  using  it  as  the  piston  of  a  suction- 
pump  ;  this  action  produces  a  vacuum  in  the  front  of  the 
mouth,  as  the  result  of  which  the  cheeks  are  drawn  inwards, 
the  lips  at  the  same  time  being  closed  all  round,  excepting 
a  small  space  in  front  which  is  placed  under  water.  Such 
is  the  method  in  both  horse  and  ox ;  in  the  former  animal 
the  head  is  extended  while  drinking,  the  ears  are  drawn 
forward  at  each  swallow  and  during  the  interval  fall  back. 
The  cause  of  this  motion  is  not  clear,  but  is  i)robably  due 
to  the  movement  of  air  in  the  guttural  pouches.  Lapping 
in  the  dog  is  performed  by  curling  the  tongue  in  such  a 
way  as  to  convert  it  into  a  spoon.  Sucking,  like  drinking, 
is  produced  by  the  animal  creating  a  vacuum  in  the  mouth 
by  closing  the  lips,  decreasing  the  size  of  the  tongue  in 
front  and  increasing  it  behind,  the  dorsum  being  applied  to 
the  roof  of  the  mouth.  The  foal  places  the  tongue  beneath 
the  nipple  and  curls  it  in  from  each  side  ;  by  this  means  he 
protects  it  from  the  lower  incisors  and  gets  a  better  hold. 

Mastication  is  performed  between  the  molar  teeth  ;  the 
movements  which  the  jaws  undergo,  to  admit  of  this  being 
carried  out,  depend  upon  the  class  of  animal.  In  the  dog 
they  are  very  simple,  being  only  a  depression  and  elevation 
of  the  jaw ;  this  motion  means  a  simple  temporo-maxillary 
articulation,  and  such  is  met  with  in  this  animal.  In 
the  horse  and  ox  the  movement  is  not  only  up  and  down, 
but  lateral,  and  some  say  even  from  front  to  rear.  This 
necessitates  a  complex  joint  capable  of  affording  a  consider- 
able amount  of  play,  and  this  is  provided  by  a  disc  of 
cartilage  being  placed  between  the  articulation,  which 
accommodates  itself  to  the  varying  movements  of  the  joint 
in  the  horse,  ox,  and  sheep,  and  also  saves  the  part  from 
jar.  In  herbivora,  therefore,  we  find  the  cartilage  exten- 
sively developed,  whilst  in  carnivora  it  is  small  and  simple. 
The  character  of  the  movement  occurring  in  the  temporo- 
maxillary  articulation  of  herbivora  during  mastication  is 
as  follows.  During  rotatory  movement,  or  lateral  displace- 
ment, one  of  the  articulating  heads  remains  as  a  fixed  point 


DIGESTION  135 

simply  turning  on  its  centre,  whilst  its  fellow  describes  an 
arc ;  this  is  why  the  movement  can  only  occur  on  one  side 
at  a  time  (Gamgee).  During  mastication  the  contents  of 
the  orbital  fossie  are  observed  in  the  horse  to  be  alternately 
ascending  and  descending.  This  movement  is  due  to  the 
coronoid  process  of  the  lower  jaw,  the  fossa  being  pushed 
up  as  it  comes  forward  and  depressed  as  it  recedes.  The 
muscles  which  bring  about  this  important  lateral  move- 
ment of  the  jaws,  which  in  the  ox,  owing  to  the  freedom 
of  the  articulation,  may  be  termed  rotatory,  are  the  two 
pterygoids,  especially  the  internal.  The  herbivora  can  only 
masticate  on  one  side  at  a  time  ;  when  tired  on  one  side 
the  process  is  reversed  and  the  opposite  molars  take 
on  the  crushing.  It  is  surprising  the  length  of  time  an 
animal  will  carry  on  mastication  on  one  side ;  even  as  long 
as  an  hour  has  been  observed  in  the  horse  by  Colin. 
Gamgee  noticed  that  in  the  ox  the  first  stroke  of  the  molars 
is  in  the  opposite  direction  to  the  regular  action  which 
follows ;  thus  if  masticating  from  right  to  left  the  first 
stroke  is  made  from  left  to  right.  It  is  important  to 
note  that  in  those  animals  where  a  single-sided  lateral  or 
rotatory  movement  in  mastication  is  necessary,  the  upper 
jaw  is  always  wider  than  the  lower ;  this  we  can  under- 
stand, for  if  both  were  the  same  width  the  molar  teeth 
would  not  meet  each  other  when  the  jaws  were  crossed 
for  lateral  mastication.  This  extra  width  of  the  upper  over 
the  lower  jaw,  in  conjunction  with  the  peculiarity  of  masti- 
cation, explains  why  the  molar  teeth  of  the  horse  and  other 
herbivora  wear  with  sharp  chisel  edges  (see  Fig.  34). 

In  the  horse  mastication  is  slow  and  as  a  rule  well  per- 
formed ;  he  takes  from  five  to  ten  minutes  to  eat  one  pound 
of  corn,  and  fifteen  to  twenty  minutes  to  eat  one  pound 
of  hay.  In  the  ox  mastication  is  imperfectly  performed  to 
start  with,  but  the  material  is  eventually  brought  back  to 
the  mouth  by  the  process  of  rumination,  and  undergoes 
thorough  re-mastication.  In  the  dog  mastication  is  imper- 
fectly performed  ;  after  a  few  hasty  snaps  of  the  jaw  the 
material  is  swallowed. 


136     A  MANUAL  OF  VETERINAKY  PHYSIOLOGY 

Opening  the  month  is  equivalent  to  depressing  the  lower 
jaw,  for  the  upper  takes  no  share  in  the  process.  The 
muscles  which  open  the  mouth  are  comparatively  small, 
for  very  little  effort  is  required,  the  sterna-  and  stylo- 
maaillaris  and  dhjastricns  perform  this  function.  On  the 
other  hand,  the  closing  of  the  jaws  in  mastication  is  a 
difficult  task,  and  for  this  purpose  very  powerful  muscles 
exist,  they  are  the  masseters,  temporals  and  ■pteri/fio'ids. 
In  the  dog  the  temporal  muscles  are  considerably  developed, 
whilst  in  herbivora  the  masseters  are  the  largest. 

The  nerves  employed  in  mastication  are  the  sensory 
fibres  of  the  fifth  which  convey  to  the  brain  the  impulse 
resulting  from  the  presence  of  food  in  the  mouth,  while 
the  motor  fibres  of  the  same  nerve  supply  the  needful 
stimulus  to  all  the  muscles  of  mastication  excepting  the 
digastricus,  which  receives  its  motor  supply  from  the 
seventh  pair. 

The  process  of  Deglutition  is  usually  described  as  occur- 
ring in  three  stages.  The  first  stage  practically  comprises 
carrying  the  food  back  to  the  base  of  the  tongue  and  press- 
ing it  against  the  soft  palate ;  it  is  a  simple  process  and 
readily  understood.  In  the  second  stage  the  act  is  complex, 
for  the  bolus  or  fluid  has  to  cross  the  air  passage  and  must 
be  prevented  from  falling  into  the  nasal  chambers,  or  finding 
its  way  down  the  trachea.  To  accomplish  this  the  soft  palate 
is  raised  and  so  closes  the  nasal  chambers,  the  tongue  at 
the  same  time  being  carried  backwards,  while  the  larynx  and 
pharynx  are  advanced.  This  movement  causes  the  base  of 
the  tongue  to  press  on  the  epiglottis  and  close  the  larynx, 
which  is  further  secured  by  the  arytenoid  cartilages  and 
vocal  cords  coming  close  together.  The  bolus  or  fluid  can 
now  safely  pass  towards  the  pharynx,  being  grasped  tightly 
by  the  pharyngeal  muscles  and  pressed  into  the  oesophagus. 
In  the  third  act  of  swallowing  the  food  is  carried  down  the 
oesophagus  by  a  continuous  wave  of  contraction,  which 
starts  at  the  pharynx  and  ends  at  the  stomach.  Chauveau 
points  out  that  owing  to  its  extreme  length,  the  soft  palate 
of  the  horse  passes  completely  into  the  pharynx  during  the 


DIGESTION  187 

second  act  of  deglutition.  The  length  of  the  soft  palate 
prevents  food  or  water  being  returned  by  the  mouth  when 
once  they  have  entered  the  pharynx,  so  that  in  vomiting,  or 
in  cases  of  sore  throat,  the  food,  water,  or  other  material 
is  returned  by  the  nostrils. 

The  action  of  the  epiglottis  in  the  closure  of  the  glottis 
has  been  much  discussed.  We  have  described  it  as  being 
forced  over  the  opening  by  the  base  of  the  tongue  and  the 
advancing  larynx  ;  but  the  epiglottis  is  not  essential  to 
swallowing,  for  an  animal  can  swallow  when  it  has  been 
removed,  and  even  when  one  of  the  arytenoid  cartilages 
has  been  excised.  With  a  finger  in  the  larynx  it  can  easily 
be  demonstrated  that  the  part  tightly  and  forcibly  closes 
during  the  second  stage  of  swallowing,  the  vocal  cords 
and  arytenoids  being  brought  so  close  together  that  the 
glottis  is  perfectly  air-tight.  It  has  been  pointed  out  that 
animals  usually  swallow  with  a  flexed  neck,  as  in  this 
position  the  epiglottis  is  behind  the  soft  palate  and  in  the 
most  favourable  position  to  be  applied  over  the  glottis  ;  it 
has  also  been  shown  that  when  the  head  is  extended  the 
epiglottis  is  in  the  mouth,  viz.,  anterior  to  the  soft  palate. 
We  have  found  it  in  this  position  in  the  horse,  and  judging 
from  the  fact  that  in  a  state  of  nature  the  horse  and  ox 
swallow  with  an  extended  and  not  with  a  flexed  neck,  it  is 
probable  that  in  feeding  oft'  the  ground  the  epiglottis  is 
anterior  to  the  soft  palate.  During  the  third  stage  of 
deglutition  the  bolus  can  be  seen  slowly  travelling  down 
the  channel  of  the  neck ;  if  liquid  however  be  passing,  the 
movement  is  very  rapid,  for  as  many  as  sixty  swallows  may 
be  made  in  a  minute.  Both  in  eating  and  drinking  the  third 
act  of  deglutition  can  occur  against  gravity ;  this  is  because 
it  is  a  muscular  act.  The  whole  process  of  deglutition  is 
considerably  assisted  by  the  salivary  secretion.  When  this 
has  been  experimentally  diverted  swallowing  only  occurs 
with  difliculty  and  very  slowly. 

The  oesophagus  of  the  horse  is  found  to  differ  consider- 
ably from  that  of  most  other  animals.  It  is  composed  for 
the   greater   part   of    its   length    of    red   striated   muscle, 


138     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

while  at  and  near  its  termination  the  previously  thin 
muscular  coat  becomes  very  thick  and  rigid,  and  the  red 
gives  way  to  pale  non-striped  muscle ;  further,  the  lumen 
of  the  tube  becomes  very  narrow.  The  thick  terminal  end 
of  the  oesophagus  of  the  horse  is  always  closely  contracted, 
so  that  if  cut  through  close  to  the  stomach  no  material  can 
escape  ;  this  is  one  explanation  why  horses  vomit  with  such 
difficulty.  In  the  ox,  sheep,  and  dog,  the  tube  is  com- 
posed of  red  muscle  throughout  ;  it  terminates  in  a  dilated 
end  at  the  stomach,  and  owing  to  its  thin  distensible  walls 
even  bulky  material  can  pass  along  it ;  what  the  ox  and 
dog  can  swallow  with  ease  would  certainly  *  choke '  the 
horse. 

The  first  stage  of  deglutition  is  voluntary,  but  the  re- 
maining processes  are  quite  involuntary,  and  are  brought 
about  by  the  stimulation  of  a  centre  in  the  medulla  known 
as  the  swallowing  centre.  By  means  of  ingoing  or  afferent 
nerves  supplied  by  branches  of  the  fifth  and  the  superior 
laryngeal,  the  centre  is  made  acquainted  with  the  fact  that 
food  is  present  in  the  fauces.  A  reflex  act  is  now  set  up 
in  the  centre  and  an  impulse  conveyed  to  the  muscles  of 
the  part  by  outgoing  or  efferent  nerves,  furnished  by  the 
pharyngeal  plexus  (composed  of  the  vagus  and  glosso- 
pharyngeal) to  the  constrictor  muscles  of  the  pharynx,  by 
the  hypoglossal  to  the  tongue,  and  by  the  recurrent  laryn- 
geal to  the  muscles  which  close  the  glottis.  The  glosso- 
pharyngeal is  the  inhibitory  nerve  of  deglutition ;  if  the 
central  end  be  stimulated  it  is  impossible  to  produce  the 
act  of  swallowing.  Swallowing  may  be  induced  without 
the  presence  of  food  in  the  fauces  ;  touching  the  rim  of  the 
glottis  will  produce  it,  as  also  will  pouring  fluids  into  the 
trachea,  or  even  touching  the  interior  of  the  trachea  as  far 
down  as  the  bronchi.  The  swallowing  centre  also  presides 
over  the  oesophagus,  and  the  peristaltic  wave  from  the 
pharynx  to  the  stomach  is  produced  by  impulses  sent  out 
from  this  centre  through  the  vagus.  This  wave  is,  there- 
fore, not  due  to  the  nerve  handing  on  a  contraction  by 
direct  conduction  from  one  layer  of  the  muscular  wall  of 


DIGESTION  139 

the  oesophagus  to  the  next.  Hence,  when  once  started  it  is 
not  arrested  either  by  ligaturing  or  dividing  the  cesophagus, 
though  section  of  the  cesophageal  nerves  prevents  it.  It 
is  not  uncommon  in  watching  a  bohis  pass  down  the 
neck  of  the  horse  to  see  it  suddenly  come  to  a  standstill, 
and  then  slowly  pass  on  again  after  probably  an  attempt 
to  ascend.  This  is  generally  due  to  absence  of  saliva. 
In  rumination  and  in  vomiting  the  wave  runs  upwards 
from  the  stomach  to  the  pharynx. 

The  Saliva. 

During  the  process  of  mastication  the  food  becomes 
mixed  in  the  mouth  with  a  fluid  known  as  saliva,  the 
secretion  of  which  occurs  in  three  distinct  pairs  of  glands. 
The  method  by  which  it  is  formed  is  important  to  under- 
stand, as  much  the  same  process  occurs  in  other  secretory 
glands  which  we  have  not  the  same  opportunity  of  watch- 
ing during  their  activity. 

Classification  of  Salivary  Glands. — The  three  glands  which 
secrete  saliva  are  the  parotid,  submaxillary,  and  sublingual ; 
these  are  structurally  divided  into  two  groups,  mucous  and 
serous  (or  albuminous)  glands,  the  submaxillary  and  sub- 
lingual being  types  of  the  first,  the  parotid  the  type  of  the 
other.  The  salivary  glands  in  the  herbivora  are  of  con- 
siderable size,  the  submaxillary  and  sublingual  being  well 
developed  in  the  ox,  while  in  the  horse  they  are  rudimentary. 
According  to  Colin,  there  is  no  relationship  between  the 
weight  of  the  glands  and  the  amount  of  fluid  they  secrete  ; 
the  parotid  in  all  cases  secretes  more  than  the  others.  In 
the  horse  it  is  only  four  times  heavier  than  the  sub- 
maxillary, but  it  secretes  twenty-four  times  as  much 
saliva ;  in  the  ox  the  parotid  is  not  so  large  as  the  sub- 
maxillary, but  its  secretion  is  four  or  five  times  greater. 

Amount  of  Secretion. — Colin  places  the  daily  secretion  of 
saliva  in  the  horse  at  84  lbs.,  and  in  the  ox  at  112  lbs., 
though  the  amount  will  depend  on  the  dryness  of  the  food 
consumed  ;    thus    hay  absorbs  more  than   four  times  its 


140     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

weight  of  saliva,  oats  rather  more  than  their  own  weight, 
and  green  fodder  half  its  own  weight. 

Physical  and  Chemical  Characters.  —  Mixed  saliva  is  an 
alkaline,  opalescent,  or  slightly  turbid  fluid  which  readily 
froths  when  shaken.  On  standing  exposed  to  the  air  it 
throws  down  a  deposit  of  carbonate  of  lime  due  to  the  loss 
of  its  carbonic  acid.  It  has  a  specific  gravity  of  1005  in 
the  horse,  and  1010  in  the  ox.  Examined  microscopically 
saliva  is  seen  to  contain  epithelial  scales  and  salivary 
corpuscles.  The  latter  are  small  round  granular  cells 
which  seem  to  be  altered  leucocytes  and  are  probably 
derived  from  the  soft  palate.  About  'G  per  cent,  of  the 
saliva  consists  of  mineral  matter,  and  "2  per  cent.,  more 
or  less,  of  organic  matter,  the  latter  consisting  of  mucin 
(which  gives  saliva  its  well-known  viscidity  and  ropiness), 
and  small  amounts  of  proteid  substances  the  nature  of 
which  has  not  been  exactly  determined.  Mucin  belongs  to 
a  peculiar  group  of  proteid  bodies  combined  with  a  carbo- 
hydrate, for  which  see  Appendix.  Ptyalin  or  salivary 
diastase  is  the  most  interesting  organic  constituent  of 
saliva  in  man,  but  it  is  doubtful  if  it  exists  in  the  herbivora, 
and  under  any  circumstances  its  amount  has  not  been  de- 
termined. Ptyalin  is  also  absent  from  the  saliva  of  the 
dog.  The  salts  of  saliva  are  principally  carbonate  of  lime, 
alkaline  chlorides,  and  phosphates  of  lime  and  magnesia. 
A  substance  known  as  sulphocyanide  of  potassium  has  been 
found  in  minute  quantities  in  the  saliva  of  the  human 
subject,  but  is  absent  from  that  of  the  horse.  The  gases 
of  the  saliva  are  principally  carbonic  acid,  with  traces  of 
oxygen  and  nitrogen ;  there  is  no  body  fluid  which  contains 
so  much  carbonic  acid  as  saliva  (65  vols,  per  cent.).  The 
three  salivas  have  different  physical  properties : — Parotid 
saliva  is  watery,  clear,  and  free  from  mucin,  but  contains 
a  small  quantity  of  proteid ;  submaxillary  and  sublingual 
saliva  are  viscid,  especially  the  latter.  In  man  the  parotid 
saliva  is  rich  in  ptyalin. 

Colin  has  observed  certain  peculiarities  in  the  secretion 
of   saliva  in    herbivora  which   deserve  careful   attention. 


DIGESTION 


141 


He  demonstrated  that  the  secretion  from  the  parotids  is 
unilateral,  the  gland  on  that  side  of  the  mouth  on  which 
the  animal  is  masticating  secreting  two  or  three  times  as 
much  as  its  fellow ;  the  submaxillary  and  sublingual  glands, 
on  the  other  hand,  secrete  equally,  no  matter  on  which 
side  mastication  is  being  performed.  Further,  the  parotids 
secrete  during  rumination,  the  unilateral  secretion  still 
being  maintained,  whilst  the  submaxillary  and  sublingual 


Fig.  35. — Apparatus  employed  by  Colin  in  Experiments  on  the 
Secretion  of  Parotid  and  Submaxillary  Saliva. 


glands  are  during  this  process  in  a  state  of  rest.  In  a 
fasting  horse  the  parotids  are  quiescent,  while  in  the  ox 
they  are  active.  Observations  tend  to  show  that  in  the 
former  animal  during  fasting  the  mouth  is  kept  moist  by 
secretions  from  the  sublingual,  palatine,  labial  and  molar 
glands.  The  glands  of  the  mouth  are  extensively  developed 
in  the  horse,  particularly  the  palatine,  and  some  large  ones 
close  to  the  epiglottis ;  their  secretion  is  extremely  viscid. 
Neither  the  sight  of  food  nor  the  introduction  into  the 
mouth  of  sapid  substances,  produces  any  effect  on  the 
salivary  secretion  from  the  parotid  of   the  horse  ;    sapid 


142     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

substances,  however,  stimulate  submaxillary  secretion. 
The  apparatus  used  in  these  experiments  is  shown  at 
Fig.  35. 

The  use  of  the  saliva  in  herbivora  is  to  assist  in  mastica- 
tion and  swallowing,  stimulating  the  nerves  of  taste,  and 
in  ruminants  assisting  in  rumination.  According  to  our 
observations  on  the  horse,  saliva  has  no  chemical  action  on 
the  raw  starch  of  its  food,  and  this  is  not  surprising  when  we 
remember  that  the  starch  grains  are  enclosed  in  an  enve- 
lope of  cellulose,  a  substance  on  which  saliva  has  no  action. 
So  intimately,  however,  is  salivary  secretion  associated 
with  starch  conversion,  that  it  is  not  possible  to  pass  over 
without  further  notice  the  action  produced  on  starch  in 
man,  and  according  to  some  observers  in  horses  and  cattle, 
by  the  presence  of  ptyalin  in  the  saliva. 

The  starch  found  in  plants  exists  in  the  form  of  granules 
possessing  a  shape  peculiar  to  the  species,  these  granules 
are  enveloped  in  a  tough  envelope  of  cellulose ;  before  the 
true  starch,  the  firantdose  contained  in  the  cellulose 
envelope,  can  be  reached  the  cellulose  must  be  traversed. 
For  this  reason  some  animals,  like  man,  cannot  digest  raw 
starch,  but  by  cooking,  the  starch  (granulose)  is  liberated 
and  free  to  be  acted  upon ;  on  the  other  hand,  the 
herbivora  are  capable  of  digesting  raw  starch,  perhaps 
because  they  can  digest  cellulose. 

If  boiled  starch  be  mixed  with  filtered  human  saliva 
and  kept  at  a  temperature  of  95°  F.,  in  a  short  time 
the  characteristic  reaction  of  a  blue  colour  with  iodine 
disappears,  and  a  reddish  colour  is  formed  on  the  addition 
of  this  reagent,  indicating  the  presence  of  a  substance 
known  as  erythrodextrin.  At  this  time  the  fluid  which 
before  was  sugar-free,  now  contains  distinct  evidence  of  its 
presence  ;  by  continuing  the  action  of  the  saliva  it  is  shortly 
found  that  the  red  colour  on  the  addition  of  iodine  has 
disappeared,  and  the  fluid  gives  evidence  of  containing  a 
considerable  proportion  of  sugar.  But  analysis  shows  that 
for  the  amount  of  starch  employed  the  full  amount  of  sugar 
has  not  been  obtained ;  in  other  words,  there  is  a  second 


DIGESTION  143 

substance  present  besides  sugar,  which  is  produced  as  the 
result  of  the  action  of  the  saHva,  and  to  this  the  name 
achroodextrin  has  been  given ;  it  is  formed  from  erythro- 
dextrin.  The  sugar  formed  from  starch  by  the  action  of 
saHva  is  not  grape-sugar  but  maltose  ;  gkicose  (dextrose  or 
grape-sugar)  only  being  found  in  small  quantities  if  at  all. 
This  action  of  the  saliva  on  starch  is  described  as  the 
Amylolytic  action  ;  it  is  due  to  the  presence  of  PtyaUn  which 
plays  the  part  of  a  ferment.  The  process  is  permanently 
destroyed  by  a  high,  inhibited  by  a  low  temperature,  re- 
tarded by  a  slightly  acid  or  alkaline  medium,  and  destroyed 
by  free  hydrochloric  acid.  If  starch  be  boiled  with  a  dilute 
acid,  conversion  into  sugar  occurs.  The  difference  between 
the  action  of  boiling  acid  on  starch  and  of  saliva  is  that 
the  latter  can  only  produce  maltose  whereas  the  acid  pro- 
duces dextrose. 

The  view  we  hold  as  to  the  non-amylolytic  action  of 
saliva  in  herbivora  is  not  supported  by  other  observers ; 
Ellenberger  *  distinctly  states  that  both  the  parotid  and 
submaxillary  secretions  of  the  horse  and  ox  can  convert 
starch  into  sugar,  but  in  the  case  of  the  horse  it  is  only 
the  saliva  first  secreted  by  the  glands  after  a  rest  which 
possesses  this  property ;  as  secretion  proceeds  the  power  is 
nearly  lost.  In  the  pig,  according  to  this  observer,  all  the 
salivary  glands  are  starch  converting  ;  in  the  rabbit  the 
submaxillary  has  no  action  while  the  parotid  is  energetic ; 
in  the  cat,  dog,  horse,  sheep,  and  ox  the  action  is  very  feeble 
or  entirely  absent.  Meade  Smith  f  states  that  the  saliva 
of  the  horse  W'ill  convert  crushed  raw  starch  into  sugar  in 
fifteen  minutes,  and  that  the  process  is  continued  in  the 
stomach ;  he  further  adds  that  the  saliva  of  the  horse  will 
convert  cane  into  grape-sugar.  In  ruminants  he  believes 
starch  conversion  takes  place  both  in  the  mouth  and  rumen. 
Though  we  do  not  accept  these  views,  we  shall  shortly 
endeavour  to  show  how^  starch  is  converted  into  sugar  in 
the  stomach  of  the  horse.     It  is  interesting  in  this  respect 

*  '  Physiologie  der  Haussaugethiere.' 
t  '  Physiology  of  the  Domestic  Animals.' 


144     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

to  note  that  in  man  starch  conversion,  brought  about  by 
the  action  of  ptyalin,  is  also  now  recognised  as  taking  place 
in  the  stomach  from  the  swallowed  saliva,  in  fact,  that  the 
bulk  of  the  conversion  takes  place  there,  and  not  in  the 
mouth. 

Secretion  of  Saliva. — The  mechanism  concerned  in  the 
secretion  of  saliva  deserves  careful  attention,  for  the 
reason  that  it  throws  considerable  light  on  other  secretory 
processes.  The  subject  has  been  worked  out  by  so  many 
competent  observers  that  the  leading  points  are  beyond 
all  doubt ;  the  submaxillary  gland  of  the  dog  has  afforded 
the  desired  information,  and  there  is  reason  to  believe  that 
the  same  process  holds  good  for  the  parotid  and  other 
glands,  both  of  this  animal  and  of  herbivora. 

The  chief  point  in  the  secretion  of  saliva  is  that  it  is 
controlled  by  the  nervous  system,  and  is  not  directly 
dependent  upon  any  mere  increase  in  the  blood  pressure  in 
the  gland.  Afferent  nerves,  viz.,  the  gustatory  division  of 
the  fifth  and  the  glosso-pharyngeal,  convey  from  the  mouth 
to  the  medulla  a  certain  impulse,  which  by  means  of 
efferent  nerves  is  conveyed  to  the  gland  and  secretion 
results.  The  efferent  nerve  of  the  submaxillary  gland  of 
the  dog  is  supplied  by  the  chorda  tympani,  a  small  branch 
given  off  by  the  seventh  cranial  nerve,  which  enters  the 
gland  at  its  hilum  and  suj)plies  the  vessels  with  dilator 
and  the  cells  with  secretory  fibres.  The  second  nerve 
supplying  the  submaxillary  gland  is  a  branch  of  the 
sympathetic,  which  spreads  out  and  invests  with  constrictor 
fibres  the  walls  of  the  artery  supplying  the  part  (Fig.  36). 
Thus  the  chorda  tympani  supplies  the  gland  with  secretory 
fibres  and  the  walls  of  the  vessels  with  dilator  fibres,  while 
the  sympathetic  supplies  the  vessels  with  constrictor  fibres, 
and  only  a  few  secretory  fibres. 

If  the  tongue  or  the  lingual  branch  of  the  fifth  or  glosso- 
pharyngeal nerves  be  stimulated  secretion  of  saliva  results ; 
if  the  sympathetic  nerve  be  divided  and  the  tongue  then 
stimulated  secretion  follows,  but  if  the  chorda  tympani  be 
previously  divided  no  secretion  follows  on  stimulation  of 


DIGESTION 


145 


the  tongue,  lingual,  or  glosso-pliaryngeal  nerves.  If  the 
chorda  be  stimulated  the  vessels  dilate,  the  gland  becomes 
red,  the  blood  flowing  from  the  veins  is  arterial  in  tint,  and 
the  veins  pulsate  ;  in  addition  to  this,  there  is  an  abundant 
secretion  of  watery  saliva  poor  in  solids.  When  the  sym- 
pathetic is  stimulated,  exactly  the  reverse  is  observed — viz., 
the  vessels  constrict,  in  consequence  of  which  the  gland 

y.syvv.  n^ 


rTi.f-' 


Fig.  36. — Diagrammatic  Representation  of  the  Submaxillary 
Gland  of  the  Dog  with  its  Nerves  and  Bloodvessels 
(Foster). 

(The  dissection  has  been  made  with  the  animal  on  its  back,  and  is 
very  diagrammatic.) 

The  submaxillary  gland  {sm.  (/Id.)  occupies  the  centre  of  the  figure  ;  the 
bloodvessels  supplying  it,  derived  from  the  carotid  artery  a.car., 
are  seen  on  the  left,  whilst  the  duct  from  the  gland  s.md.,  in  which 
a  canula  is  inserted,  is  on  the  right  of  the  figure. 

The  chorda  tyrapani  nerve  cJi.t".,  running  in  company  with  the  lingual 
branch  of  the  fifth  n.l'.,  is  seen  to  the  right  and  below ;  after 
running  together  the  two  nerves  separate,  the  chorda  tympani 
ch.t.  running  along  the  submaxillary  duct  to  the  gland.  Close  to 
where  the  two  nerves  separate  is  the  submaxillary  ganglion  sm.gl. 

The  sympathetic  nerve  supply  is  shown  in  the  figure  to  the  left  and 
above,  the  fibres  being  derived  from  the  superior  cervical  ganglion 
gl.cer.s.  and  coursing  along  the  bloodvessels  to  enter  the  gland. 

The  bloodvessels  leading  from  the  gland  fall  into  the  jugular  vein  v.j. 

The  arrows  indicate  the  direction  of  the  nervous  impulses  during  the 
reflex  act,  ascending  to  the  brain  by  the  lingual  and  descending  by 
the  chorda. 

10 


IJG     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

becomes  pale,  only  a  small  quantity  of  extremely  viscid  saliva 
flows  which  is  rich  in  solids,  the  blood  in  the  veins  becomes 
very  dark  in  colour,  and  the  blood-stream  slows  to  such  an 
extent  that  if  the  veins  leading  from  the  gland  be  cut,  the 
flow  from  them  is  less  than  from  a  gland  at  rest.  That 
the  increased  flow  of  blood  to  the  gland  produced  by 
stimulating  the  chorda  is  not  the  essential  cause  of  the 
secretion,  is  proved  by  the  fact  that  the  pressure  of  the 
saliva  in  the  duct  of  the  gland  is  higher  than  the  blood 
pressure  within  the  vessels.  Further,  if  before  stimulating 
the  chorda  some  atropin  be  injected,  stimulation  of  the 
nerve  still  produces  to  the  full  all  the  vascular  changes, 
but  not  a  trace  of  saliva  is  secreted.  Hence,  secretion  is 
not  due  merely  to  increased  blood  pressure.  This  atropin 
experiment  proves  the  existence  in  the  chorda  of  two  sets 
of  nerves,  viz.,  secretory  and  vaso-dilator ;  owing  to  the 
action  of  atropin  the  secretory  nerves  are  paralysed,  whilst 
the  vaso-dilators  are  not.  And  in  the  sympathetic  two  sets 
of  nerves  can  similarly  be  demonstrated,  secretory  and 
vaso-constrictor,  though  it  is  most  likely  that  in  the 
majority  of  animals  the  secretory  fibres  in  the  sympathetic 
are  few  in  number.  Pilocarpin  is  antagonistic  to  atropin 
and  produces  a  profuse  flow  of  saliva. 

A  peculiar  phenomenon  is  observed  in  connection  with 
salivary  secretion  after  division  of  the  chorda.  Though 
the  gland  is  cut  off  from  its  secretory  nerve,  yet  one  or  two 
days  after  section  a  secretion  appears,  and  may  continue 
for  some  weeks  until  the  gland  undergoes  atrophy.  Thig 
is  known  as  '  paralytic  secretion.' 

Heidenhain's  view  of  the  action  of  secretory  nerves  is 
that  a  gland  is  supplied  with  a  trophic  or  nutritive  nerve 
which  excites  the  formation  of  the  organic  constituents  of 
the  secretion,  and  a  secretory  nerve  which  controls  the 
secretion  of  water  and  inorganic  salts.  The  cranial  nerves 
are  chiefly  secretory,  whilst  the  sympathetic  are  trophic, 
hence  stimulation  of  the  chorda  gives  a  watery  saliva  poor 
in  solids,  whilst  stimulation  of  the  sympathetic  gives  a 
scanty  saliva  rich  in  solids. 


DIGESTION  147 

The  method  by  which  secretion  in  the  parotid  gland  is 
carried  out  differs  in  no  essential  respect  from  that  of  the 
submaxillary.  The  nerves  supplying  the  parotid  are  the 
glosso-pharyngeal  (the  action  of  which  corresponds  to  the 
chorda  of  the  submaxillary)  and  the  sympathetic.  In  the 
glosso-pharyngeal  are  dilator  fibres,  and  in  the  sympathetic 
constrictor  fibres  for  the  bloodvessels,  while  both  trunks 
contain  secretory  nerves. 

It  will  be  observed  that  no  reference  has  been  made  to 
the  nerve  ganglia  in  connection  with  salivary  secretion. 
Ganglia  are  a  collection  of  cells  in  the  course  of  a  nerve. 


l^'-^  '\ 


Fig.  37. — Changes  in  the  Cells  of  the  Living  Parotid  (Serous 
Gland)  during  Secretion. 

A,  At  rest;  B,  in  the  first  stage  of   secretion;    C,  after   prolonged 
secretion  (Foster,  after  Langley). 

If  these  cells  be  paralysed  by  nicotine,  as  was  first  shown 
by  Langley,  stimulation  of  the  nerve  does  not  produce 
a  secretion. 

The  changes  occurring  in  the  cells  of  the  salivary  glands 
during  secretion  depend  upon  the  type  of  gland.  We  will 
therefore  describe  separately,  from  Langley's  observations, 
the  changes  in  the  cells  of  a  serous  gland  such  as  the 
parotid,  and  in  those  of  a  mucous  gland  of  which  the  sub- 
maxillary is  a  type.  We  select  Langley's  observations, 
since  he  examined  the  living  gland  and  not  one  simply 
hardened  and  stained.  During  the  stage  of  rest  in  a  living 
serous  gland,  the  cells  are  found  to  be  filled  with  a  quantity 
of  granular  material,  and  the  outline  of  each  individual  cell 
is  indistinct ;  the  lumen  of  the  gland  is  also  occluded,  and 
no  nucleus  can  be  observed  in  the  cells ;  in  other  words,  the 
^land  is  charged  with  its  secretory  products  (Fig.  37,  A). 

10— g 


148    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

During  activity  the  cells  get  rid  of  their  granular  material, 
which  gradually  passes  towards  the  centre  of  the  acinus  or 
lumen,  leaving  each  cell  with  a  clear  outer  edge,  whilst  that 
edge  next  the  lumen  is  still  granular  (Fig.  37,  B).  In  an 
exhausted  condition  the  cells  are  smaller  and  remarkably 
clear,  only  a  few  granules  being  left  in  them  on  the  inner 
edge,  whilst  the  lumen  is  now  distinct  and  large,  and 
the  nuclei  are  clearly  seen  occupying  a  central  position 
(Fig.  37,  C). 

If  a  mucous  gland  at  rest  be  examined  under  like  con- 
ditions, the  cells  are  found  filled  with  granules  much  larger 


,i^. 


Fig.    38. — Cells   from    Mucous    Gland   (Submaxillary   Gland  of 

THE  Dog).     (Foster.) 

a,  From  loaded  gland  ;  b,  from  discharged  gland ;  a',  b',  treated  with 

dilute  acetic  acid ;  a',  from  loaded ;  b',  from  discharged  gland. 

than  those  of  a  serous  gland,  and  a  nucleus  is  seen  occu- 
pying one  edge  of  the  cell  (Fig.  38,  a).  During  activity  the 
granules  are  passed  into  the  lumen  of  the  gland,  but  they 
do  not  leave  behind  them  in  the  cells  the  same  clear  space 
seen  in  the  serous  cell  (Fig.  38,  b).  If  the  cells,  while  in  an 
active  condition,  be  acted  upon  by  water  or  dilute  acetic 
acid,  the  granules  swell  up  and  become  transparent  owing 
to  the  mucin  they  contain,  and  a  delicate  network  is  seen  to 
pervade  the  cell  (Fig.  38,  a').  A  similar  appearance  is  pro- 
duced in  the  exhausted  cell  (Fig.  38,  b'),  excepting  that  less 
transparent  mucin  is  seen  and  more  granular  substance, 
while  the  nucleus  of  the  exhausted  irrigated  gland  is  seen 
passing  towards  the  centre  of  the  cell  instead  of  remaining 


DIGESTION  149 

close  to  the  outer  wall.  Though  we  have  spoken  of  these 
granules  as  mucin,  in  the  gland  they  are  not  really  mucin, 
but  the  mother  substance  of  it,  viz.,  miicigen,  which  during 
the  act  of  secretion  is  converted  into  mucin.  The  same 
holds  good  for  the  serous  type  ;  the  granules  in  the  resting 
gland  are  the  precursors  of  the  ferment  or  the  zymogen  of 
the  secretion,  from  which  the  secretion  is  actually  formed 
at  the  moment  it  is  poured  out. 

The  outcome  of  the  changes  above  described  proves  that 
the  organic  elements  found  in  the  salivary  secretion  are 
manufactured  by  the  cells  in  the  glands  ;  the  inorganic 
constituents  are  either  the  result  of  filtration  or  secretion. 
Experiments  made  by  Langley  and  Fletcher  go  to  prove 
that  even  water  and  salts  are  the  result  of  an  act  of  cell 
secretion,  and  not  of  mere  transudation. 


Section  2. 

Stomach  Digestion. 

Important  digestive  changes  in  the  food  of  the  lower 
animals  take  place  in  the  stomach.  It  is  not  a  matter  for 
surprise  to  iind  that  the  size  and  shape  of  this  organ  varies 
with  the  species  of  animal ;  we  should  expect  to  meet  with 
a  simple  stomach  in  the  dog,  and  complex  arrangement  in 
vegetable  feeders.  It  seems  remarkable  that  any  animal 
should  possess  a  laboratory  capable  of  converting  grass, 
hay,  and  grain  into  muscle  and  fat ;  and  it  is  evident  that 
the  conversion  of  vegetable  into  animal  tissues  must  be  a 
more  complex  process  than  the  conversion  of  animal 
tissues  into  the  living  structure  of  an  animal  body.  But 
it  is  curious  to  observe  that  a  complex  stomach  for  a 
vegetable  feeder  is  by  no  means  a  necessity ;  the  stomach 
of  the  ruminant  and  the  simple  stomach  of  the  horse 
could  not  be  in  greater  contrast,  whilst  the  resulting 
laboratory  processes  are  practically  identical.  So  far  as 
vegetable  food  is  concerned,  it  does  not  matter  whether 


150    A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

the  solution  and  absorption  of  its  readily  soluble  matters 
comes  before  maceration,  or  whether  maceration  pre- 
cedes the  extraction  of  the  readily  soluble  substances. 
If  maceration  comes  first,  as  in  ruminants,  bulky  gastric 
compartments  are  provided  for  the  purpose,  and  the  sub 
sequent  intestinal  canal  is  small.  If  the  simple  stomach 
comes  first,  bulky  intestines  for  the  purpose  of  maceration 
follow ;  in  both  cases  ample  provision  is  made  for  the 
maceration  necessary  for  the  solution  of  the  cell  wall  and 
fibrous  portion  of  plants.  The  dog  with  its  simple  stomach 
and  simple  intestines  offers  no  difficulty  to  our  under- 
standing. He  lives  on  flesh  and  converts  it  into  flesh  ; 
it  is  not  very  clear  why  he  has  both  a  stomach  and 
intestines,  for  the  whole  process  of  digestion  is  simple, 
and  could  be  readily  carried  out  single-handed  by  the 
intestines.  In  fact,  the  stomach  of  the  dog  has  been 
removed  experimentally  and  the  animal  remained  in  health. 

For  simplicity  in  construction  the  stomach  of  the  dog 
occupies  one  end  of  the  scale,  for  complexity  the  gastric 
reservoirs  of  the  ox  occupy  the  other,  whilst  between  the 
two  comes  the  stomach  of  the  omnivorous  pig,  partaking  of 
some  of  the  characters  of  the  carnivora  and  ruminant  and 
belonging  to  neither. 

Stomach  Digestion  in  the  Horse. — The  subject  of  stomach 
digestion  in  the  horse  has  been  worked  out  by  means  of 
feeding  experiments,  as  it  has  been  found  impossible  to 
establish  a  gastric  fistula  in  this  animal  owing  to  the 
distance  the  stomach  lies  from  the  abdominal  wall ;  pure 
gastric  juice  has,  therefore,  never  been  obtained  from  the 
horse. 

The  first  peculiarity  to  be  noticed  in  soliped  digestion  is 
that  the  stomach  is  rarely  empty  ;  it  is  only  when  horses 
have  purposely  been  deprived  of  food  for  not  less  than 
twenty-four  hours  that  an  empty  stomach  can  be  obtained. 
On  the  other  hand,  feeding  experiments  show  that  very 
shortly  after  food  arrives  in  the  stomach  it  commences  to 
pass  out,  and  the  difficulty  thus  presented  to  the  observer 
in   reconciling   these  opposed  facts  is  at  first  sight   con- 


DIGESTION  151 

siderable.  It  is  perfectly  true  that  food  does  pass  out  early, 
it  is  equally  true  that  it  is  long  retained,  these  opposite 
conditions  being  the  result  of  the  periods  of  digestion.  When 
food  enters  an  empty  stomach  it  passes  towards  the  pylorus, 
where  it  meets  with  a  fluid  of  an  alkaline  or  neutral  re- 
action which  has  come  from  the  mouth.  As  more  food  is 
consumed  an  acid  fluid  is  secreted  in  the  stomach,  and 
material  commences  to  pass  out  at  the  pylorus  into  the 
bowel,  the  amount  passing  out  not  equalling  at  present  the 
amount  passing  in.  Thus  the  stomach  becomes  gradually 
distended,  and  when  two-thirds  full,  which  is  the  condition 
in  which  the  most  active  digestion  occurs,  the  amount 
passing  out  will,  if  more  food  be  taken,  equal  the  amount 
being  swallowed,  so  that  we  have  a  stream  of  partly 
peptonized  chyme  streaming  out  of  the  right  extremity, 
while  a  corresponding  bulk  of  ingesta  is  entering  the  inert 
left  sac.  In  fact,  the  stomach  may  during  feeding  allow 
two  or  three  times  the  bulk  of  food  to  pass  out  which 
remains  in  it  when  the  meal  is  finished.  Let  us  now 
suppose  that  the  *  feed '  is  finished.  At  once  the  passage 
of  chyme  into  the  duodenum  ceases,  or  becomes  so  slowed 
down  that  only  small  quantities  of  food  pass  out,  and 
so  gradually  does  this  occur  that  it  will  be  many  hours 
before  the  stomach  is  really  empty,  though  had  the  process 
continued  as  it  commenced,  it  would  not  have  contained 
anything  at  the  end  of  an  hour.  This  condition  of  stomach 
digestion  in  the  horse  may  be  variously  modified,  depending 
on  the  nature  of  the  food,  the  quantity  given,  the  form  in 
which  it  is  given,  the  order  in  which  one  food  follows 
another,  and  w^hether  water  be  given  before  or  after 
feeding.  All  these  are  points  requiring  our  attention, 
but  before  giving  it  we  must  briefly  look  at  the  stomach 
itself. 

The  mean  capacity  of  a  horse's  stomach  is,  according  to 
Colin,  from  25  to  30  pints,  or  from  '5  to  '63  of  a  cubic 
foot ;  these  figures  were  obtained  from  a  very  large  number 
of  observations,  and  give  the  extreme  size  of  the  organ 
when  distended  ;  the  viscus  is  under  the  best  conditions  for 


152     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


digestion  when  it  contains  about  17i  pints,  or  is  distended 
to  two-thirds  of  its  capacity.  The  mucous  membrane  of 
the  stomach  of  the  horse  is  peculiar ;  one  portion  of  it, 
practically  half,  is  a  continuation  of  the  membrane  of  the 
oesophagus,  this  ends  abruptly  and  is  succeeded  by  the 
villous  coat  which  extends  to  the  pylorus.  It  is  in  this 
latter  coat  that  a  true  digestive  juice  is  secreted,  though 
not  from  the  entire  surface,  for  on  examining  the  villous 
membrane  it  is  found  to  differ  greatly  in  appearance,  the 


PYL. 


CARD. 


L.S 


CUT. 


BOU 


FUN  D  . 

Fig.  39. — Longitudinal  Section  of  the  Stomach  of  the  Horse. 

CARD.,  Cardia  ;  pyl.,  pylorus  ;  l.s.,  left  sac  ;  r.s.,  right  sac  ;  cut.,  cuti- 
cular  coat  ;  vil  ,  villous  coat  ;  sou.,  boundary  line  between  the 
cuticular  and  villous  portions  ;  fund.,  fundus  of  the  stomach.  The 
dotted  surface  indicates  the  area  for  the  secretion  of  gastric  juice. 

fundus  being  channelled,  furrowed,  and  velvety,  whilst  the 
pyloric  portion  is  smooth.  It  is  in  the  fundus  only  where 
true  gastric  juice,  viz.,  pepsin  and  acid,  is  secreted;  in  the 
smooth  pyloric  mucous  membrane  only  pepsin  is  formed. 
The  area  of  the  fundus-secreting  surface  is  about  one  square 
foot.  Fig.  39  shows  the  relative  position  of  the  various 
parts  of  the  mucous  membrane  of  the  stomach  of  the  horse ; 
the  drawing  accurately  indicates  the  shape  of  the  stomach, 
the  position  of  the  inlet  and  outlet,  and  the  direction  and 
position  of  the  various  areas.  A  very  remarkable  amount 
of  mucin  is  secreted  by  the  villous  sac  of  the  stomach, 


DIGESTION  153 

.and  forms  over  the  inner  surface  of  the  viscus  a  thick 
gelatinous  firmly  adherent  coating  like  white  of  egg,  which 
cannot  be  washed  away  even  by  a  powerful  jet  of  water. 

The  pyloric  orifice  of  the  stomach  is  usually  large  and 
open,  and  there  is  a  distinct  pyloric  ring ;  behind  this  the 
duodenum  is  dilated,  and  the  gut  comports  itself  in  such 
a  singular  manner  (which  has  a  very  important  bearing 
on  the  pathology  of  the  organ)  that  mention  must  be  made 
of  it  here.  From  the  pylorus  the  duodenum  curves  down 
and  then  up  again,  forming  a  letter  U ;  so  much  does  this 


■1^ 
Fig.  40. — Longitudinal   Section  of  the   Stomach  of  the  Horse, 

SHOWING    THE    SyPHON    TrAP   OF   THE    DuODENUM. 

ce,  ffisophagus  ;  2^U->  pylorus  ;  d,  left  sac  ;  v,  fundus  ;  duo.,  duodenum. 

remind  one  of  a  well-known  form  of  trap  used  in  drainage, 
that  we  have  described  it  as  the  syphon  trap  of  the  duode- 
num (Fig.  40).  The  use  of  this  trap  appears  to  be  to 
regulate  the  passage  of  material  from  the  stomach  into  the 
intestines.  Our  observations  have  shown  that  its  i^resence 
in  all  probability  influences  rupture  of  the  stomach,  for  the 
more  distended  the  large  bowels  become,  the  greater  the 
pressure  exercised  on  the  duodenum,  and  in  cases  of  severe 
tympany  the  passage  from  the  stomach  to  the  intestines  is 
completely  cut  ofl*.  Should  fermentation  still  continue 
in  the  stomach,  the  contents  can  neither  escape  into  the 
oesophagus,  nor  into  the  bowel,  and  the  coats  of  the  viscus 


154     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

may  be  completely  ruptured  under  the  intense  strain.  It 
was  mentioned  on  p.  138  that  the  cesophagus  of  the  horse 
near  its  termination  changes  from  red  to  pale  muscle  and 
for  several  inches  increases  enormously  in  thickness.  It 
is  this  thickened  contracted  end  of  the  oesophagus  which 
completely  seals  the  stomach  anteriorly ;  nothing  can  be 
forced  out  by  this  passage,  not  even  after  death  or  under 
great  pressure. 

The  physiological  points  of  interest  in  the  structure  of 
the  horse's  stomach  are :  1,  that  it  is  small ;  2,  that  it  is 
not  in  contact  with  the  abdominal  wall,  but  rests  on  the 
colon ;  3,  that  the  outlet  and  inlet  are  situated  close 
together ;  4,  that  the  cardia  is  tightly  contracted ;  5,  that 
only  a  portion  of  its  surface  is  capable  of  secreting  a 
digestive  fluid ;  6,  that  there  are  remarkable  differences  in 
the  character  and  nature  of  the  various  regions  of  its 
mucous  membrane. 

We  can  now  consider  the  stomach  digestion  of  the  two 
chief  foods  used  for  horses,  viz.,  hay  and  oats. 

Digestion  of  Hay. — Hay,  as  we  have  shown,  mixes  in  the 
mouth  with  four  times  its  bulk  of  saliva,  and  after  a  very 
perfect  grinding  passes  into  the  stomach.  If  the  stomach 
be  empty  it  is  of  no  size  and  the  material  lies  in  the 
pyloric  region  ;  as  the  viscus  gradually  fills,  the  gastric 
juice  begins  to  act,  and  chyme  commences  to  pass  into  the 
intestines  probably  in  a  very  imperfectly  elaborated  form. 
Assuming  the  animal  to  have  finished  eating  the  hay,  we 
now  find  the  output  into  the  intestine  becomes  small  and 
slow.  The  gastric  juice  has  an  opportunity  of  acting  more 
thoroughly  upon  the  ingesta,  which  turn  yellow  on  that 
surface  which  is  in  contact  with  the  villous  wall,  the  com- 
pression of  the  stomach  on  the  contents  causing  them  to 
become  distinctly  moulded  into  a  mass  the  shape  of  the 
viscus.  Owing  to  gravity  there  is  more  fluid  towards  the 
pylorus  than  elsewhere,  and  for  the  same  reason  the  greater 
curvature  in  all  probability  is  fuller  than  the  lesser.  The 
material  in  the  stomach  is  perfectly  comminuted,  resembles 
firm  green  and  yellow  fteces,  and  the  smell  is  peculiar,  like 


DIGESTION  155 

sour  tobacco.  The  yellowness  is  due  to  the  gastric  juice, 
and  is  consequently  more  marked  towards  the  pylorus  ;  the 
portion  coloured  green  is  the  part  as  yet  unacted  upon  by  the 
juice.  The  entire  surface  of  the  stomach  and  its  contents  are 
now  acid,  excepting  at  the  cardia,  where  it  may  occasionally 
be  alkaline  from  swallowed  saliva  ;  the  acidity  is  greater  at 
the  fundus  than  at  the  cardia.  This  general  acidity  shows 
that  a  diffusion  of  the  gastric  juice  must  have  been  going 
on.  There  is  no  evidence  of  any  churning  motion,  the 
cake-like  condition  into  which  the  hay  is  compressed,  in 
spite  of  its  four  equivalents  of  saliva,  is  due  to  the  com- 
pression of  the  material  by  the  stomach  walls. 

The  duration  of  stomach  digestion  of  hay  is  variable,  but 
we  quote  one  or  two  of  Colin's  experiments.  A  horse 
received  5i  lbs.  of  hay  which  he  took  two  hours  to  eat ; 
at  the  end  of  that  time  he  was  destroyed,  and  the  stomach 
contained  2'2  lbs. ;  thus  in  two  hours  he  had  digested 
3'3  lbs.  Another  horse  received  5^  lbs.  hay,  and  was  de- 
stroyed three  hours  from  the  time  of  commencing  to  feed ; 
in  the  stomach  were  found  1*54  lbs.,  so  that  in  three  hours 
this  horse  had  digested  3*96  lbs.  In  the  third  hour  (during 
which  time  he  was  not  feeding),  judging  from  the  first 
experiment,  he  had  digested  only  "66  lb.,  whereas  the 
previous  rate  of  digestion  for  the  first  two  hours  was  at  the 
rate  of  1'65  lbs.  per  hour. 

To  return  to  onr  previous  statement,  when  the  animal  is 
no  longer  feeding  the  rate  of  digestion  at  once  becomes 
reduced,  and  it  is  probable  that  several  hours  must  elapse, 
assuming  no  further  food  be  given,  before  the  stomach 
completely  empties  itself.  This  period  may  be  fifteen, 
eighteen,  twenty-four  or  even  thirty-six  hours.  We  starved 
a  horse  for  twenty-four  hours,  and  at  6  a.m.  gave  him 
6  lbs.  of  dried  grass  ;  he  was  destroyed  at  3  p.m.,  and  the 
stomach  still  contained  2A  lbs.  ;  in  nine  hours,  therefore, 
only  3i  lbs.  had  been  digested.  In  another  observation 
carried  out  under  similar  conditions,  only  1  lb.  had  been 
digested  in  four  hours  and  three-quarters.  Of  4  lbs.  hay 
given  only   1  lb.   11  ozs.  were  digested  in  six  hours  ;  of 


156     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

3|  lbs.  hay,  2^  lbs.  were  digested  in  five  and  a  half  hours ; 
while  in  another  observation,  of  4  lbs.  hay,  2  lbs.  12  ozs. 
were  digested  in  five  hours. 

Colin's  elaborate  researches  furnish  us  with  very  complete 
data  on  the  question  of  hay  digestion  in  the  horse.  He 
fed  fourteen  horses  on  hay,  and  destroyed  two  of  them  at 
regular  intervals  ;  each  animal  received  5*5  lbs.  of  hay,  and 
digestion  was  counted  from  the  time  they  were  fed.  Here 
are  the  results : 


Amount  of  Hay  given  5'5  Lbs. 


lbs. 

Ihs. 

d  digested  3-37  ; 

the  second,  3-08 

3-83 

4-24 

4-04 

3-56 

4-32 

5-03 

4-10 

4-55 

4-01 

4-35 

4-87 

4-44 

From  this  it  is  seen  that  the  rate  of  digestion  during  the 
first  two  hours  is  rapid  and  then  falls  off,  so  that  even  at 
the  end  of  eight  hours  there  is  still  something  left  in  the 
stomach.  The  second  horse  in  the  five  hours'  observation 
had  very  nearly  digested  the  whole  of  the  ration,  but  this 
is  an  exception.  There  is  no  doubt  that  it  is  extremely 
difficult  to  get  the  stomach  to  empty  itself.  We  fed  a  horse 
on  dried  grass  and  destroyed  it  eighteen  hours  later ;  there 
was  still  a  small  quantity  of  food  in  the  stomach.  In 
another  case  the  stomach,  after  fifteen  hours,  was  found 
empty.  In  a  third  case  a  horse  was  given  grass  twice  at 
intervals  of  twenty-four  hours  ;  he  was  destroyed  eighteen 
hours  after  eating  his  last  feed,  and  a  handful  of  grass  was 
still  found  in  the  stomach. 

Digestion  of  Oats. — We  have  now  to  consider  the  digestion 
of  oats,  and  here  again  we  still  observe  the  same  fact  noted 
under  that  of  hay,  viz.,  that  the  stomach  commences  to 
pass  its  contents  into  the  intestine  during  feeding,  and  that 
this  slackens  considerably  when  no  more  food  is  entering 


DIGESTION  157 

the  viscus.     Colin  fed  six  horses  on  5*5  lbs.  of  oats  each, 
and  destroyed  them  at  certain  intervals. 

lbs.  lbs. 

After  2  hours,  one  horse  had  digested  27  ;  a  second,  2*5 

„     4         „  „  „  3-1         „  3-4 

„     6         „  „  „  3-5         „  3-0 

We  have  observed  in  a  horse  which  had  received  2  lbs. 
of  oats,  and  was  destroyed  twenty  hours  later,  that  the 
stomach  had  not  completely  emptied  itself.  In  another 
experiment  four  hours  after  feeding  on  one  pound  of  oats, 
6  ozs.  were  recovered  from  the  stomach. 


horse  received 

And 

was  destroy 

ed  in 

Amount  digested 

lbs.  oats. 

]ioi(rs. 

lbs.  ozs. 

4 

- 

- 

4 

- 

2       3 

8 

- 

- 

4^ 

- 

1     111 

4 

- 

- 

4 

- 

2       4 

3 

- 

- 

3| 

- 

2       2.^ 

3 

- 

- 

4 

- 

— 

4 

. 

- 

4 

- 

1  m 

3 

- 

- 

6^ 

- 

2       6h 

4 

- 

4 

3       0 

4 

. 

. 

4 

- 

0     12 

The  last  horse  is  included  to  illustrate  a  point  of  some 
importance  in  the  feeding  of  animals.  For  eighteen  months 
this  horse  had  never  tasted  corn,  having  been  fed  on  a 
patent  food  ;  a  sudden  change  in  diet  is  the  explanation 
why  he  only  digested  12  ozs.  of  oats  in  four  hours.  It 
will  be  observed  that  the  fifth  horse  in  this  series  digested 
nothing,  even  at  the  end  of  four  hours  ;  we  can  only  account 
for  this  by  the  fact  that  the  animal  was  in  a  strange  place 
where  the  feeding  experiment  was  carried  out,  and  was  of  a 
very  nervous  disposition. 

Arrangement  of  Food  in  the  Stomach. — An  interesting 
practical  and  physiological  study  is  the  effect  of  feeding 
horses  on  different  foods  in  succession.  When  hay  is  given 
first  and  oats  afterwards,  the  hay  is  found  close  to  the 
greater  curvature  and  pylorus,  and  the  oats  in  the  lesser 
curvature  and  cardia  ;  no  mixing  has  occurred,  both  foods 


158     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

are   perfectly  distinct,  and   a   sharp  line   of   demarcation 
exists  between  them  (Fig.  41,  L).     During  digestion  mixing 
occurs  at  the  pylorus  but  nowhere  else ;  no  matter  what 
compression  the  contents  have  undergone  as  the  result  of 
gastric  contractions,  the  foods  always  remain  distinct.     The 
presence  of  the  oats,  however,  causes  the  hay  to  pass  out 
more  rapidly  than  it  would  have  done  had  it  been  given 
alone.     Colin  observed  that  half  the  hay,  but  only  one- 
fourth  or  one-sixth  of  the  oats,  would,  under  these  con- 
ditions, pass  into  the  intestine  in  two  hours.     Ellenberger 
has  shown  that  when  hay  and  oats  are  given  in  this  order, 
a  portion  of  the  oats  may  pass  out  into  the  bowel  by  the 
lesser  curvature  without  entering  either   the   left  sac   or 
fundus  of  the  stomach  (see  Fig.  41,  L).     When  oats  are 
given  first,  followed  by  hay  (Fig.  41,  XL),  the  oats  com- 
mence to  pass  out  before  the   hay,  but  the  presence  of 
the  hay  causes   the  oats  to  pass   more  quickly  into  the 
intestines  than   they   otherwise   would   have   done.     If  a 
horse   be  fed  on   three  or  four  foods   in   succession   they 
arrange  themselves  in  the  stomach  in  the  order  in  which 
they  arrived,  viz.,  they  do  not  mix.     The  first  enters  the 
greater  curvature,  the  last  the  lesser  curvature,  and  it  is 
only  at  the  pylorus  that  any  mixing  occurs  under  ordinary 
conditions  (Fig.  41,  III.).    This  regular  arrangement  of  the 
different  foods  in  layers  is  only  disturbed  when  a  horse  is 
watered  after  feeding ;  under  these  circumstances  the  con- 
tents are  mixed  together  and  digestion  thereby  impeded. 
Apart  from  this,  the  influx  of  a  considerable  quantity  of 
fluid   into   a   stomach   already  containing  as  much  as  it 
should  hold,  means  that  material  is  washed  out  of  it  into 
the  small  and  large  intestines,  and  this  may  set  up  irrita- 
tion and  colic.     By  watering  a  horse  after  feeding  more 
than   half   the   food   may  at  once  be  washed  out  of   the 
stomach.    The  water  which  a  horse  drinks  does  not  remain 
in  the  stomach,  but  passes  immediately  into  the  small  in- 
testines, and  in  the  course  of  a  few  minutes  finds  its  way 
into  the  caecum  ;  hence  the  golden  rule  of  experience  that 
horses  should  be  watered   first   and  fed  afterwards.     We 


DIGESTION 


159 


may  summarise  these  facts  by  saying  that  in  a  succession 
of  foods  the  first  consumed  is  the  first  to  pass  out.  That 
does  not  mean  to  say  that  the  whole  of  it  passes  out 
before  any  portion  of  the  succeeding  food  enters  the  bowel, 


III. 


Fig.  41. — Longitudinal  Section  of  the  Horse's  Stomach,  showing 
THE  Arrangement  of  the  Food  according  to  the  Order  in 
WHICH  it  was  received  (Ellenberger). 

In  each  case  ce  is  the  oesophagus ;  inj,  pylorus  ;  d,  the  left  sac ;  v,  the 
fundus.  I.  Hay  first,  followed  by  oats  :  b,  the  hay ;  a,  the  oats ; 
the  latter  are  passing  along  the  lesser  curvature  and  escaping  with 
the  hay  at  the  pylorus.  II.  Oats  first,  followed  by  hay  :  a,  the 
oats  ;  h,  the  hay.  III.  The  order  of  three  successive  feeds  ;  c,  the 
first  feed ;  h,  the  second  ;  a,  the  third. 


160    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

for  we  have  shown  that  after  a  time,  at  the  pylorus,  they 
mix  and  pass  out  together  ;  but  the  actual  influence  of 
giving  a  food  first  is  to  cause  it  to  pass  out  first.  The 
practical  application  of  this  fact,  according  to  Ellenberger, 
is  that  when  foods  are  given  in  succession,  the  least 
albuminous  should  be  given  first.  This  appears  to  dis- 
tinctly reverse  the  English  practice  of  giving  oats  first  and 
hay  afterwards,  but  perhaps  only  apparently  so,  for  experi- 
ment shows  that  the  longer  digestion  is  prolonged,  the 
more  oats  and  the  less  hay  pass  out,  so  that  some  hay 
(under  ordinary  circumstances  a  moderate  quantity)  is 
always  left  in  the  stomach  until  the  commencement  of  the 
next  meal.  The  presence  of  this  hay  from  the  previous  feed 
may  prevent  the  corn  of  the  succeeding  meal  from  passing 
out  too  early.  According  to  Ellenberger,  in  order  that 
horses  may  obtain  the  fullest  possible  nutriment  from  their 
oats,  hay  should  be  given  first  and  then  water ;  this  carries 
some  of  the  hay  into  the  bowel  and  after  a  time  the  oats  are 
to  be  given.  The  remaining  hay  now  passes  into  the  bowel 
and  the  oats  remain  in  the  stomach.  This  does  not  accord 
with  English  views  of  watering  and  feeding  horses,  which 
have,  however,  stood  the  test  of  prolonged  practical 
experience. 

The  appearance  of  the  food  after  it  has  been  in  the  stomach 
depends  upon  the  period  of  digestion.  We  have  previously 
drawn  attention  to  the  fact  that  an  hour  or  two  after  hay 
has  been  taken  the  material  is  found  in  a  finely  chopped 
condition,  firm,  one  may  almost  say  dry,  in  places,  though 
towards  the  pylorus  it  is  liquid.  This  hay  contains  between 
four  and  five  parts  of  saliva  ;  it  is  yellow  in  colour  where 
the  gastric  juice  has  attacked  it,  but  of  rather  a  greenish 
tint  elsewhere,  and  it  has  a  peculiar  odour.  Several  hours 
after  feeding,  the  stomach  is  found  to  contain  a  variable 
quantity  of  watery  fluid  discoloured  by  the  hay  which  is 
left  behind,  part  of  which  may  be  found  floating  on  the 
fluid.  At  other  times,  when  the  stomach  is  empty,  the 
fluid  is  viscid,  contains  numerous  gas  bubbles,  and  is  of  an 
amber  or  yellow  tint ;    this   particular  fluid  is  no  doubt 


DIGESTION  161 

saliva  and  mucin,  with  possibly  a  little  bile,  the  result  of  a 
reflux  from  the  bowel.  When  oats  alone  have  been  given 
the  contents  of  the  stomach  are  found  liquid,  the  fluid 
being  creamy  in  consistency  and  colour ;  the  oats  are 
swollen,  soft,  and  their  interior  exposed ;  towards  the  end 
of  digestion  the  creamy  fluid  is  replaced  by  the  frothy 
yellow  one.  With  both  hay  and  oats,  and  also  other  foods, 
there  is  a  peculiar  sour-milk-like  smell  from  the  contents 
of  the  stomach,  more  marked  with  bran  and  oats  than  with 
hay,  the  latter,  as  previously  mentioned,  smelling  like  sour 
tobacco. 

The  reaction  of  the  contents  of  the  stomach  is  strongly 
acid  ;  this  acid  reaction  may  be  obtained  on  the  cuticular 
as  well  as  the  villous  portion  of  the  lining,  and  is  very 
persistent ;  the  cuticular  membrane  even  after  prolonged 
washing  gives  an  acid  reaction.  The  acidity  is  derived 
entirely  from  the  juice  secreted  by  the  villous  membrane  of 
the  fundus.  Our  observations  on  this  subject  do  not  agree 
with  those  of  Ellenberger,  who  says  that  during  the  first 
hour  of  digestion  the  contents  of  the  stomach  may  be 
alkaline ;  acidity,  he  states,  then  commences  in  the  fundus 
and  extends  to  the  cardia,  though  for  some  time  the  pro- 
portion of  fundus  acidity  is  three  or  four  times  greater 
than  that  of  the  cardia  ;  in  the  course  of  five  or  six  hours 
the  proportion  of  acid  throughout  the  stomach  is  equal. 
When  the  stomach  is  empty,  as  after  a  few  days'  starva- 
tion, its  reaction  is  neutral  or  alkaline.  We  have  observed 
extreme  alkalinity  towards  the  pylorus  under  these  con- 
ditions, due  no  doubt  to  the  regurgitation  of  bile  and 
pancreatic  fluid. 

The  Stomach  Acids. — It  is  not  necessary  here  to  enter  into 
any  detail  as  to  the  nature  of  the  gastric  acids  ;  both 
in  the  horse  and  man  a  considerable  amount  has  been 
written  to  prove  that  the  acidity  depends  upon  lactic  or 
hydrochloric  acids,  and  it  is  possible  that  both  these  views 
may  be  reconciled.  Ellenberger  and  Hofmeister  are  of 
opinion  that  shortly  after  a  meal  lactic  acid  predominates 
in  the  horse's  stomach  to  be  replaced  by  hydrochloric  acid 

11 


162     A  MANUAL  OF  VETEEINAEY  PHYSIOLOGY 

some  four  or  five  hours  after  the  commencement  of  feeding. 
These  observers  found  that  the  nature  of  the  acid  depended 
upon  the  region  of  the  stomach,  the  period  of  digestion,  and 
the  character  of  the  food ;  oats  induced  an  outpouring  of 
hydrochloric  acid,  whilst  hay  favoured  the  organic  acids. 

The  following  are  Ellenberger's  views  on  the  nature  of 
the  stomach  acids  :  In  the  contents  of  the  stomach,  hydro- 
chloric, lactic,  butyric  and  acetic  acids  may  be  found,  the 
two  latter  in  insignificant  quantities  only.  In  flesh  feeders 
HCl  predominates,  "25  per  cent.,  and  lactic  acid  is  found,  in 
small  quantities.  In  vegetable  feeders  lactic  acid  at  first 
predominates,  '4  per  cent.,  and  later  HCl  is  present  in  small 
quantities  ;  lactic  acid  exists  throughout  the  whole  stomach, 
but  predominates  in  the  right  and  left  sacs,  whilst  hydro- 
chloric acid  principally  exists  in  the  fundus  region.  Lactic 
is  the  first  digestive  acid  employed,  but  towards  the  end  of 
a  long  digestion  hydrochloric  exists  throughout  the  whole 
stomach.  The  amount  of  lactic  acid  found  in  the  stomach 
of  the  horse  during  the  first  hours  of  digestion  is  con- 
siderable. 

Having  gone  carefully  into  the  question  of  the  presence 
of  hydrochloric  acid  and  organic  acids  in  the  stomach  con- 
tents, we  can  only  say  that,  no  matter  at  what  period  of 
digestion  observations  have  been  made,  we  have  only  two 
or  three  times  succeeded  in  finding  hydrochloric  acid  in  the 
stomach  of  the  horse,  and  are  convinced  that  lactic  is  the 
chief,  if  not  the  sole,  digestive  acid  in  this  animal. 

The  Secretion  of  Gastric  Juice  is  accomplished  in  certain 
glands  known  as  the  gastric.  In  man  these  are  divided 
into  cardiac  and  pyloric,  each  having  not  only  a  different 
structure  but  a  separate  function.  In  the  horse  cardiac 
glands  are  impossible  owing  to  the  presence  of  the  cuticular 
coat ;  but  it  has  been  shown  that  the  villous  coat  contains 
glands  corresponding  to  cardiac,  which  are  principally 
situated  in  the  greater  curvature,  at  the  fundus  of  the 
stomach,  and  extending  over  a  limited  area,  described  on 
p.  152  as  not  larger  than  1  square  foot  (Fig.  39).  The 
two  kinds  of  gland  employed  in  the  production  of  gastric 


DIGESTION 


163 


juice  are  both  found  in  the  villous  coat,  the  one  in  the 
fundus,  the  other  in  the  pyloric  portion,  though  Ellen- 
berger  states  that  he  has  found  fundus  glands  in  the 
pyloric  region.  They  are  simple  or  divided  tubes  lying  side 
by  side,  and  opening,  generally  in  groups,  on  the  surface  of 


Duct. 


Gland. 


'  Duct. 


Chief  Cells. 


Parietal  Cells. 
Gland. 


Ptloric  Gland. 


SCALE  lOQJi 

Cardiac  or  Fundus  Gland. 


Fig.  42. — The  Gastric  Glands  after  Heidenhain  (Wallei!). 


the  mucous  membrane  by  means  of  a  shallow  depression 
in  the  coat.  These  depressions  can  readily  be  seen  studded 
over  the  tunic  of  the  fundus,  giving  it  a  rough  appearance 
owing  to  the  elevation  of  the  mucous  membrane  between 
the  openings  of  the  glands  ;  in  the  pyloric  region  the  mem- 
brane is  as  smooth  as  that  found  in  the  intestine.     Each 

11—2 


164     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

gland  consists  of  a  body,  neck,  and  mouth,  and  is  lined 
■with  cells ;  it  is  in  respect  of  the  cellular  contents  that  the 
pyloric  and  fundus  glands  differ. 

The  cells  of  the  fundus  gland  (Fig.  42)  are  small,  poly- 
hedral, granular,  and  nucleated,  and  line  the  lumen  of  the 
gland  ;  they  are  called  the  inincvpal,  central  or  chief  cells. 
Scattered  amongst  the  principal  cells,  but  existing  in  larger 
numbers  at  the  neck  of  the  gland  than  at  its  base,  are  found 
certain  large  cells  (oval,  granular,  and  nucleated),  which 
from  their  position  relative  to  the  lumen  of  the  gland  are 
called  imrictal,  maniwal,  or  horcler  cells.  These  cells  are 
distinctive  of  the  fundus  glands,  and  they  stain  readily  with 
aniline  blue. 

The  pyloric  gland  (Fig.  42)  below  its  neck  has  but  one 
variety  of  cell — viz.,  the  cylindrical — containing  a  nucleus  at 
its  attached  edge.  The  duct  is  lined,  above  the  neck,  by  the 
ordinary  epithelium  of  the  stomach,  and  the  same  remark 
applies  to  the  fundus  glands  ;  it  is  from  this  epithelium 
that  the  mucus  is  secreted.  The  important  distinction 
between  the  fundus  gland  with  its  principal  and  parietal 
cells,  and  the  pyloric  gland  with  only  its  principal  cells,  is 
that  the  former  secretes  both  the  pepsin  and  acid  of  the 
gastric  juice  (the  acid  being  separated  from  the  blood  by 
the  parietal  cells),  whilst  the  pepsin  only  is  formed  by  the 
principal  cells.  The  pyloric  glands,  on  the  contrary,  only 
secrete  pepsin  and  no  acid. 

We  have  previously  mentioned  that  the  cells  of  the 
salivary  glands  undergo  certain  changes  in  appearance,  the 
result  of  rest  and  activity ;  the  same  remark  applies  to  the 
gastric  follicles,  in  which  the  general  type  of  changes  during 
secretory  activity  is  very  closely  allied  to  those  already 
described.  Langley  has  found  that  in  the  active  state  the 
granules  decrease  in  number,  the  cells  becoming  clear,  and 
capable  of  differentiation  into  a  clear  outer  and  a  granular 
inner  zone,  just  as  we  have  seen  in  the  parotid  gland ;  during 
rest  the  entire  cell  became  granular.  The  parietal  cells 
during  digestion  were  found  to  increase  in  size  but  did  not 
characteristically  lose   their  granules.     The   central   cells 


DIGESTION  165 

secrete  both  the  pepsin  and  rennin  ferments,  but  in  neither 
case  do  these  exist  as  such  in  the  cells,  but  as  a  mother 
substance  or  zymogen  of  the  ferments.  The  formation  by 
the  parietal  cells  of  a  free  acid  from  the  alkaline  blood  is 
a  special  chemical  change,  the  result  of  selective  powers 
possessed  by  the  cells.  In  those  animals,  such  as  the  dog, 
yielding  hydrochloric  acid,  the  cells  very  possibly  form  it 
by  an  inter-action  of  the  sodium  chloride  and  sodium 
dihydrogen  phosphate  of  the  blood. 

Mucin  is  secreted  by  mucous  glands  found  in  the  deep 
layers  of  the  villous  membrane,  especially  in  the  region  of 
the  fundus  ;  the  epithelial  cells  lining  the  excretory  ducts 
of  the  gastric  glands  also  take  part  in  the  process.  The 
amount  of  mucin  formed  in  the  stomach  of  the  horse  is 
remarkable  ;  it  adheres  to  the  villous  coat  like  unboiled 
white  of  egg,  and  cannot  be  washed  away  even  by  a 
powerful  jet  of  water.  The  amount  secreted  is  unknown 
but  must  be  considerable ;  less  is  formed  during  hunger 
than  during  activity,  and  there  is  less  in  ruminants  than  in 
horses. 

Gastric  Juice. — It  is  only  lately  that  a  pure  sample  of 
gastric  juice  (but  not  from  the  horse)  has  been  available 
for  analysis.  Most  of  the  previous  secretions  examined 
have  been  a  mixture  of  saliva,  gastric  juice,  and  perhaps 
other  substances.  Pawlow  devised  a  method  by  which  the 
stomach  of  the  dog  could  be  rendered  available  for  physio- 
logical enquiry,  and  a  pure  secretion  was  obtained  (see 
Figs.  43  and  44). 

Pure  gastric  juice  in  the  dog  is  as  colourless  as  water, 
thin,  transparent,  and  of  strongly  acid  reaction.  Chemically 
it  consists  of  acid  and  enzymes,  the  acidity,  which  is  due  to 
hydrochloric,  being  about  '46  or  "56  per  cent.  The  enzymes 
are  pepsin  and  rennin ;  the  former  is  unable  to  act  excepting 
in  an  acid  medium,  and  furnishes  the  only  example  in  the 
body  of  this  necessary  combination.  How  far  the  gastric 
juice  of  other  animals  resembles  that  of  the  dog  in  com- 
position and  appearance  we  do  not  know  owing  to  the 
difficulty  in  obtaining  it  pure,  but  in  all  cases  an  acid  and 


166    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

an  enzyme  are  present.  The  enzyme  is  invariably  pepsin, 
but  the  acid  is  not  always  hydrochloric.  The  amount  of 
juice  secreted  is  uncertain,  in  the  dog  some  700  c.c.  (24^  ozs.) 
have  been  collected  in  a  few  hours,  from  which  we  may 
perhaps  imagine  that  a  considerable  amount  is  formed  in 
the  stomach  of  the  larger  animals. 

The  gastric  juice  of  the  dog  withstands  putrefaction  for  a 
long  time  ;  it  may  be  kept  for  months  without  undergoing 
any  important  change ;  not  so  with  herbivora ;  the  mixed 


pylorus  ^y^^\[\y /  /oesophagus 

Plexus  gastricuy^  \\jf  ^A  i   Plexus  gastricus 

anterior  vagi/  ^  -       Jl^     /    Posterior  vagi. 


Fig.  43. — Pawlow's  Stomach  Pouch  (Stewart). 

J.,  £,  line  of  incision  ;  C,  flap  for  forming  the  stomach  pouch.  At  the 
base  of  the  flap  the  serous  and  muscular  coats  are  preserved,  and 
only  the  mucous  membrane  divided,  so  that  the  branches  of  the 
vagus  going  to  the  pouch  are  not  severed. 

gastric  fluids  of  the  horse  rapidly  putrefy.  The  antiseptic 
properties  of  the  dog's  juice  are  attributed  to  its  hydro- 
chloric acid ;  if  this  is  so  it  is  additional  evidence  against 
the  acid  of  herbivora  being  hydrochloric.  There  appears  to 
be  no  reason  why  lactic  acid  should  not  be  formed  by  the 
marginal  cells  of  the  fundus  glands,  but  an  important 
source  of  lactic  supply  in  herbivora  is  the  carbohydrate 
of  their  food. 

Pepsin  is  of  a  proteid  nature,  though  very  little  is  known 
of  it  chemically.  It  best  exhibits  its  action  at  a  temperature 
of  the  interior  of  the  body  (37°  to  •40°  C.) ;  a  low  temperature 


DIGESTION 


167 


retards  its  activity,  while  it  is  destroyed  at  a  high  one. 
The  ordinary  commercial  product  is  very  impure  ;  it  is  an 
extract  of  the  mucous  membrane  of  the  stomach,  to  which 
starch  or  milk  sugar  has  been  added.  In  physiological 
work  a  glycerine  extract  of  the  mucous  membrane  of  the 
stomach  suffices,  to  which  is  added  some  dilute  HCl. 
Glycerine  has  the  power  of  extracting  the  ferments  both 


Muscularil 


Fig.  44.  — Pawlow's  Stomach  Pouch  (Stewart). 

(S',  the  completed  pouch  ;   F,  cavity  of  the  stomach  ;  A,  A,  the 
abdominal  wall. 

from  the  stomach  and  other  portions  of  the  digestive  tract 
such  as  the  pancreas.  The  action  of  pepsin  is  almost 
wholly  if  not  entirely  confined  to  the  proteid  constituents 
of  food.  It  converts  the  insoluble  proteids  into  soluble 
ones  not  by  direct  transformation  but  by  several  stages. 
The  products  intermediate  between  proteid  and  peptone 
have  received  certain  names  suggestive  of  differences  in 
their  chemical  nature,  but  as  to  all  of  this  a  good  deal  of 
doubt  and  speculation  exists. 


168    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

In  the  following  table  the  various  stages  of  conversion 
are  indicated  in  the  order  in  which  they  are  found  to  occur 
as  determined  by  small  differences  in  the  chemical  tests, 
such  as  solubility  or  colour  reaction,  yielded  by  the 
peptonized  product.  The  table  is  the  one  drawn  up  by 
Kuhne. 

1.  The  proteid  as  consumed,  or  native  albumin. 

2.  Acid  albumin  or  syntonin. 

3.  Primary  proteoses. 

4.  Secondary  proteoses. 

5.  Peptones. 

The  proteid  having  reached  the  stage  of  peptones  is  now 
capable  of  being  absorbed,  but  the  conversion  from  proteid 
to  peptone  is  a  most  complex  one,  during  which  the  large 
proteid  molecule  is  converted  into  simpler  products  of  an 
infinitely  smaller  molecular  weight,  while  so  great  is  the 
complexity  that  the  resulting  product,  peptone,  is  in  all 
probability  a  group  of  compounds,  rather  than  a  single  one, 
which  only  resemble  each  other  in  their  solubility  and  their 
definite  reaction  to  certain  chemical  tests.  Rennin  is  the 
second  enzyme  present  in  the  gastric  juice.  Commercially 
it  is  used  in  the  manufacture  of  cheese,  an  infusion  of  the 
mucous  membrane  of  the  stomach  being  sufficient  to 
produce  the  needful  change  in  the  milk.  There  appears 
no  necessity  for  adult  animals  to  possess  this  ferment  in 
their  Juice  after  weaning,  as  milk  does  not  form  an  article 
of  diet  unless  we  except  the  chemically  altered  milk  given 
to  the  pig.  In  the  young  animal  rennin  plus  acid  causes 
milk  to  clot  rapidly.  The  clotting  under  rennin  resembles 
blood-clotting.  The  clot  contracts  after  a  time,  squeezing 
out  a  yellowish  fluid  known  as  whey,  and  furthermore  it  is 
definitely  known  that,  as  in  blood-clotting,  a  calcium  salt  is 
necessary  to  the  process  of  milk-clotting.  In  fact  two 
distinct  steps  are  recognised  as  taking  place,  first  the 
formation  of  a  substance  known  as  paracasein,  by  the 
action  of  rennin  on  casein,  and  secondly  the  action  on  the 
paracasein  of  the  lime  salts  of  the  milk  forming  a  curd. 


DIGESTION  169 

If  milk  be  deprived  of  its  calcium  salts,  no  clotting  occurs 
on  the  addition  of  rennin,  from  which  it  is  considered  that 
the  calcium  salts  are  of  more  importance  than  the  ferment. 
Rennin  takes  no  part  in  the  digestive  process ;  once  the 
curd  is  formed  its  digestion  is  carried  out  by  pepsin. 

Other  ferment  actions  of  the  gastric  juice  have  been 
described,  such  as  fat-  and  stareh-splitting,  but  of  their 
existence  there  is  very  little  evidence.  Proteid  digestion  is 
the  essential  duty  of  the  stomach,  while  in  all  vegetable 
feeders  maceration  of  the  vegetable  fibres  is  begun  in  the 
stomach  as  a  preliminary  measure.  Still  in  all  animals 
a  stomach  is  not  essential  to  life ;  in  the  dog  for  example  it 
may  be  removed  experimentally,  for  as  we  shall  see  later 
on,  proteid  digestion  is  provided  for  elsewhere.  But  in  the 
herbivora,  especially  ruminants,  a  stomach  is  essential. 
The  chief  value  of  the  stomach  in  those  animals  which  can 
be  proved  to  live  without  it  lies  in  the  preparation  of  the 
food  for  subsequent  digestion  in  the  small  intestines,  for  it 
is  quite  undoubted  that  proteid  previously  acted  upon  by 
gastric  juice  is  far  more  thoroughly  handled  by  the 
pancreatic  fluid  than  proteid  not  so  previously  acted  upon. 

The  secretion  of  gastric  juice  has  but  recently  been  proved 
to  be  under  the  control  of  the  nervous  system,  and  the 
secretory  fibres  are  contained  in  the  vagus.*  Stimulation 
of  the  peripheral  end  of  the  divided  nerve  causes  after  a 
short  delay  a  flow  of  fluid.  The  cause  of  the  latent  period 
is  unknown.  It  can  be  shown  in  the  dog  that  mastication, 
swallowing,  taste,  odour,  etc.,  are  direct  excitants  of  the 
secretions,  for  they  cause  a  copious  production  of  gastric 
juice,  though  not  if  the  vagus  has  been  previously  divided. 
If  the  cpsophagus  of  a  dog  be  divided  and  the  upper  section 
brought  outside  the  wound,  the  animal  may  be  indulged 
in  a  meal  which  never  enters  the  stomach,  but  which, 
nevertheless,  produces  a  profuse  secretion  of  gastric  juice. 
Mechanical  stimulation  of  the  mucous  membranes  of  the 
stomach   has   no   efi'ect   in   producing   secretion.     Certain 

*  Pawlow,  '  The  Work  of  the  Digestive  Glands.'  Translated  by 
Thompson,  1902. 


170    A  MA.NUAL  OF  VETERINARY  PHYSIOLOGY 

foods  in  the  case  of  the  dog,  such  as  meat  extracts,  are  most 
effective  stimulants,  while  ])read  and  white  of  egg  are  found 
to  have  no  effect  if  introduced  directly  into  the  stomach, 
though  they  operate  reflexly  through  mastication  and 
taste.  Finally,  Pawlow,  to  whom  all  this  work  is  due, 
believes  that  the  quantity  and  quality  of  the  gastric  juice 
will  be  found  to  depend  on  the  character  of  the  food,  so 
that  while  in  some  cases  an  economical  production  is 
arrived  at,  in  others  a  stronger  or  weaker  fluid  is  poured 
out  depending  upon  the  work  to  be  done,  the  regulation  of 
which  is  probably  a  specific  action  on  the  part  of  the  food 
itself. 

Such,  briefly,  is  the  case  as  it  stands  at  present.  If  the 
above  proves  to  be  correct,  we  have  in  our  hands  a  most 
likely  explanation  of  some  of  the  digestive  troubles  of  the 
horse. 

There  are  other  changes  occurring  in  the  stomach  inde- 
pendently of  peptonizing  or  of  gastric  juice.  If  a  horse  be 
fed  on  oats  and  the  stomach  fluid  examined,  it  will  be 
found  to  contain  an  abundance  of  sugar.  The  sugar  is 
produced  from  the  starch  of  the  grain,  and  is  not,  according 
to  our  observations,  the  result  of  the  action  of  saliva. 
Abundant  saliva  exists  in  the  stomach,  but  it  will  be 
remembered  that  in  the  horse  we  have  never  succeeded  in 
getting  it  to  give  any  evidence  of  starch  conversion.  The 
question,  therefore,  is.  What  is  the  cause  of  this  formation 
of  sugar  ?  It  has  been  shown  that  oats  may  yield  a  starch- 
converting  ferment,  and  the  view  that  the  grain  provides 
its  own  enzyme  for  the  conversion  of  starch  into  sugar  may 
be  provisionally  accepted  as  the  explanation  of  the  pre- 
sence of  sugar  in  the  stomach  of  the  horse.  The  whole  of 
the  starch  is  not  thus  converted,  for  distinct  evidence  of 
unaltered  starch  can  be  obtained  in  the  first  portion  of  the 
small  intestines.  Further,  some  of  the  starch  is  no  doubt 
converted  into  lactic  acid,  and  the  presence  of  this  acid  in 
the  proportion  of  2  per  cent,  does  not  in  any  way  inhibit 
the  amylotic  action.  If  oats  provide  their  own  starch- 
converting  enzyme,  we  see  the  strongest  argument  against 


DIGESTION  171 

boiled  food  for  horses,  a  practice  we  believe  to  be  deleterious 
or  even  dangerous. 

Fats  are  not  acted  upon  in  the  stomach,  though  the 
envelope  surrounding  the  fat  globule  is  digested,  and  the 
fat  set  free. 

Cellulose  fermentation  is  considered  by  Tappeiner  to 
occur  in  the  left  sac  of  the  stomach,  and  when  marsh-gas 
has  been  found  in  this  organ,  it  results  from  cellulose 
decomposition.  Brown*  has  shown  that  the  destruction 
of  the  cell-wall  of  oats  and  barley  occurs  in  the  stomach, 
where  it  is  dissolved  by  a  cyto-hydrolytic  ferment  j^re- 
existcnt  in  the  gram  ;  the  changes  occur  with  extraordinary 
rapidity  in  the  stomach  of  the  horse.  The  researches  of 
this  observer  on  a  cellulose- dissolving  ferment  are  of  the 
greatest  interest  to  the  veterinary  physiologist,  and  of  con- 
siderable practical  importance. 

Periods  of  Stomach  Digestion. — Stomach  digestion  in  the 
horse  has  been  divided  by  Ellenberger  and  Hofmeister  into 
certain  periods  corresponding  to  definite  chemical  changes 
in  the  food.  For  example,  it  is  said  that  during  the  two 
first  periods,  which  between  them  last  two  and  three  hours, 
starch  conversion,  lactic  acid  fermentation,  and  proteid  con- 
version to  a  limited  extent  occur.  In  the  third  period 
mixed  digestion  of  starch  and  proteid  occurs,  while  in  the 
fourth  and  last  period  only  proteid  digestion  takes  place. 
The  third  and  fourth  periods  may  together  last  four  hours 
and  upwards.  We  must  be  careful  to  avoid  regarding 
these  periods  as  based  on  some  rigid  law  ;  they  are  very 
variable  in  duration,  due  to  causes  we  have  previously  con- 
sidered, and  run  imperceptibly  into  each  other.  With  this 
caution  we  give  the  following  periods  at  which  gastric 
digestion  is  said  by  Ellenberger  and  Hofmeister  to  be  at 
its  maximum  in  the  horse : 

After  a  moderate  feed  digestion  is  at  its  height  in  o  or  4  hours. 
,,         full  „  „  „  „  6  to   8      „ 

,,  an  immoderate  ,,  ,,  delayed  still  longer. 

*  '  On  the  Search  for  a  Cellulose-dissolving  Enzyme,'  H.  J.  Brown, 
F.R.S.,  Journal  of  the  Chemical  Society,  1892,  p.  352. 


172     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Stomach  Digestion  in  Ruminants. — The  Rumen  or  first 
gastric  reservoir  is  a  viscus  of  enormous  proportions, 
capable  in  the  ox  of  holding  60  gallons.  It  is  divided  into 
four  sacs  by  means  of  very  thick  muscular  pillars,  and 
the  whole  is  lined  by  a  well  developed  mucous  membrane, 
in  part  covered  by  leaf-like  papillse.  The  mucous  mem- 
brane, it  is  said,  contains  some  small  glands  which  are  not 
considered  to  provide  any  digestive  secretion.  The  rumen 
is  in  connection  with  the  reticulum,  and  by  means  of  the 
oesophageal  groove  with  the  omasum.  All  solid  food  on 
first  coming  from  the  mouth  is  received  by  the  rumen  and, 
judging  by  the  contents  of  this  compartment,  much  of  the 
fluid  which  is  swallowed  must  also  find  its  way  there  ;  it 
has  been  proved  by  the  experiments  of  Flourens  that  fluid 
may  find  its  way  from  the  oesophagus  into  all  four  stomachs 
at  one  and  the  same  time.  The  amount  of  fluid  in  the 
rumen  is  important  from  a  digestive  point  of  view,  since 
rumination  is  impossible  unless  a  large  proportion  of  water 
exists  in  this  cavity.  The  fluid  found  in  the  rumen  consists 
of  the  water  which  has  been  consumed,  of  the  amount  of 
saliva  swallowed,  and  of  the  amount  existing  in  the  food ; 
but  much  of  it  is  saliva,  of  which  the  ox  secretes  enormous 
quantities. 

The  contents  of  the  rumen  are  alkaline,  which  is  pro- 
bably owing  to  the  saliva  ;  in  appearance  they  resemble 
food  which  has  been  coarsely  ground.  This  mass  is  slowly 
and  deliberately,  not  energetically,  revolved  within  the 
stomach,  the  material  at  the  posterior  part  being  gradually 
forced  upwards  and  forwards  and  so  a  complete  mixing 
occurs.  The  churning  movement  is  brought  about  by  the 
extremely  powerful  muscular  pillars  of  the  organ,  which  are 
so  arranged  as  to  separate  it  into  various  sacs ;  these 
pillars,  when  they  contract,  shorten  the  rumen  in  its  two 
diameters,  and  press  the  contents  towards  the  opening  of 
the  oesophagus.  Fermentation  may  also  assist  to  mix  the 
contents,  owing  to  the  evolution  of  gas  during  the  process. 
It  is  due  to  the  churning  movement  that  the  '  hair  balls,' 
found  in  the  rumen  of  cattle,  are  formed. 


DIGESTION 


173 


The  essential  function  of  the  rumen  is  to  retain  the  food 
for  remastication,  to  macerate  all  fibrous  substances  and 
to  fit  them  for  cellulose  digestion,  which  here  takes  place 
possibly  under  the  influence  of  ferments  contained  in  the 
food  itself.  The  amount  of  cellulose  digested  in  the 
rumen  has  been  estimated  at  between  60  and  70  per  cent. 
Ellenberger  is  of  opinion  that  in  addition  to  the  functions 
named,  other  digestive  changes  occur  ;  he  says  that  carbo- 
hydrates are  digested  by  means  of  enzymes  contained  in  the 
food,  and  in  this  way  starch  and  cane  sugar  are  converted 


Fig.  45. — The  Gastric  CoaiPARTiiENTs  and  True  Stomach  of  Rumi- 
nants (Colin). 

C,  The  oesophagus  ;  A,  A,  B,  B,  the  rumen  ;  D,  the  reticulum  ;  E,  the 
omasum  ;  F,  the  abomasum. 

into  maltose.  Proteids  are  also  slowly  converted  into  pep- 
tones, not  through  any  true  peptic  ferment  but  by  some 
enzyme  provided  by  the  food.  The  result  of  the  decomposi- 
tion of  cellulose  is  the  production  of  a  considerable  quantity 
of  gas.  The  rumen  never  empties  itself;  even  after  pro- 
longed starvation  it  contains  food.  In  young  ruminants 
digpstion  occurs  principally  in  the  fourth  stomach,  the 
other  compartments  being  rudimentary  ;  when  the  young 
animal  is  placed  on  solid  food  it  is  remarkable  how  soon 
these  compartments  develop,  and  the  process  of  remastica- 
tion is  established. 


174     A  MANUAL  OF  VETEKINAKY  PHYSIOLOGY 

The  Reticulum  or  second  gastric  reservoir  is  a  small  one. 
Its  interior  is  arranged  like  a  honeycomb,  in  the  cells  of 
which  foreign  bodies  such  as  stones,  sand,  nails,  etc.,  may 
be  found.  The  contents  of  this  compartment  are  fluid  and 
alkaline,  the  fluid  being  derived  from  that  swallowed,  and 
from  the  rumen ;  the  alkaline  reaction  is  due  to  the  saliva, 
for  so  far  as  we  know,  the  mucous  membrane  possesses  no 
secretory  activity.  The  fluid  in  the  reticulum  is  of  use  in 
rumination,  and  is  forced  into  the  oesophagus  by  a  contrac- 
tion of  the  walls  of  the  viscus  ;  in  order  that  fluid  may  be 
retained  in  this  compartment  the  openings  out  of  it  are 
situated  considerably  above  the  base  of  the  organ,  and 
further,  the  reticulum  is  so  situated  relatively  to  the 
rumen  that  it  receives  the  overflow  of  fluid  from  that  com- 
partment when  it  contracts. 

Ellenberger  is  of  opinion  that  the  reticulum  regulates 
the  passage  of  food  from  the  first  to  the  third  compart- 
ment, and  from  the  rumen  to  the  oesophagus.  In  trans- 
ferring the  contents  of  the  rumen  to  the  omasum,  the 
reticulum  contracts  and  forces  the  material  into  the  open 
oesophageal  groove.  That  the  reticulum  is  capable  of 
energetic  contraction  is  specially  noted  by  Colin,  whose 
observations  on  the  physiology  of  the  stomach  in  rumi- 
nants were  mainly  carried  out  by  means  of  an  opening 
in  the  abdominal  wall.  Flourens  showed  that  the  reticulum 
was  not  essential  to  rumination,  for  he  excised  it  in  a  sheep 
and  rumination  was  not  interfered  with. 

The  Omasum,  or  third  compartment,  is  peculiar ;  its 
physiology  has  been  elaborately  worked  out  by  Ellenberger. 
This  authority  says  that  it  possesses  no  secreting  power ; 
that  its  function  is  to  compress  and  triturate  the  food 
which  it  crushes  between  its  powerful  muscular  leaves, 
rasping  the  ingesta  down  by  means  of  its  papilla.  The 
contents  of  this  sac  are  always  dry,  due  to  the  fluid  portion 
being  squeezed  off  and  flowing  into  the  fourth  stomach  by 
the  action  of  gravity,  through  a  passage  formed  in  the 
lesser  curvature  of  the  organ.  The  food  may  find  its  way 
into  the  omasum,  either  directly  from  the  oesophagus  after 


DIGESTION  175 

remastication,  or  from  the  first  or  second  compartments.  It 
is  probable  that  its  chief  source  of  supply  is  directly  from 
the  oesophagus,  the  omasum  being  drawn  forwards  towards 
it  by  a  contraction  of  the  pillars  of  the  oesophageal  groove, 
by  which  means  communication  with  the  rumen  and  reti- 
culum is  cut  off.  Normally  the  reaction  of  the  contents  of 
the  omasum  is  neutral ;  if  found  acid  it  is  due  to  regurgita- 
tion from  the  true  stomach.  It  is  peculiar  in  possessing  a 
separate  source  of  nerve  supply,  stimulation  of  the  pneumo- 
gastric  producing  contraction  of  all  the  other  compartments 
but  this. 

The  Abomasum  is  the  true  digestive  stomach,  and  is  the 
only  compartment  secreting  gastric  juice.  In  the  abomasum 
proteids  are  converted  into  peptones,  the  region  of  the 
cardia  being  in  this  respect  more  active  than  the  pylorus. 
Ellenberger  states  that  starch  is  also  digested,  and  that 
this  precedes  proteid  digestion.  In  the  fourth  stomach  of 
the  calf  a  milk-curdling  ferment  (rennin)  exists,  which  has 
already  been  dealt  with. 

Stomach  Digestion  in  the  Pig. — The  stomach  of  the  pig  is 
peculiar ;  it  is  a  type  between  the  carnivorous  and  rumi- 
nant, and  is  divided  by  Ellenberger  and  Hofmeister  into 
five  distinct  regions,  which  do  not  all  possess  the  same 
digestive  activity. 

The  gastric  juice  of  the  pig  contains  for  the  first  hour  or 
two  of  digestion  lactic,  and  afterwards  hydrochloric  acid ; 
pepsin  is  present,  and,  it  is  said,  a  ferment  which  converts 
starch  into  sugar.  In  the  pig,  according  to  the  above 
observers,  the  process  of  digestion  is  not  the  same  in  all 
regions  of  the  viscus ;  one  may  contain  hydrochloric  acid, 
another  lactic ;  one  may  be  abundant  in  sugar,  while  this 
may  be  absent  elsewhere.  The  first  stage  of  digestion  is 
one  of  starch  conversion ;  the  second  stage  is  the  same 
only  more  pronounced ;  the  third  is  one  of  starch  and 
proteid  conversion,  both  processes  occurring  at  the  cardia, 
but  only  proteid  conversion  taking  place  at  the  fundus  ; 
lactic  acid  is  present  in  the  former  and  both  lactic  and 
hydrochloric  acid  in  the  latter.     In  the  fourth  stage  starch 


176     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

conversion  is  nearly  complete,  hydrochloric  acid  pre- 
dominates in  all  the  regions,  and  proteid  conversion  is 
general. 

Stomach  Digestion  in  the  Dog. — Very  complete  knowledge 
of  the  physiology  of  the  dog's  stomach  exists,  for  nearly  all 
the  work  carried  out  to  elucidate  the  physiology  of  the 
human  stomach  has  been  effected  on  the  dog,  and  has, 
more  or  less,  been  already  embodied  in  the  previous  pages 
in  dealing  with  gastric  juice. 

A  flesh  diet  requires  very  little  saliva  and  practically  no 
mastication,  but  its  digestion  is  slow,  in  spite  of  the  fact 
that  it  is  taken  in  a  form  closely  allied  to  that  in  which  it 
is  assimilated.  Colin  states  that  it  takes  a  dog  twelve  hours 
to  digest  an  amount  of  meat  which  it  could  eat  at  one  meal. 
The  substances  most  difficult  of  digestion  are  tendons  and 
ligaments,  but  their  digestion  is  facilitated  by  boiling  ;  liver 
and  flesh  are  best  given  raw  as  cooking  interferes  with  their 
digestibility.  The  gastric  juice  of  the  dog  contains  pepsin 
and  hydrochloric  acid  '46  to  "56  per  cent.,  and  it  has  been 
shown  that  it  is  possessed  of  considerable  activity,  and 
certain  peculiarities  which  have  been  dealt  with  on  p.  165. 

Absorption  from  the  Stomach. — The  needful  changes 
having  occurred  in  the  stomach — and  we  now  refer  prin- 
cipally to  the  stomach  of  the  horse — the  next  step  is  to 
inquire  into  the  proportion  of  food  so  altered  as  to  be 
rendered  fit  for  absorption. 

Experiment  shows  that  in  the  stomach  40  to  50  per  cent, 
of  the  carbo-hydrates  have  been  converted  into  sugar,  whilst 
40  to  70  per  cent,  of  the  proteids  are  converted  into  pep- 
tones ;  when  food  has  been  long  in  the  stomach,  not  more 
than  10  per  cent,  of  the  proteids  escape  being  peptonized. 
In  ruminants  probably  the  greater  part  of  the  food  sub- 
stance is  acted  upon  in  the  gastric  compartments  and 
stomach,  leaving  comparatively  little  for  the  intestines  to 
perform. 

In  spite  of  the  changes  which  occur  in  the  stomach,  it 
has  been  proved  by  the  experiments  of  Colin  that  no 
absorption  occurs  from  tJiis  organ  in  the  horse.     It  would  be 


DIGESTION  177 

useless  to  recapitulate  all  bis  experiments ;  they  were 
generally  performed  with  strychnine,  and  he  found,  that 
so  long  as  the  pylorus  was  securely  tied,  no  symptoms  of 
poisoning  occurred  when  the  alkaloid  was  introduced  into 
the  stomach,  no  matter  how  long  it  was  left  there,  but 
that  when  the  ligature  was  untied,  and  the  contents  of 
the  stomach  passed  into  the  intestines,  poisoning  rapidly 
followed.  These  remarkable  results  were  obtained  by 
him  so  often,  and  under  such  varying  conditions,  as 
to  leave  no  doubt  as  to  the  accuracy  of  the  observations. 
Strychnine  experiments  are  not  altogether  free  from  objec- 
tion, but  as  matters  stand  we  can  only  surmise  that  no 
absorption  of  sugar  or  peptones  occurs  in  the  stomach. 
It  is  certainly  very  remarkable  what  becomes  of  the  pep- 
tones ;  we  have  never  found  any  in  the  stomach  contents, 
no  matter  at  what  period  of  digestion  the  examination  was 
made,  and  if  they  are  not  absorbed  in  the  stomach  they 
must  pass  very  rapidly  into  the  intestines  and  enter  the 
vessels  at  once,  as  no  peptone  can  be  found  in  the  small 
intestines.  Colin  attributes  the  absence  of  absorption  from 
the  stomach  of  the  horse  to  the  small  area  of  the  mucous 
membrane,  which,  he  says,  cannot  be  secreting  gastric 
juice  and  absorbing  at  the  same  time.  In  the  empty 
stomach  he  attributes  the  non-absorption  of  poisons  to  the 
thick  layer  of  tenacious  mucus  v.hich,  as  we  have  previously 
mentioned,  covers  the  villous  stomach  of  the  horse.  Colin's 
experiments  also  show  that  there  is  little  or  no  absorption 
from  the  abomasum  of  ruminants.  On  the  other  hand, 
there  is  absorption  from  the  stomach  of  the  dog  and  pig. 
Recent  experiments  on  the  dog  show  that  absorption  does 
not  take  place  readily  from  the  stomach.  Water  taken 
alone  is  practically  not  absorbed  at  all ;  sugars  and  peptones 
are  absorbed  only  when  in  sufficient  concentration,  while 
fats  are  not  absorbed. 

Self-digestion  of  the  Stomach. — A  question  which  for  a 
long  time  gave  rise  to  an  energetic  discussion,  was  the 
reason  why  the  stomach  during  life  does  not  digest  itself, 
seeing  that  the  action  of   its   secretion  is  so  potent  that 

12 


17.S     A  MANUAL  OF  YETEEINARY  PHYSIOLOGY 

portions  of  living  material,  legs  of  frogs,  ears  of  rabbits, 
etc.,  if  introduced  into  it  are  readily  digested,  also  that 
post-mortem  digestion  of  the  stomach  in  some  animals  is 
far  from  rare.  It  is  believed  that  the  gastric  epithelium 
forms  an  antibody,  known  as  antipepsin,  which  neutralizes 
the  digestive  action  on  the  living  wall.  This  view  is  the 
outcome  of  recent  studies  in  immunity  (see  p.  26).  We 
have  never  yet  met  with  post-mortem  digestion  of  the 
stomach  in  the  horse ;  whether  this  be  due  to  the  horse's 
acid  being  mainly  or  wholly  lactic  cannot  be  definitely 
stated. 

The  Gases  of  the  Stomach. — The  nature  of  these  largely 
depends  upon  the  food — for  example,  green  food  is  most 
productive  of  gas  owing  to  the  active  fermentation  it 
undergoes.  Traces  of  oxygen,  a  quantity  of  carbonic 
acid,  and  variable  amounts  of  marsh-gas,  sulphuretted 
hydrogen,  hydrogen,  and  nitrogen  are  found.  The  oxygen 
and  nitrogen  are  derived  from  the  swallowed  air,  the 
carbonic  acid  is  derived  from  the  fermentation  of  the  food, 
and  the  action  of  acids  on  the  saliva,  whilst  the  marsh-gas 
is  obtaimed  by  the  decomposition  of  cellulose. 

The  gases  from  the  intestines  of  the  horse  and  rumen 
of  the  ox  are  very  commonly  inflammable,  and  burn  with  a 
pale  blue  flame.  This  is  due  to  marsh-gas,  which  may  be 
readily  ignited  when  mixed  with  a  due  proportion  of 
oxygen. 

Vomiting. — Yomiting  amongst  solipeds  and  ruminants  is 
rare,  but  the  act  is  common  in  the  dog  and  pig. 

The  reasons  given  as  to  why  the  horse  does  not  ordinarily 
vomit  are  various:  (1)  the  thickened  and  contracted  cardiac 
extremity  of  the  oesophagus ;  (2)  the  oblique  manner  in  which 
the  latter  enters  the  gastric  walls;  (3)  the  dilated  pylorus 
lying  close  to  the  contracted  cardia,  so  that  compression  of 
the  stomach  contents  forces  them  into  the  duodenum ;  (4)  the 
cuticular  coat  thrown  into  folds  over  the  opening  of  the 
cardia  ;  (5)  muscular  loops  encircling  the  cardia,  the  con- 
traction of  which  keeps  the  opening  tightly  closed ;  (6)  the 
stomach  not    l^eing   in   contact  with  the  abdominal  wall. 


DIGESTION  179 

All  these  and  other  reasons  have  been  assigned  as  the 
cause  of  non-vomiting  in  the  horse.  Yet  on  turning  to 
ruminants,  which  also  normally  do  not  vomit,  we  find  the 
stomach,  gastric  compartments,  and  oesophagus  freely 
communicating  ;  the  largest  reservoir  lies  in  contact  with 
the  abdominal  wall,  the  cardia  is  freely  open,  the  oesophagus 
is  of  great  size,  and,  still  stranger,  the  animal  possesses  the 
ability,  under  the  control  of  the  will,  to  bring  up  food  from 
the  stomach  as  a  normal  condition,  and  yet  cannot  vomit ! 
It  is  evident,  therefore,  that  all  these  theories  are  not  suffi- 
ciently satisfactory  to  account  for  the  absence  of  vomiting, 
and  we  are  bound  to  suppose  that  the  vomiting  centres 
in  the  medulla  of  both  horse  and  ox  are  either  only  rudi- 
mentary or  very  insensitive  to  ordinary  impressions. 

Vomition  in  the  horse  is  no  doubt  seriously  interfered 
with  by  the  thickened  oesophagus,  contracted  cardia,  and 
the  arrangement  of  the  muscular  fibres.  The  folds  of 
mucous  membrane  filling  up  the  orifice  could  offer  no 
serious  obstruction  to  a  distended  stomach,  for  we  know 
that  even  when  this  membrane  is  dissected  away  post- 
mortem, a  stomach  will  burst  rather  than  allow  fluid  or  air 
pumped  in  at  the  pylorus  to  escape  at  the  cardia,  unless 
the  muscular  fibres  surrounding  it  be  partly  divided. 
Vomition  in  the  horse  is  generally  indicative  of  ruptured 
stomach,  and  much  has  been  written  as  to  whether  vomit- 
ing occurs  before  or  after  rupture.  From  no  inconsiderable 
experience  of  these  cases,  we  have  arrived  at  the  conclusion 
that  it  may  occur  at  either  time,  and  that  a  horse  may 
vomit  though  a  rent  seven  or  eight  inches  long  exists  in 
the  stomach  wall. 

Dilatation  of  the  cardia  and  oesophagus  is  essential  to 
the  act  of  vomition  in  the  horse,  and  in  all  cases  where 
vomiting  occurs  during  life,  the  cardia  is  so  dilated  that  two 
or  three  fingers  may  readily  be  introduced  into  it.  It  is 
perfectly  possible  for  a  horse  to  vomit  and  recover  (show- 
ing that  it  had  not  a  ruptured  stomach),  and  it  is  not 
unusual  to  have  attempts  at  or  actual  vomition  when  the 
small   or   large   intestines   are  twisted.     Vomiting  in  the 

12—2 


180    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

horse  is  not  as  a  rule  attended  by  any  distressing  symp- 
toms ;  the  ingesta  dribble  away  from  one  or  both  nostrils  ; 
occasionally  an  effort  is  made  on  the  part  of  the  patient, 
the  head  being  depressed  to  facilitate  expulsion,  but  more 
than  this  is  very  rarely  seen.* 

It  is  important  to  notice  in  connection  with  the  subject 
of  vomiting  that  agents  such  as  tartar  emetic,  ipecacuanha, 
and  apomorphia,  which  excite  vomiting  by  their  action  on 
the  cerebral  centre,  have  no  effect  on  the  horse  or  rumi- 
nants, nor  does  the  horse  vomit  as  the  result  of  sea-sick- 
ness, though  he  suffers  extremely  from  it.  Why  he  should 
vomit  more  often  with  a  ruptured  stomach  than  a  sound 
one  is  a  fact  we  cannot  explain. 

In  those  animals  where  vomiting  is  a  natural  process, 
the  three  important  factors  are,  the  dilatation  of  the 
cardia  by  active  contraction  of  the  longitudinal  fibres  of 
the  oesophagus,  pressure  on  the  walls  of  the  stomach  by  a 
contraction  of  the  diaphragm  and  abdominal  muscles,  and 
closure  of  the  pylorus.  But  there  is  some  evidence  to 
show  that  the  stomach  itself  is  not  passive  ;  it  is  true 
Majendie  produced  vomiting  after  he  had  replaced  the 
stomach  by  a  bladder,  but  under  normal  conditions  there 
appears  no  reason  why  the  stomach  wall  should  remain 
quiescent,  and  in  the  cat  it  has  been  observed  that  during 
vomiting  a  strong  contraction  of  the  pyloric  end  of  the 
stomach  occurred,  shutting  it  off  from  the  cardiac  portion. 
We  may  here  have  one  explanation  of  ruptured  stomach  in 
the  horse. 

Rumination. 

The  physiology  of  rumination  has  been  principally  worked 
out  in  France  by  Flourens  and  Colin,  and  our  knowledge  of 
this  singular  process  is  based  almost  entirely  on  their  observa- 
tions.    (Esophageal  Groove. — The  cesophagus  in  ruminants 

*  The  only  case  of  vomiting  we  have  seen  in  the  horse  which  re- 
sembled that  presented  by  the  human  subject  was  in  a  case  of 
volvulus  of  the  small  bowels.  The  horse  was  lying  on  his  chest  with 
the  nose  extended,  the  inf,'esta  gushed  in  a  stream  froixi  both  nostrils, 
and  a  sound  accompanied  the  effort. 


DIGESTION 


181 


enters  and  passes  through  the  rumen,  formmg  a  singular 
groove  or  channel  known  as  the  cesophageal,  which  on  the  left 
communicates  with  the  first  and  second  compartments,  and 
by  an  opening  on  the  right  and  inferiorly,  with  the  third 
compartment  (Figs.  46  and  47).  In  this  way  food  coming 
down  the  cesophagus  may  enter  either  of  the  first  three 
reservoirs,  the  choice  being  determined,  as  we  shall  presently 
point  out,  by  the  condition  in  which  it  is  swallowed. 

The  oesophageal  groove  possesses  two  lips  or  pillars,  the 


Fig.  46.— Diagram  of  the  ffisoPHAGEAL  Groove  (Carpenter). 

(E,  Oesophagus  entering  the  stomach  ;  c,  its  cardiac  opening ;  rp,  right 
pillar  of  oesophageal  groove  ;  lp,  left  pillar  of  the  same  ;  o,  opening 
into  the  omasum  ;  (Eg,  oesophageal  groove  extending  from  c  to  o, 
aboat  7  inches  in  length.  To  the  right  of  the  figure  is  the  rumen, 
to  the  left  the  reticulum. 

anterior  being  formed  by  the  reticulum,  the  posterior  by 
the  rumen.  The  lips  are  thin  above,  and  thick  below  where 
they  overlap ;  normally  they  lie  in  apposition  in  such  a 
way  as  to  conceal  the  groove,  but  in  both  Figs.  46  and  47 
they  are  intentionally  separated  in  order  to  show  the 
arrangement.  These  pillars  are  composed  of  involuntary 
muscular  fibres  arranged  longitudinally  and  transversely, 
by  which  means  the  groove  can  be  shortened  and  con- 
stricted.    By  a  contraction  of  the  pillars  the  omasum  may 


182     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

be  shut  off  from  the  lirst  and  second  compartments,  and 
broup;ht  nearly  in  apposition  with  the  ccsophagus  ;  or  by 
their  relaxation  the  first  and  second  may  be  made  to  com- 
municate with  the  third  compartment.  When  the  pillars 
are  relaxed  the  cESophagus  communicates  more  directly  with 
the  rumen  and  reticulum.  Anothei-  function  of  the  groove 
was  said  to  be  to  cut  off  a  pellet  of  food  pressed  into  it  by  a 
contraction  of  the  rumen  and  reticulum,  the  pellet  or  bolus 


Fig.  47. — Longitudinal  Section  of  the  Rumen  and  Reticulum 
TO  SHOW  the  Position  of  the  CEsophageal  Groove  in  the 
Living  Animal. 

R-zt,  rumen  ;  the  lettering  is  placed  on  the  muscular  pillars,  which  are 
held  apart.  R^  reticulum.  (E,  a'sophapjus.  R^j,  right  pillar  ;  L^?, 
left  pillar  :  both  lield  widely  apart  to  show  G,  the  groove.  Om, 
opening  leading  to  the  omasum. 


being  then  passed  into  the  oesophagus  for  remastication. 
Colin  has  shown  that  if  the  lips  of  the  canal  be  stitched 
together  rumination  may  still  occur,  so  the  theory  that  the 
bolus  is  formed  between  these  lips  is  not  correct,  and  this 
view  is  supported  by  the  stomach  of  the  llama,  which  only 
possesses  one  pillar. 

Colin's  description  of  the  mechanism  of  rumination  is  as 


DIGESTION  183 

follows.  During  the  churning  movement  the  food  is  gently 
pressed  against  the  lips  of  the  groove,  when,  by  a  spasmodic 
contraction  of  the  diaphragm  and  abdominal  muscles,  some 
of  the  liquid  from  the  reticulum  and  some  of  the  solid  from 
the  rumen  is  carried  up  the  oesophagus,  while  the  latter, 
by  the  contraction  of  its  funnel-shaped  extremity,  cuts  off 
the  bolus,  and  by  its  reversed  peristaltic  action  conveys 
it  to  the  mouth.  In  passing  under  the  velum  palati  the 
liquid  portion  is  squeezed  out  and  is  at  once  reswallowed, 
travelling  to  the  third  compartment,  while  the  solid  mass 
undergoes  grinding.  After  the  bolus  is  reswallowed  it 
may  either  return  to  the  rumen,  or,  if  in  a  finely  com- 
minuted condition,  it  passes  at  once  from  the  oesophagus 
into  the  third  compartment.  The  reticulum  appears  to  be 
only  a  convenient  accessory  to  rumination,  for,  as  previously 
mentioned,  Flourens  excised  it  without  interfering  with  the 
process  of  rumination.  During  the  process  of  rumination 
the  parotid  glands  secrete,  but  not  the  submaxillary  or 
sublingual. 

Eumination  is  a  reflex  nervous  act,  the  centre  for 
which  probably  lies  in  the  medulla.  The  process  can  only 
be  performed  by  means  of  the  united  action  of  the  dia- 
phragm, walls  of  the  stomach  and  abdominal  muscles. 
Hence,  if  the  phrenics  be  divided  rumination  is  carried 
out  with  great  difficulty,  and  only  by  an  extra  effort  of  the 
abdominal  muscles ;  if  the  vagi  be  divided  the  walls  of 
the  stomach  are  paralyzed  and  the  process  cannot  go  on  ; 
if  the  spinal  cord  be  divided  in  the  mid-dorsal  region  the 
abdominal  walls  are  paralysed  and  rumination  can  no 
longer  occur.  The  condition  of  the  stomach  and  its 
contents  also  exercises  an  important  influence  ;  rumination 
can  only  take  place  when  the  organ  contains  a  fair  amount 
of  food  and  a  considerable  quantity  of  liquid.  The  ascent 
of  the  food  in  the  cesophagus  can  be  distinctly  seen  in  the 
neck,  and  sounds  may  be  heard  on  auscultation  due  to  the 
passage  of  the  bolus  with  its  fluid  admixture,  and  the 
friction  of  the  rumen  against  the  diaphragm.  The  amount 
of  each  bolus  has  been  estimated  by  Colin  at  3|  to  4  ozs. ; 


184     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

its  formation  in  the  stomach  and  ascent  occupies  about 
three  seconds,  and  its  descent  after  remastication  one  and 
a  half  seconds  ;  its  remastication  occupies  about  fifty 
seconds.  On  these  data  CoHn  has  calculated  that  at  least 
seven  hours  out  of  the  twenty-four  are  required  for  the 
process  of  rumination. 

Movements  of  the  Stomach  begin  very  shortly  after  food 
is  received.  Waves  of  peristalsis  travel  from  the  middle 
of  the  organ  towards  the  pylorus ;  these  waves  become 
stronger  as  digestion  proceeds,  and  their  function  is  to 
press  the  peptonized  food  against  the  pylorus.  The 
pylorus  is  kept  tightly  closed,  and  only  relaxes  to  allow 
a  stream  of  chyme  to  be  ejected,  which  occurs  with  con- 
siderable force.  The  left  or  o'sophageal  end  of  the  stomach 
in  all  animals  plays  but  a  passive  part,  and  may  be  re- 
garded in  animals  with  a  single  stomach  more  in  the  light 
of  an  oesophageal  dilatation,  a  characteristic  particularly 
indicated  in  the  horse.  There  is  very  little  movement  in 
the  left  end  of  the  stomach,  and  this  permits  starch  con- 
version to  go  on  undisturbed,  especially  in  the  last  portions 
of  food  swallowed. 

It  is  prol)able  that  in  all  animals  with  a  single  stomach 
churning  movements  are  unnecessary,  and  it  is  certain 
they  do  not  occur  in  the  horse,  for  in  feeding  on  three  or  four 
different  foods  they  are  all  found  arranged  in  strata  in 
the  stomach,  in  the  order  of  their  arrival.  In  ruminants, 
on  the  other  hand,  other  movements  are  clearly  indicated  ; 
the  immense  muscular  pillars  of  the  rumen  are  capable 
of  rotating  the  contents,  and  the  formation  of  balls  in  the 
rumen,  from  hair  swallowed  when  licking  the  body,  is  most 
suggestive  of  rotatory  movement.  Eber  of  Dresden  says 
that  in  the  ox  the  rumen  normally  contracts  a  little  more 
than  three  times  in  two  minutes. 

The  relaxation  of  the  pylorus  is  a  distinct  mechanism ;  it 
only  occurs  when  material  is  ready  to  pass  out,  and  not 
with  every  contraction  wave  which  passes  over  the  organ. 
Yet  this  statement  must  be  modified  in  the  case  of  the 
horse,  where,  as  we  have  shown,  owing  to  the  small  size  of 


DIGESTION  185 

the  stomach,  and  the  bulky  nature  of  the  food,  an  amount 
passes  out  at  the  pylorus  equal  to  that  received  at  the 
cardia.  Liquid  foods  readily  pass  the  pylorus,  and  prob- 
ably most  liquids  pass  rapidly  out  of  the  stomach.  It  is 
especially  so  in  the  horse,  in  which  animal  the  water  as 
consumed  sweeps  directly  through  the  stomach,  and  may, 
on  auscultation,  be  heard  passing  along  the  duodenum  to 
the  large  intestines. 

The  movements  of  the  stomach  are  excited  by  the 
presence  of  food,  or  any  irritation  applied  to  the  mucous 
membrane.  These  movements  are  rendered  more  energetic 
by  stimulation  of  the  vagus,  but  even  when  all  the  nerves 
going  to  the  part  are  divided,  the  stomach  can  still  contract, 
which  is  probably  due  to  the  ganglia  contained  in  its  walls. 
The  stomach  is  in  fact  an  automatic  organ.  Both 
pneumogastrics  supply  the  stomach,  the  nerves  being 
non-medullated.  In  addition  it  obtains  sympathetic  fibres 
from  the  solar  plexus,  to  which  the  right  vagus  also  sends 
some  fibres  (see  Fig,  55,  p.  206).  In  the  wall  of  the 
stomach  are  found  ganglia  with  which  both  the  vagus  and 
sympathetic  communicate.  The  vagus  may  be  regarded  as 
the  motor  nerve  of  the  stomach,  while  the  sympathetic  is 
mainly  inhibitory  ;  stimulation  of  the  vagus  leads  to  con- 
traction of  the  stomach  walls,  stimulation  of  the  sympathetic 
causes  dilatation  of  a  contracted  stomach  and  relaxation  of 
the  pylorus.  The  vagus  supplies  the  bloodvessels  with 
dilator  fibres,  whilst  the  sympathetic  supplies  them  with 
constrictor  fibres.  Section  of  the  vagus  in  the  horse  causes 
paralysis  of  the  stomach  and  in  other  animals  ;  if  the  move- 
ments are  not  abolished,  they  are  certainly  diminished. 
The  result  of  stomach  paralysis  is  that  nothing  passes 
on  to  the  intestines,  so  that  in  the  horse  even  large 
poisonous  doses  of  strychnia  may  thus  fail  to  cause 
death  by  lying  inert  in  the  stomach.  This  experiment 
demonstrates  the  uselessness  of  giving  medicine  by  the 
mouth  in  many  cases  of  digestive  trouble  in  the  horse ; 
the  material  lies  in  the  stomach  owing  to  paralysis 
of    the    organ,    and    is    never    absorbed.     The    secretory 


186     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

nerves   of   the   gastric   glands    have   been   dealt    with    on 
p.  169. 

The  nervous  mechamsm  of  tJie  stomach  of  ruminants  is 
derived  mainly  from  the  vagus,  excepting  for  the  third 
compartment,  which  has  a  separate  and,  at  present,  un- 
known source  of  supply.  Stimulation  of  the  vagus  was 
found  by  Ellenberger  to  produce  energetic  contraction  of 
the  reticulum,  slow  kneading  movements  of  the  rumen, 
and  slower  and  later-appearing  peristaltic  contractions  of 
the  abomasum,  but  no  contraction  of  the  omasum.  Section 
of  both  vagi  was  found  to  paralyse  the  oesophagus,  rumen, 
and  reticulum,  followed  by  tympany  of  the  rumen.  Ellen- 
berger could  not  obtain  any  effect  on  the  stomach  move- 
ments by  stimulating  the  sympathetics. 

Section  3. 
Intestinal  Digestion. 

The  chyme  which  is  poured  from  the  stomach  into  the 
small  intestines  meets  there  with  three  digestive  fluids, 
viz.,  the  succus  entericus,  the  bile,  and  the  pancreatic  juice. 

The  Succus  Entericus  is  prepared  by  the  glands  of  the 
small  intestines ;  in  the  duodenum  the  glands  of  Brunner 
are  found,  whilst  the  follicles  of  Lieberkiihn  are  met  with 
throughout  the  whole  of  the  small  and  large  intestines. 
Lieberkiihn's  crypts  supply  a  considerable  proportion  of 
intestinal  juice,  while  the  secretion  from  the  glands  of 
Brunner  is  scanty.  Brunner' s  glands,  which  are  very 
large  in  the  horse,  are  arranged  on  the  same  principle 
as  the  gastric  glands,  while  those  of  Lieberkiihn  are 
tubular  glands,  amongst  the  cylindrical  epithelial  cells  of 
which  numerous  mucus-forming  goblet  cells  may  be  found. 

At  one  time  it  was  considered  that  the  succus  entericus 
was  a  comparatively  unimportant  fluid,  the  chief  function 
of  which  was  to  neutralise  the  acid  chyme  ;  Colin,  however, 
showed  that  in  the  horse  it  had  a  distinctly  digestive  effect. 
It  is  now  known  that  though  a  pure  secretion  of  Lieber- 
kiihn's crypts  has  little  or  no  digestive  action  excepting 


DIGESTION  187 

on  starch,  an  extract  of,  and  juice  squeezed  from  the 
intestinal  wall  has  a  most  important  function.  The  Lieber- 
kiihn  fluid  is  quantitatively  small  in  amount,  and  alkaline 
in  reaction  due  to  carbonate  of  soda.  The  intestinal  extract, 
on  the  other  hand,  contains  three  enzymes,  and  in  addition 
a  peculiar  chemical  substance  of  remarkable  properties. 
The  enzymes  are  : 

1.  Enterokinase,  which  converts  the  trypsinogen,  the 
mother  substance  of  the  pancreatic  proteolytic  enzyme,  into 
trypsin. 

2.  Erepsin,  also  a  proteolytic  ferment,  which  supplements 
the  work  of  trypsin,  acting  on  deutero  -  albumoses  and 
peptones,  breaking  them  up  into  amido-acids  and  hesone 
bases. 

3.  Inverting  ferments,  converting  double  sugars  which 
cannot  be  utilized  by  the  tissues  into  single  sugars  which 
can.     Of  inverting  ferments  there  are  three  : 

Maltase,  converting  maltose  and  dextrin  into  dextrose. 

Invertase,  converting  cane  -  sugar  into  dextrose  and 
levulose. 

Lactase,  converting  milk-sugar  into  dextrose  and  galac- 
tose. 

Finally,  the  intestinal  fluid  contains  secretin,  which  is  not 
a  ferment  but  a  chemical  substance  found  in  the  walls  of 
the  small  intestines ;  this  when  taken  into  the  blood 
possesses  the  singular  property  of  causing  the  secretion 
of  pancreatic  juice. 

Enterokinase  and  secretin  will  be  dealt  with  in  our 
consideration  of  the  pancreas. 

Intestinal  Digestion  in  the  Horse. — The  contents  of  the 
stomach  are  neutralised  by  the  pancreatic  and  biliary 
secretions  immediately  or  shortly  after  they  leave  the 
stomach.  So  much  is  this  the  case  that  on  the  duodenal 
side  of  the  pylorus  the  reaction  of  previously  acid  chyme  is 
neutral,  and  a  few  inches  along  the  duodenum  it  is  alkaline; 
this  alkaline  reaction  is  at  first  faint,  but  becomes  more 
marked  as  the  ileum  is  approached.  Ellenberger  describes 
the  contents  of  the  small  intestines  as  being  acid  in  the 


188  A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

first  two-thirds  of  their  length,  then  neutral  as  far  as  the 
ileum,  where  they  become  alkaline  ;  we  have  only  once 
found  them  otherwise  than  alkaline  throughout.  He  further 
states  that  in  the  fasting  horse  the  contents  are  alkaline, 
but  that  in  the  digesting  animal,  whether  horse,  ox,  or 
sheep,  they  are  acid,  the  acidity  decreasing  after  passing 
the  common  duct,  and  becoming  decidedly  alkaline  at  the 
posterior  portion  of  the  small  intestine.  This,  as  we  have 
said,  does  not  agree  with  our  experience  in  the  horse ;  it  is 
usual  to  find  the  contents  of  the  duodenum  next  the  pylorus 
neutral,  and  from  this  point  the  bowel  is  faintly  alkaline, 
the  reaction  increasing  in  intensity  up  to  the  ileum,  where 
the  contents  are  always  markedly  alkaline.  We  have  only 
once  found  the  small  bowels  acid  in  the  horse,  no  matter 
what  diet  has  been  given,  or  at  what  period  of  digestion  the 
examination  has  been  made ;  a  neutral  or  faintly  alkaline 
reaction  in  the  anterior  part  of  their  course,  and  marked 
alkalinity  in  the  posterior  portion,  is  doubtless  the  rule 
rather  than  the  exception. 

The  arrangement  of  the  small  intestines  suspended  or 
dangling  in  festoons  from  the  spine  through  the  medium 
of  a  very  delicate  membrane  is  a  construction  the  ad- 
vantages of  which  are  not  very  apparent.  It  appears  to 
invite  trouble.  The  long  mesentery  is  considered  to  favour 
volvulus,  ])ut  no  doubt  the  chief  cause  of  this  latter  trouble 
is  tympany.  If  the  bowels  be  artificially  distended  with 
air,  loops  of  them  behave  in  such  a  way  as  would  lead  to 
twist  in  the  living  animal. 

Physical  Characters  of  the  Chyme. — The  chyme  having 
passed  into  the  bowel  its  appearance  at  once  changes,  for 
the  acid  albumin  is  precipitated  by  the  alkaline  secretion 
found  there.  It  is  now  observed  that  the  material  consists 
of  clots  floating  or  suspended  in  a  yellowish  fluid,  extremely 
slimy  in  nature,  and  resembling  in  appearance,  through  its 
precipitated  albumin,  nasal  mucus  suspended  in  fluid.  The 
proportion  of  mucin  must  be  considerable  judging  from  its 
ropiness  when  poured  from  one  vessel  to  another,  and  this 
mucus    is    probably   largely   derived   from   the    stomach. 


DIGESTION  189 

Throughout  the  small  intestmes  the  character  of  the  chyme 
is  as  follows,  viz.,  a  yellow,  frothy,  precipitated,  slimy 
fluid,  the  material  from  the  anterior  part  of  the  intestinal 
canal  having  a  peculiar  mawkish  smell,  whilst  that  from 
the  region  of  the  ileum  is  of  a  distinctly  faecal  odour ;  the 
latter  is  due  to  indol  and  skatol  formed  putrefactively 
during  pancreatic  digestion.  In  the  ileum  the  proportion 
of  fluid  material  is  considerably  reduced  in  amount,  and 
the  character  of  the  ingesta  may  now  be  recognised,  which 
was  previously  almost  impossible. 

Function  of  the  Ileum. — As  the  flow  of  material  into  the 
small  intestines  is  controlled  by  a  sphincter,  so  is  the  flow 
out  of  it.  The  ileum  is  a  remarkably  thick  and  powerful 
bowel,  it  is  always  found  contracted  and  containing  material 
which  is  dry  compared  with  that  found  in  the  anterior 
portion  of  the  intestine.  One  of  the  functions  of  the  ileum 
is  to  control  the  passage  of  material  into  the  caecum.  Colin 
describes  the  chyme  in  the  horse  as  circulating  between 
the  i^ylorus  and  ileum,  viz.,  that  it  is  poured  backwards 
and  forwards  in  order  to  expose  it  sufficiently  to  the 
absorbent  surface ;  this  necessitates  a  reversed  peristaltic 
action.  He  says  that  were  it  not  for  this  the  material 
could  not  be  acted  upon  and  absorbed,  as  the  passage  of 
fluid  through  the  small  intestines  is  very  rapid.  It  would 
have  been  impossible  to  reason  out  that  the  fluid  material 
of  the  small  intestines  was  passed  to  and  fro  between  the 
stomach  and  the  ileum,  exposed,  as  Colin  expresses  it, 
twenty  times  over  to  the  absorbent  surface  of  the  bowels. 
This  observation  must  have  been  made  as  the  result  of  his 
examination  of  the  living  animal,  and  there  can  be  no  doubt 
of  its  correctness. 

Experiment  shows  that  water  will  pass  from  the  stomach 
to  the  caecum  in  from  five  to  fifteen  minutes.  By  applying 
the  ear  over  the  duodenum,  as  it  passes  under  the  last  rib 
on  the  right  side,  the  water  which  a  horse  at  that  moment 
is  drinking  may  be  heard  rushing  through  the  intestines 
on  its  way  to  the  caecum.  One  is  always  struck  by 
the  fact  that  the  small  intestines  are  never  seen  full,  in 


190     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

fact,  are  often  practically  empty,  from  which  we  judge 
either  that  material  passes  very  rapidly  through  them,  or 
that  only  small  amounts  of  chyme  are  propelled  into  them 
at  a  time.  The  contents  are  always  in  a  liquid  condition 
excepting  at  the  ileum,  the  fluid  being  derived  from  the 
secretions  poured  into  and  those  originating  in  the  bowel. 
That  active  absorption  goes  on  in  the  intestines  is  proved 
by  the  difference  in  the  physical  characters  of  the  contents 
in  their  several  parts.  The  rate  at  which  the  chyme  passes 
through  the  small  intestines  varies  with  the  nature  of  the 
food,  and  the  frequency  with  which  the  horse  is  fed.  Ellen- 
berger  says  it  reaches  the  Ccnecum  six  hours  after  feeding, 
but  has  not  entirely  passed  into  this  bowel  for  twelve 
or  even  twenty  hours ;  we  have  known  it  reach  the  ctecum 
in  four  hours. 

In  the  small  intestines  the  chyme  meets  with  the  bile  and 
pancreatic  juice  ;  the  action  of  these  on  food  is  described  in 
the  chapter  dealing  with  the  liver  and  pancreas.  The 
absorption  of  chyle,  and  its  elaboration  before  reaching  the 
blood,  are  points  which  must  be  reserved  for  the  chapter  on 
'  Absorption.' 

Large  Intestines. — There  can  be  no  doubt  that  in  solipeds 
digestion  in  the  large  intestines  is  a  very  important  process, 
at  least,  we  judge  so  from  the  fact  of  their  enormous 
development.  In  many  respects  they  present  a  consider- 
able contrast  to  the  small  intestines ;  for  instance,  they  are 
always  found  filled  with  ingesta,  the  contents  are  more 
solid,  the  material  lies  a  considerable  time  in  them,  and 
there  are  no  juices  other  than  the  succus  entericus  poured 
into  the  bowel.  These  are  conditions  exactly  the  reverse 
of  those  found  in  the  small  intestines.  The  bowels  which 
are  spoken  of  as  the  large  intestines  are  the  csecum,  double 
and  single  colon,  and  the  rectum. 

The  Caecum  has  been  described  by  Ellenberger  as  a  second 
stomach ;  its  enormous  capacity  and  fantastic  shape  have 
always  rendered  it  an  intestine  of  considerable  interest 
(Fig.  48).  To  our  mind  its  most  remarkable  feature  is 
that  it  is  a  bag  the  openings  into  and  out  of  which  are  both 


DIGESTION  191 

found  at  the  upper  part  close  together  ;  the  exit,  strange  to 
say,  is  above  the  inlet,  and  the  contents  have  to  work 
against  gravity  in  order  to  obtain  an  entr}'  into  the  next 
intestine,  the  double  colon.  This  is  brought  about  by  the 
four  muscular  bands  on  the  ca?cum  (Fig.  49),  which  shorten 
the  bowel,  forcing  the  contents  upwards  towards  the  'crook.' 
The  ileum  being  closed,  the  only  available  outlet  is  into  the 
colon  (Fig.  48). 

Several  questions  suggest  themselves  regarding  the  com- 


FiG.  48. — C^OUM    OF    THE   HoRSE    IN    POSITION,  ITS   InNER  FaCE 
BEING    SEEN. 

1,  The  first  colon  ;  2,  the  ileum. 

munication  between  the  large  and  small  intestines.  It  is 
certain  that  in  order  to  get  from  the  ileum  into  the  colon 
everything  must  pass  into  or,  at  any  rate,  through 
the  caecum,  yet  we  are  assured  that  material  does  not 
remain  there  long.  Could  it  be  possible  for  the  opening 
of  the  ileum  and  that  of  the  colon  to  be  so  brought  to- 
gether that  material  might  pass  direct  from  one  into  the 
other  ?  (Fig.  50.)  Nothing  is  returned  into  the  ileum 
from  the  cascum  ;  there  must  be,  in  consequence,  a  sphincter 
keeping  the  ileum  closed,  for  when  the  caecum  contracts 


192     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

material  must  cross  the  opening  of  the  ileum  in  order  to 
reach  the  colon.  This  sphincter  is  furnished  by  the 
thickened  condition  of  the  wall  of  the  ileum.  We  see  no 
difficulty  in  believing  that  the  rigid  end  of  this  tul)e  may 
pass  its  contents  practically  direct  into  the  colon,  and  the 
slightly  funnel-shaped  arrangement  of  the  latter  would 
readily  admit  the  rigid  nozzle  of  the  ileum. 

The  contents  of  the  caecum  are  always  fluid,  some- 
times quite  watery,  occasionally  of  the  colour  and  consist- 
ence of  pea-soup,  in  which  condition  they  are  full  of  gas 


Fig.  49. — Schematic  Arrangkment  of  the  Longitudinal  Muscular 
Bands  of  the  C^cum. 

Bands  1  and  2  are  one,  and  form  a  complete  sling  for  the  bowel  ; 
band  4  runs  from  the  cttcum  to  the  pelvic  fiexure  of  the  colon. 
It  is  a  remarkable  band,  and  doubtless  intimately  connected  with 
the  mechanism  which  brings  about  the  passage  of  material  from 
caecum  to  colon. 

bubbles  ;  when  watery  the  fluid  is  generally  brownish  in 
colour,  with  particles  of  ingesta  floating  about  in  it.  The 
reaction  of  the  contents  is  always,  alkaline  ;  all  observers 
are  agreed  on  this  point.* 

The  caicum  is  most  admirably  arranged  as  a  receptacle  for 
fluids,  and  though  absorption  undoubtedly  occurs  from  it,  and 
digestion  of  cellulose  takes  place  in  it,  yet  we  believe  its  chief 
function  is  the  storing  up  of  water  for  the  wants  of  the  body 
and  the  digestive  requirements,  as  it  is  absolutely  certain 

*  We  once  found  the  ciecum  acid. 


DIGESTION 


193 


that  digestion  in  the  horse  can  only  be  properly  carried  out 
when  the  contents  are  kept  in  a  fairly  fluid  condition.  We 
do  not  say  that  the  cnecum  produces  no  digestive  changes 
in  the  food,  for  we  have  stated  that  the  contents  are 
occasionally  of  the  consistence  of  pea-soup,  but  we  consider 
its  digestive  function  subordinate  to  its  water-holding  one. 
Ellenberger  views  the  caecum  as  a  bowel  for  the  digestion 
of  cellulose,  where  by  churning,  maceration,  and  decom- 
position, this  substance  is  dissolved  and  rendered  fit  for 


y" 


-.-.2 


Fig.  50. — The  Opening  of  the  Ileum  and  Colon  in  the  C^cum. 

1,  The  ileum  ;  2,  the  colon.  In  the  figure  the  openings  are  represented 
close  together,  but  even  when  stretched  apart  they  are  less  than 
4  inches  distant. 

absorption,  and  he  likens  it  to  the  stomach  of  ruminants 
and  the  crop  of  birds.  He  further  considers  that  the  caecum 
exists  owing  to  the  small  size  of  the  stomach,  and  the 
rapidity  with  which  the  contents  are  sent  along  the  small 
intestines.  His  experiments  demonstrated  that  the  entire 
'  feed  '  reached  the  caecum  between  12  and  24  hours  after 
entering  the  stomach,  that  it  remained  24  hours  in  the 
caecum,  and  that  during  this  time  10  to  30  per  cent,  of  the 
cellulose  disappeared. 

13 


194     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

The  digestion  of  cellulose  is  no  doubt  a  very  important 
matter,  especially  as  we  know  that  the  poorer  the  food  the 
more  cellulose  digested ;  but  we  are  not  prepared  to  admit 
that  food  necessarily  remains  in  the  csecum  24  hours,  and  we 
believe  that  cellulose  digestion  occurs  principally,  though  not 
entirely,  in  the  colon,  and  further,  that  it  is  not  absolutely 
necessary  the  material  should  remain  in  the  caecum,  but 
that  it  may  pass  on  at  once  to  the  colon.  Our  experiments 
on  digestion  have  shown  that  ingesta  may  reach  the  caecum 
3  to  4  hours  after  entering  the  mouth,  and  we  are  quite 
clear  on  the  point  that  oats  may  travel  some  considerable 
distance  along  the  colon  in  4  hours  from  the  time  of  being 
consumed,  though  this  is  regarded  as  exceptionally  rapid. 
A  horse  which  had  never  had  maize  and  had  not  tasted  oats 
for  two  or  three  years,  was  fed  first  with  2|  lbs.  of  maize, 
and  17  hours  later  with  4  lbs.  of  oats.  The  animal  was 
destroyed  4  hours  from  the  time  of  commencing  to  eat  the 
oats.  Much  maize  and  a  few  oats  were  found  in  the  pelvic 
flexure  of  the  colon,  and  a  certain  proportion  of  maize  and 
a  quantity  of  oats  in  the  stomach.  In  21  hours  the  small 
ration  of  2^  lbs.  of  maize  was  distributed  between  the 
stomach  and  pelvic  flexure  of  the  colon,  which  is  a  very 
large  area.  In  4  hours  the  oats  reached  the  same  point 
in  the  bowel  that  the  maize  had  arrived  at ;  this  is  excep- 
tionally rapid,  but  this  experiment  supports  two  points  it  is 
desired  to  emphasize,  viz.,  the  difficulty  in  getting  the 
stomach  to  empty  itself  completely,  and  the  rapid  transit 
of  material  through  the  small  intestines. 

Colin  believes  that  in  the  caecum  starch  can  be  converted 
into  sugar,  fats  emulsified,  and  the  active  absorption  of 
assimilable  matters  occur. 

The  Colon. — The  direction  taken  by  the  colon  of  the  horse 
is  remarkable.  It  commences  high  up  under  the  spine  on 
the  right  side,  its  origin  being  very  narrow,  but  it  immedi- 
ately becomes  of  immense  size;  it  descends  towards  the 
sternum,  and  curving  to  the  left  side,  rests  on  the  ensiform 
cartilage  and  inferior  abdominal  wall.  The  colon  now 
p,scends  towards  the  pelvis,  and  here  makes  a  curve,  the 


DIGESTION 


195 


bowel  becoming  very  narrow  in  calibre :  the  pelvic  flexure 
having  been  formed,  the  intestine  retraces  its  steps  towards 
its  starting  point.  Eunning  on  top  of  the  previously 
described  portion  it  descends  towards  the  diaphragm,  gradu- 


FiG.  51. — The  Double  Colon  looked  at  from  Above  (modified 

FROM    MuLLER). 

1,  The  first  colon,  the  caecum  being  removed  ;  2,  the  pelvic  flexure,  the 
bowel  being  narrow ;  3,  the  colon  suddenly  enlarges  ;  4,  its 
diaphragmatic  flexure ;  5,  the  single  colon.  Several  of  the  bands 
are  seen  ;  note  also  the  sacculated  and  non-sacculated  portions  of 
the  bowels. 

ally  growing  larger  in  calibre,  and  then  ascends  towards  the 
loin,  being  here  of  immense  volume — in  fact,  at  its  largest 
diameter ;  it  then  suddenly  contracts,  and  forms  the  single 
colon  (Figs.  51  and  52).  The  object  of  the  difference  in 
the  volume  of   the  double   colon   appears   to   be   for  the 

13^2 


19G     A  MANUAL  OF  VETERINAKY  PHYSIOLOGY 

convenience    of    its    accommodation    in    the    abdominal 
cavity. 

The  double  colon  may  for  the  purpose  of  description  be 
divided  into  four  portions :  the  ingesta  in  the  first  and 
third  descend,  in  the  second  and  fourth  ascend.  It  is 
found  that  the  physical  characters  of  the  contents  are  not 


.^. 


1 


Fig.  52. — Position  of  the  C^cum  and  Double  Colon  on  the  Floor 
OF  THE  Abdomen  seen  from  Below. 

The  point  of  the  csecum  is  directed  towards  the  sternum. 

the  same  throughout.  In  the  first  colon  the  food  is  fairly 
firm,  and  the  particles  of  corn,  etc.,  can  be  readily  recog- 
nised ;  in  the  second  colon  the  material  is  becoming  more 
fluid,  whilst  at  the  pelvic  flexure  the  contents  are  invariably 
in  a  liquid  pea-soup-like  condition,  and  the  particles  of 
which  they  are  composed  are  not  readily  recognised.     In 


DIGESTION  197 

the  third  colon  the  material  becomes  firmer,  but  only 
slightly  so,  and  bubbles  of  gas  are  being  constantly  given 
off  from  its  surface ;  in  the  fourth  colon  the  entire  ingesta 
are  like  thick  soup,  and  the  material  composing  them  is  in 
a  finely  comminuted  condition,  the  surface  being  covered 
with  gas  bubbles.  For  the  first  foot  or  so  of  the  single 
colon  this  condition  is  maintained,  when  quite  suddenly 
the  contents  are  found  solid  and  formed  into  balls.  The 
remarkable  suddenness  of  this  change  is  invariable  in  a 
state  of  health,  and  indicates  either  most  active  absorption, 
or  that  the  contents  are  subjected  to  great  compression. 
The  entire  contents  of  the  colon  are  yellow  in  colour  or 
yellowish  green,  rapidly  becoming  brown  or  olive-green  on 
exposure  to  the  air  ;  the  colour  being  due  to  the  chlorophyll 
of  the  food.  The  contents  of  the  colon  are  normally  alka- 
line throughout ;  we  once,  however,  found  them  acid. 

Digestive  Changes. — The  changes  food  undergoes  in  the 
large  intestine  have  never  excited  the  same  interest  as 
those  in  the  small.  The  absence  of  any  secretion  from 
the  large  bowel  other  than  the  succus  may  help  to 
account  for  this,  and  may  also  assist  in  explaining 
why  the  largo  bowels  have  been  regarded  in  the  light 
of  reservoirs  for  ingesta,  rather  than  as  active  centres 
01  digestion.  As  a  matter  of  fact,  the  large  intestines  of 
the  horse  are  actively  employed  in  dealing  with  cellulose, 
not  by  means  of  any  known  enzyme  peculiar  to  the  body, 
but  rather  by  the  process  of  bacterial  disintegration,  the 
result  of  decomposition.  It  is  known  that  bacteria  may 
hydrolize  cellulose  and  render  it  fit  for  absorption.  In  the 
case  of  oats  we  mentioned,  p.  171,  that  they  probably, 
furnished  their  own  cellulose  enzyme,  but  this  has  not 
been  proved  for  all  vegetable  material.  The  cellulose  of 
hay  is,  probably,  only  extracted  after  prolonged  maceration 
in  the  large  intestines  and  the  subsequent  attack  of  bacteria. 
By  some,  it  has  been  considered  that  the  epithelial  cells  of 
the  intestine  are  capable  of  dealing  with  cellulose,  but  on 
this  point  no  definite  statement  can  be  made.  Cellulose 
yields  energy  to  the  body  on  oxidation,  but  there  is  another 


198     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

reason  for  the  extensive  preparations  made  for  its  digestion 
in  herbivora,  viz.,  the  cellulose  encloses  the  proteid,  starch, 
and  fat  of  vegetable  substances  in  a  frame-work,  and  until 
this  is  broken  down  these  substances  cannot  be  acted  upon. 
We  know  that  considerable  cellulose  solution  must  occur 
before  the  material  arrives  at  the  large  intestines,  otherwise 
neither  in  the  stomach  nor  small  intestine  could  digestion 
occupy  the  prominent  position  it  does.  The  digestion  of 
proteid,  fat  and  sugar  are  largely,  though  not  entirely, 
dealt  with  in  the  stomach  and  small  intestine,  but  there 
must  be  a  certain  amount  of  these  substances  so  firmly 
locked  up  in  their  cellulose  envelope  that  they  are  not 
liberated  until  after  prolonged  maceration  and  digestion  in 
the  large  intestines.  We  may,  therefore,  safely  assume  that 
proteid,  fat,  starch,  and  cellulose  are  capable  of  being  acted 
upon  and  absorbed  from  the  large  bowels  of  the  horse. 

As  the  result  of  cellulose  digestion  carbonic  acid  and 
marsh  gas  are  formed  in  equal  volumes.  We  have  in  our 
description  of  the  large  bowels  drawn  attention  to  the 
appearance  of  the  caecum  and  fourth  portion  of  the  double 
colon,  with  their  pea-soup-like  contents,  on  the  surface  of 
which  gas  bubbles  are  constantly  breaking.  It  may  well 
be  that  these  two  places  are  the  active  seats  of  the  final 
transformation  of  cellulose,  the  caecum  dealing  with  that 
which  has  already  been  acted  upon  in  the  stomach  and 
small  intestines,  and  the  fourth  colon  being  concerned  with 
the  more  refractory  cellulose,  which  has  required  prolonged 
maceration  in  the  large  intestines  before  becoming  capable 
of  solution.  This  is  rather  supported  by  the  remarkably 
rapid  change  in  the  character  of  the  contents  in  the  single 
colon,  the  pea-soup-like  condition  giving  way,  in  the  space 
of  a  few  inches,  to  the  appearance  presented  by  ordinary 
normal  faeces. 

The  large  intestines  cannot  exist  entirely  for  the  solution 
of  cellulose.  There  are  other  processes  going  on,  chief  of 
which  is  the  bacterial  attack  on  the  unabsorbed  proteid 
products  of  the  small  intestines.  The  small  intestine  may 
be  regarded  as  free  from  putrefactive  processes,  in  fact  it 


DIGESTION  ID!) 

is  only  towards  the  ileum  that  the  unpleasant  products  of 
pancreatic  digestion  can  be  detected.  In  the  large  intestine, 
on  the  other  hand,  putrefactive  processes  are  evident 
throughout ;  the  bacteria  are  here  engaged,  among  other 
things,  in  attacking  the  unabsorbed  products  of  proteid 
digestion  and  reducing  them  to  simpler  end-products,  such 
as  peptones,  proteoses,  amido-acids,  indol,  skatol,  phenol, 
phenyl-proprionic,  phenyl-acetic  and  fatty  acids,  with  the 
evolution  of  CO..,  Ho,  H.,S,  and  CH^.  These  end-products 
are  got  rid  of  either  through  the  fteces,  or  they  are  absorbed 
into  the  blood,  taken  to  the  kidneys,  and  combined  with 
sulphuric  acid  are  got  rid  of  through  the  urine ;  especially 
is  this  the  case  with  phenol,  indol,  and  skatol. 

As  the  material  moves .  towards  the  rectum  it  becomes 
drier  and  drier,  and  more  thoroughly  formed  into  balls  by 
the  action  of  the  bowel-sacs,  which  squeeze  the  mass  into  a 
round  or  oval  shape.  The  contents  of  this  portion  are  still 
alkaline,  or  slightly  so.  As  we  approach  the  anus  a  dis- 
tinctly acid  reaction  is  obtained  on  the  surface  of  the  faeces, 
though  at  this  time  the  interior  of  the  ball  may  be,  and 
often  is,  alkaline  ;  the  converse  of  this  may  also  be  obtained. 
In  the  rectum  the  single  balls  collect  in  masses,  to  be 
forced  out  of  the  body  at  the  next  evacuation.  The  reaction 
of  this  mass  is  acid,  and  the  colour  depends  on  the  food, 
being,  on  an  ordinary  diet,  of  rather  a  reddish-yellow  or 
brownish  tint  due  to  altered  chlorophyll. 

Absorption  from  the  single  colon  and  rectum  is  rapid ; 
the  marked  change  in  the  physical  character  of  the  fieces 
is  evidence  of  this.  Animals  may  also  be  killed  by  the 
rectal  injection  of  strychnine  ;  narcosis  can  be  produced  by 
the  rectal  administration  of  ether,  and  life  may  be  sup- 
ported, at  any  rate  for  a  short  time,  by  means  of  nutrient 
enemata. 

Intestinal  Digestion  in  Ruminants. — Though  intestinal 
digestion  is  so  important  in  the  horse,  it  would  appear 
in  ruminants  to  occupy  a  subordinate  position.  It  is 
curious  why  in  one  animal  the  changes  should  occur  at 
the  anterior,  and  in  the  other  at  the  posterior  part  of  the 


200    A  MANUAL  OF  VETERINARY  RHYSIOLOGY 


digestive  tract,  but  this  difference  in  the  arrangement  for 
digesting  cellulose  depends  upon  one  being  capable  of 
rumination  and  the  other  not.  The  rumen  of  the  ox 
corresponds  to  the  large  intestines  of  the  horse.  The 
intestines  of  the  ox  are  of  extreme  length  but  small  in 
calibre  ;  they  are  half  as  long  again  as  those  of  the  horse, 
and  it  would  appear  that  their  chief  function  is  that  of 
absorption.  Their  arrangement,  especially  that  of  the 
large  intestine,  is  most  singular.  The  small  intestines 
are   hung    in    convolutions    on    a    mesentery ;    they   are 


Fig.  53. — Schematic  Arrangement  of  the  Intestines  of  the  Ox. 

1,  The  small  bowels ;  2,  the  caecum  ;  3,  the  '  spiral '  colon  ; 
4,  the  single  colon. 

narrow  in  diameter  and  about  120  feet  in  length.  The 
large  intestines  are  about  30  feet  in  length,  also  narrow 
and  without  muscular  bands  or  puckerings  as  in  the 
horse ;  the  colon  is  arranged  in  a  remarkable  spiral  manner 
between  the  folds  of  the  mesentery  (see  Fig.  53).  It  is  in 
this  immense  length  of  absorbent  surface  that  the  food 
substances  capable  of  being  utilized  are  taken  up.  It  is 
clear,  however,  that  certain  digestive  changes  occur  in  the 
small  intestines,  into  which,  as  in  other  animals,  the 
pancreatic  and  biliary  fluids  are  poured.  Here  the  proteids 
which  have  escaped  the  stomach,  and  the  fats  and  starches 


DIGESTION  201 

are  rapidly  changed  and  rendered  fit  for  assimilation  ;  the 
altered  cellulose  in  all  probability  only  finds  its  way  here 
when  fit  for  absorption  after  its  digestion  in  the  rumen. 

Intestinal  Digestion  in  other  Animals. — In  the  pig  intestinal 
digestion  is  said  to  be  of  short  duration,  and  absorption 
very  rapid.  In  the  dog  the  material  passes  out  of  the 
stomach  slowly  and  only  in  small  quantities  into  the  small 
intestines,  which  are  usually  found  collapsed.  It  is  in  the 
small  intestines  of  this  animal  that  the  chief  digestion 
occurs,  as  the  large  bowels  are  rudimentary. 

In  the  sheep,  ox,  pig,  and  dog,  the  reaction  of  the 
contents  of  the  small  intestines  is  acid  anteriorly  and 
alkaline  towards  the  ileum ;  probably  in  all  animals  the 
contents  of  the  large  intestines  are  alkaline  in  reaction. 

Munk  gives  the  following  statistics  respecting  the  in- 
testinal canal.  In  the  tiger  and  lion  the  whole  digestive 
tract  is  3  times  the  length  of  the  body,  in  the  dog  5  times, 
man  9  times,  horse  12  times,  pig  16  times,  and  ox  20 
times.  The  comparative  shortness  of  the  intestinal  canal 
of  the  horse  is  compensated  by  its  enormous  capacity, 
which  is  352  pints ;  in  the  ox  140  pints,  pig  47  pints,  dog 
14  pints.  The  area  of  the  intestinal  tract  is  also  given  by 
the  same  observer — horse  550  square  feet,  ox  160  square 
feet,  pig  32  square  feet,  and  dog  5h  square  feet  (M'Kendrick). 

Movements  of  the  Intestines. — The  movements  of  the 
intestines  are  brought  about  by  the  involuntary  muscle 
composing  its  wall.  This  muscle  in  the  small  intestines 
is  arranged  in  two  sheets  in  a  circular  and  longitudinal 
manner,  while  in  the  large  intestines  narrow  bands  of  pale 
muscle  of  considerable  length  take  the  place  of  the  ordinary 
longitudinal  layer,  and  may  be  found  on  all  parts  where 
the  tube  is  sacculated.  In  fact,  one  function  of  the  bands 
is  to  bring  about  the  sacculated  condition  of  the  canal, 
an  important  arrangement  whereby  economy  of  space  is 
effected  with  no  loss  of  surface. 

The  sacculated  condition  of  the  double  colon  is  confined 
principally  to  the  first  and  second  and  fourth  portions.  The 
third  portion  especially  at  the  pelvic  flexure  is  free  from 


202     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

sacculations,  and  the  fourth  portion  is  not  so  liberally 
puckered  as  the  first  and  second.  On  the  first  colon  there 
are  four  bands,  on  the  second  colon  there  are  also  four, 
three  of  which  disappear  at  the  pelvic  flexure ;  on  the  third 
portion  there  is  only  one  band,  while  on  the  fourth  colon 
there  are  three  (see  Fig.  54,  also  Figs.  51  and  52).  In  the 
large  intestines  the  longitudinal  layer  of  fibres  is  confined 
to  the  muscular  bands,  so  that  the  great  bulk  of  the  wall 
consists  of  circular  muscle  only.  The  longitudinal  bands 
shorten  the  bowel,  but  the  main  work  in  pressing  the 
contents  along  is  performed  by  the  circular  layer.  The 
bands,  in  fact,  are  numerous  where  the  intestine  is  large, 

FLEXURES 
STERN/Vl.  PELVIC  DIAPHRgJ'C 


1  >■; 

\       """o 

* 

• 

C^CUM         — \ 

*6t 

-m          COLON 

COLON 

Fig.  54.— Schematic 

1          2*^*^      * 

'      COLON       I 

Arrangement 

COUON 
OF    THE 

:      4^ 

:     COUOIM 

Muscular  Bands  on 

THE  Double  Colon. 

The  colon  is  supposed  to  be  opened  out  into  a  straight  tube.  Bands 
1,  2,  and  '6  run  from  the  first  colon  to  the  pelvic  flexure;  one  of 
the  three  actually  comes  from  the  apex  of  the  caecum.  No.  4  is 
the  only  band  running  the  whole  length  of  the  bowel.  Nos.  5 
and  6  originate  in  the  region  of  the  third  colon,  and  finally  run 
on  to  the  single  colon. 

and  reduced  in  number  where  the  bowel  becomes  smaller. 
This  arrangement  suggests  that  they  may  under  suitable 
conditions  produce  an  irregularity  of  pull,  and  we  can  see 
no  other  explanation  of  displacement  of  the  large  intestines 
of  the  horse  (a  matter  dealt  with  more  fully  at  the  end  of 
this  chapter)  than  through  the  medium  of  these  muscular 
bands. 

The  muscular  movements  of  the  large  intestine  are 
slower  than  those  of  the  small  bowels ;  possibly  one  reason 
for  this  may  be  that  the  food  has  to  remain  a  longer  time 
in  contact  with  the  absorbing  surface,  viz.,  for  at  least 
forty-eight   hours,    and   for   as   long   as   four   days.     The 


DIGESTION  203 

peristaltic  movement  of  the  small  intestines  is  quite 
distinct  from  that  of  the  large ;  the  one  ends  at  the  ileum, 
the  other  begins  at  the  caecum. 

The  muscle  of  the  intestinal  wall  causes  the  movement 
known  as  2^<^ristalsis,  which  normally  passes  in  the  direction 
stomach  to  rectum.  Relatively  quick  in  the  small  intestines 
it  becomes  slower  and  more  deliberate  in  the  large,  but  the 
wave  has  always  the  one  object  in  view,  viz.,  to  press  the 
ingesta  onward.  A  wave  of  contraction  passing  the  reverse 
way,  viz.,  in  the  direction  of  rectum  to  stomach,  is  known 
as  antiperistaltic  :  such  a  movement  is  considered  abnormal, 
but  in  the  horse,  according  to  the  observations  of  Colin, 
antiperistalsis  of  the  small  intestines  is  a  natural  con- 
dition. Some  physiologists  recognize  antiperistaltic  move- 
ments of  the  large  intestines  as  being  normal  in  certain 
animals,  producing  a  to-and-fro  movement  of  the  contents, 
but  it  is  generally  thought  that  in  the  small  bowels  anti- 
peristalsis  is  only  present  under  abnormal  circumstances. 
If  antiperistalsis  be  admitted  for  the  large  bowels,  we  see 
no  dijfficulty  in  extending  it  to  the  small,  especially  in  view 
of  Colin's  positive  statement  that  it  occurs.  The  peristaltic 
wave  depends  upon  a  something  peculiar  to  the  bowel  wall, 
for  if  a  piece  of  small  intestine  has  been  experimentally 
reversed,  so  that  the  portion  originally  nearest  the  stomach 
is  made  to  occupy  a  position  farthest  away  from  it,  it  is 
found  that  the  peristaltic  wave  in  the  reversed  segment  is 
still  in  the  original  direction  instead  of  in  the  new  direction. 
The  actual  mechanism  involved  in  a  peristaltic  contraction, 
according  to  Starling  and  Bayliss,  is  as  follows :  The  circular 
muscle  on  the  stomach  side  of  the  bolus  contracts,  while 
that  on  the  far  side  is  relaxed  for  some  distance,  so  that 
the  advancing  wave  drives  the  bolus  into  a  relaxed  portion 
of  bowel.  If  a  solution  of  cocaine  or  nicotine  be  applied 
to  the  intestinal  wall  these  movements  cease,  from  which 
it  is  argued  that  they  are  probably  due  to  local  ganglia. 

Another  movement  quite  different  to  the  above  is  the 
Ijcndidar,  which  shows  itself  by  a  gentle  swaying  to  and  fro 
of  the  different  loops  of  bowel,  caused  by  a  simultaneous 


204     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

contraction  of  both  muscular  coats.  This  movement  is  not 
stopped  by  cocaine  or  nicotine,  from  which  it  is  reasoned 
that  the  nervous  gangha  have  nothing  to  do  with  it. 
These  pendular  movements,  which  are  rhythmical  and  as 
regular  as  the  heart-beat,  are  regarded  by  Starling  and 
Bayliss,  who  first  described  them,  ad  being  of  the  greatest 
importance,  as  they  cause  the  material  under  digestion  to 
be  mixed  thoroughly  with  the  secretion,  and  bring  it  in 
contact  with  the  wall  for  absorption.  While  these  rhythmic 
contractions  are  in  operation  the  food  is  not  pressed  on- 
wards, but  remains  in  the  same  region  of  the  bowel,  under- 
going, however,  repeated  divisions.  We  have  not  succeeded 
in  observing  the  pendular  movements  in  the  horse. 

In  the  first  and  third  portions  of  the  colon  the  ingesta 
travel  by  their  own  gravity ;  in  the  second  and  fourth 
portions  they  travel  against  gravity,  as  in  the  caecum.  As 
the  first  and  fourth  and  second  and  third  portions  of  the 
colon  are  united,  the  curious  results  follow  that  material 
is  passing  along  each  section  apparently  in  two  opposite 
directions.  The  frequency  of  intestinal  affections  in  the 
horse  causes  the  canal  to  be  of  exceptional  practical  interest. 
When  the  cfecum  is  found  completely  inverted  into  the 
colon,  as  if  a  hand  had  passed  through  the  colo-cnecal  open- 
ing, laid  hold  of  the  apex  of  the  caecum  and  drawn  the 
entire  bowel  within  the  first  portion  of  the  colon,  it  is  then 
that  the  question  of  muscular  movements  so  strongly  pre- 
sents itself.  Or  take  what  is  far  commoner  and  equally 
fatal,  viz.,  displacement  or  actual  twist  of  the  large  bowel, 
or  a  complete  twist  of  the  small  intestine,  leaving  the 
bowels  in  such  indescribable  confusion  that  the  parts 
cannot  be  unravelled,  even  when  removed  from  the  body ! 
It  is  impossible  to  believe  that  muscular  action  of  the 
intestines  is  free  from  all  blame  in  the  production  of  these 
lesions.  It  is  easier  to  understand  a  twist  of  the  small 
intestine  apart  from  muscular  action  than  it  is  to  under- 
stand displacement  or  actual  twist  of  the  large  intestine. 
A  loop  or  coil  of  small  intestine  may  be  so  distended  by  gas 
or  ingesta  as  to  become  twisted,  but  it  is  more  difficult  to 


DIGESTION  205 

imagine  either  of  these  conditions  producing  twist  or  dis- 
placement of  the  large  intestines,  and  it  becomes  a  question, 
as  we  have  previously  said,  how  far  the  action  of  the 
muscular  bands  of  the  bowel  may  have  a  contributing 
influence.  That  great  force  is  necessary  is  undoubted, 
bearing  in  mind  the  difficulty,  if  not  impossibility,  of 
restoring  the  parts  to  their  position  post-mortem,  or  en- 
deavouring after  death  to  reproduce  the  lesions  experi- 
mentally.    These  matters  will  be  referred  to  again. 

Nervous  Mechanism  of  the  Intestinal  Canal. — Two  distinct 
impulses  are  conveyed  by  the  intestinal  nerves,  viz.,  those 
for  contraction  and  for  inhibition.  In  the  anterior  part  of 
the  tract  the  former  function  is  mainly  or  entirely  carried 
out  by  the  vagus,  stimulation  of  which  is  found  to  cause 
active  contraction  of  the  small  intestines.  Contraction  of 
the  large  intestines  is  effected  through  branches  of  nerves 
which  issue  from  the  sacral  portion  of  the  cord,  and  pass 
with  the  nervi  erigentes  to  the  hypogastric  plexus.  From 
this  plexus  fibres  run  in  the  coats  of  the  large  intestines, 
producing  on  stimulation  much  the  same  results  as  the 
vagus,  viz.,  active  contraction  of  both  circular  and  longi- 
tudinal coats. 

Stimulation  of  certain  branches  of  the  sympathetic  nerve 
stops  or  inhibits  the  contractions  produced  by  stimulation 
of  the  vagus,  hence  the  term  'inhibitory.'  The  inhibitory 
nerves  of  the  small  intestine  are  derived  from  the  dorso- 
lumbar  portion  of  the  cord,  pass  by  the  rami  communi- 
cantes  {re,  Fig.  55)  to  the  main  sympathetic  chain,  Sij., 
and  from  thence  through  the  large  and  small  splanchnic 
nerves  to  the  solar  plexus,  from  which  the  final  distribution 
to  the  intestines  is  made.  The  inhibitory  fibres  for  the 
large  intestines  are  derived  mainly  from  the  lumbar  cord 
through  re.  and  Sy.  (Fig,  55)  to  the  inferior  mesenteric 
ganglion.  From  this  ganglion  inhibitory  fibres  are  given 
off  to  both  longitudinal  and  circular  coats. 

Contractions  of  the  bowels  and  peristalsis  can  occur  after 
all  nerves  leading  to  the  intestines  have  been  divided  ;  this 
points  to  the  existence  of  local  ganglia,  and  such  may  be 


206     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


RV 


Oe 


Ret 


Fig.  55. — Diagram  to  illustrate  the  Nerves  of  the  Alimentary 
Canal  of  the  Dog  (Foster). 

(The  figure  is  very  diagrammatic  and  does  not  represent  the  anatomical 

relations.) 

Oe.  to  Bet.  The  alimentary  canal  from  the  oesophagus  to  the  rectum. 

LV.  Left  vagus  nerve  ending  on  the  front  of  the  stomach,  rl.  Re- 
current laryngeal  supplying  upper  part  of  oesophagus.  E.  V.  Right 
vagus  joining  left  vagus  in  the  oesophageal  plexus  Oe.  pZ.,  supplying 
the  posterior  part  of  the  stomach,  continued  as  B'.V'.  to  join  the 
solar  plexus,  Sol.  2^1-,  here  represeiited  by  a  single  ganglion,  and 
connected  through  x  with  the  inferior  mesenteric  ganglion  (or 
plexus),  G.  m.  i.  a,  «,  a,  branches  from  the  solar  plexus  to  stomach 
and  small  intestines,  and  h  from  the  mesenteric  ganglion  to  the 
large  intestines. 

Spl.  Large  splanchnic  nerve  arising  from  the  thoracic  ganglia  of  the 
sympathetic  8y.  and  rami  communicantes  r.c.  of  the  dorsal  nerves. 

Spl.mi.  Small  splanchnic  nerve.  Both  the  large  and  small  splanchnics 
join  the  solar  plexus  and  thence  make  their  way  to  the  alimentary 
canal,  supplying  the  small  intestine  with  inhibitory  impulses. 

G.m.i.  Inferior  mesenteric  ganglion  formed  by  nerves  running  from 
the  dorsal  and  lumbar  cord.  From  this  ganglion  inhibitory  nerves 
are  given  off  to  the  large  intestines. 

n.e.  Nervi  erigentes  arising  from  the  sacral  cord  and  proceeding  to  the 
hypogastric  plexus.  PI.  hyp.  From  this  plexus  impulses  of  a 
motor  kind  are  supplied  to  the  large  intestines. 


DIGESTION  207 

found  in  the  intestinal  wall.  The  intestinal  movements 
are  automatic  and  self-regulated,  though  they  can  be  pro- 
voked by  both  chemical  and  mechanical  stimuli.  The 
normal  stimulus  to  peristalsis  is  the  passage  of  ingesta 
along  the  canal.  In  the  dog  even  the  sight  of  food  is 
said  to  promote  peristalsis.  Gases  such  as  COo,  H2S, 
and  CH4,  and  organic  acids  such  as  acetic,  proprionic, 
caprylic,  etc.,  act  as  stimuli  and  promote  contraction, 
which  is  a  fortunate  circumstance,  as  they  are  normal  to 
the  bowel  in  consequence  of  bacterial  activity.  Oxygen  gas, 
on  the  other  hand,  inhibits  movements,  and,  as  a  matter 
of  fact,  we  know  that  oxygen  gas  normally  does  not  exist, 
or  only  in  traces,  in  the  gaseous  contents  of  the  bowels. 
Cutting  off  the  blood-supply  to  the  bowels  causes  violent 
contractions,  which  occur  again  when  the  circulation  is 
re-established ;  the  former  is  of  interest  in  those  cases  of 
twist  where  the  blood-supply  is  wholly  or  partly  interfered 
with. 

Under  normal  conditions  the  mind  is  not  conscious  of 
peristaltic  movements,  but  when  these  become  very  energetic 
pain  is  produced.  Under  the  influence  of  nervous  excite- 
ment rapid  and  frequent  evacuations  of  the  bowels  may 
take  place  in  both  cattle  and  horses.  So  rapid  may  the 
evacuations  be  that  in  the  horse,  in  a  short  time,  the  whole 
of  the  rectum  and  single  colon  are  unloaded.  Ordinary 
exercise  is  always  an  important  cause  of  peristalsis,  and 
hence  an  actual  means  of  unloading  the  rectum. 

As  previously  remarked,  the  normal  stimulus  to  peri- 
stalsis is  the  presence  of  ingesta  in  the  canal.  In  the 
feeding  of  herbivora  bulk  is  essential,  they  cannot  live 
in  a  state  of  health  on  concentrated  food  alone.  Their 
intestines  need  bulk,  if  only  in  order  to  maintain  peri- 
stalsis. Bunge  has  shown  that  if  cellulose  be  withheld  from 
the  diet  of  rabbits  they  die  from  intestinal  obstruction.  It 
is  the  cellulose  and  lignin  in  the  diet  of  herbivora  which 
largely  provide  the  needful  stimulus  to  peristalsis. 

Gases  of  the  Intestines. — The  largest  amount  of  gas  found 
in  the  intestinal  canal  is  in  the  caecum  and  colon  ;  the 


208     A  MANUAL  OF  VETERINAliY  PHYSIOLOGY 

small  intestines  naturally  contain  very  little,  frequently 
none,  whatever  is  formed  there  being  probably  rapidly 
passed  into  the  large  bowels.  In  the  large  intestines 
marsh-gas  commonly  exists,  forming  with  carbonic  acid 
the  bulk  of  the  gases  present.  The  pathological  conditions 
arising  in  the  large  bowels  of  horses,  and  in  the  rumen  of 
cattle,  as  the  result  of  fermentation — particularly  of  green 
food — and  the  enormous  size  to  which  these  animals  may 
in  consequence  be  distended,  are  matters  of  common 
clinical  experience.  In  both  horse  and  ox  the  gas  may 
generally  be  ignited  a  short  distance  away  from  the 
cannula  which  has  been  passed  to  give  relief,  the  marsh- 
gas  igniting  readily  on  meeting  with  the  proper  proportion 
of  oxygen.  The  whole  of  the  chemical  changes  in  the 
intestinal  canal  are  carried  on  in  the  absence  of  oxygen ; 
the  gases  which  are  produced  depend  mainly  on  the  nature 
of  the  food,  green  material  producing  marsh -gas  and 
carbonic  acid,  leguminous  matters  producing  sulphuretted 
hydrogen  and  hydrogen. 

The  Faeces. — The  faeces  consist  of  that  portion  of  the  food 
which  is  indigestible,  together  with  that  part  which  though 
digestible  has  escaped  absorption ;  mixed  with  these  are 
w^ater,  colouring  substances,  mucin,  organic  matters  in 
great  variety,  inorganic  salts,  bile  pigment,  volatile  fatty 
acids,  remains  of  digestive  fluids,  organisms,  etc. 

The  composition  of  the  fseces  depends  largely  on  the 
diet.  The  following  table  from  Gamgee*  can  only  give  a 
general  idea  of  their  nature  : 

Approximate  Composition  of  the  F^ces  of  the 


Water 

Horse. 
70-0 

Cow. 
84-0 

Sheep. 
58-0 

Pig. 
80-0 

Organic  matter 

21-0 

13-6 

36-0 

17-0 

Mineral     ,, 

3-0 

2-4 

6-0 

3-0 

100-0         100-0         1000         1000 

Considerable  differences  exist  amongst  animals  in   the 
consistency  of  the  faeces  ;  they  are  moderately  firm  in  the 

"^  '  Our  Domestic  Animals  in  Health  and  Disease,'  p.  253. 


DIGESTION  209 

horse,  pultaceous  in  the  ox,  and  hard  in  the  sheep.  These 
differences  depend  upon  the  amount  of  fluid  they  contain. 
In  the  pig  they  are  human-Hke  and  very  offensive ;  in  the 
dog  they  are  soft  or  hard,  dark  or  Hght,  depending  on  the 
diet,  the  mineral  matter  of  bones  producing  the  Hght- 
coloured  excreta.  It  is  necessary  to  remember  that  the 
proportion  of  fluid  in  the  fieces  does  not  depend  upon  the 
amount  of  water  which  is  drunk,  but  rather  on  the  character 
of  the  food,  the  activity  of  intestinal  peristalsis,  and  the 
energy  with  which  absorption  is  carried  on  in  the  digestive 
canal.  Succulent  green  food  in  horses  produces  a  liquid  or 
pultaceous  motion ;  other  foods,  such  as  hay  and  chaff, 
have  a  constipating  effect,  the  faeces  being  large  and  firm ; 
excess  of  nitrogenous  matter  in  the  food  produces  extreme 
foetor  of  the  dejecta,  and  frequently  diarrhcea,  probably  due 
to  putrefactive  processes.  Nervous  excitement  frequently 
induces  a  free  action  of  the  bowels,  accompanied  by  liquid 
fieces. 

Faeces  always  float  in  water  so  long  as  cohesion  is  main- 
tained. The  colour  of  the  faeces  in  the  horse  is  yellowish 
or  brownish-red,  in  the  ox  greenish-brown  ;  they  rapidly 
become  darker  on  exposure  to  the  air.  When  the  animal 
is  grass-fed  the  faeces  are  green,  and  when  a  horse  is  fed 
wholly  on  corn  they  become  very  yellow  and  like  wet  bran 
in  appearance.  The  colour  of  the  faeces  of  animals  re- 
ceiving hay  or  grass  is  due  to  altered  chlorophyll.  The 
faeces  of  the  horse  are  moulded  into  balls  in  the  single 
colon.  They  are  always  acid  in  reaction,  the  acidity 
probably  depending  upon  the  development  of  some  acid 
from  the  carbo-hydrates  of  the  food. 

Faeces  contain  lignin  amongst  the  indigestible  portion 
of  the  ingesta,  a  proportion  of  cellulose,  husks  of  grains, 
the  downy  hair  found  on  the  kernel  of  oats,  vegetable 
tubes  and  spirals,  starch  and  fat  granules,  gums,  resins, 
chlorophyll,  etc. ;  unabsorbed  proteid,  carbo-hydrate  and 
fatty  matters ;  products  of  digestive  fermentation,  such  as 
lactic,  malic,  butyric,  succinic,  acetic,  and  formic  acids ; 
leucin,  tyrosin,  indol,  skatol,  and  phenol ;  biliary  matters 

14 


210     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

and  altered  bile  pigment — stercobilin — which  gives  the 
colour  to  the  dejecta  in  the  dog  but  not  in  herbivora ;  and, 
lastly,  mineral  matter  in  varying  proportions.  In  the  dog 
portions  of  muscle  fibre,  fat  cells,  tendinous  and  fibrous 
tissue,  are  found  in  animals  fed  on  flesh. 

Of  the  morfianic  matter  silica  exists  in  largest  amounts 
in  herbivora,  then  potassium  and  phosphates ;  sodium, 
calcium,  magnesium,  and  sulphates,  form  a  smaller  but 
still  important  proportion.  The  horse  excretes  but  little 
phosphoric  acid  by  the  kidneys,  but  considerable  quantities 
pass  with  the  fseces  in  the  form  of  ammonio-magnesium 
phosphate.  This  salt  is  derived  principally  from  the  oats 
and  bran  of  the  food,  and  it  frequently  forms  calculi 
through  collecting  in  the  colon  and  becoming  mixed  with 
organic  substances.  Other  intestinal  calculi  are  formed 
from  lime  deposits  in  the  bowel,  while  collections  of  the 
fine  hairs  from  the  kernels  of  oats  become  encrusted  with 
ammonio-magnesium  phosphate  and  form  oat- hair  calculi. 
In  the  Persian  wild  goat  and  certain  antelopes  intestinal 
concretions  are  found  known  as  Bezoar  stones,  formerly 
much  used  in  medicine  and  as  antidotes  to  poison.  There 
are  two  varieties  of  calculi,  one  olive  green,  the  other 
blackish  green.  The  first  melts  when  heated,  emits 
aromatic  fumes,  and  consists  chiefly  of  an  acid  allied  to 
cholalic  acid.  The  chief  constituent  of  the  second  variety 
is  an  acid  derivative  of  tannic  acid,  which  indicates  their 
origin  from  food  substances.  Stomach  calculi  have  not 
been  unknown  in  the  horse,  while  in  cattle,  as  the  result 
of  licking  each  other,  '  hair  balls '  are  common  objects. 

The  following  table  by  Roger  gives  the  mineral  composi- 
tion of  the  fctces  in  every  100  parts  of  the  ash  :* 


Sodium  chloride 

Horse. 
•03 

Ox. 
■23 

Sheep. 
•14 

Potassium 

11-30 

2-91 

8^32 

Sodium 

1-98 

•98 

3^28 

Lime 

4-63 

5-71 

1815 

Magnesium 

3-84 

11-47 

5-45 

*  Quoted  by  Ellenberger. 


DIGESTION  211 

Horse.  Ox.  Sheep. 

Oxide  of  iron      -  -  1-44  5-22  210 

Phosphoric  acid  -         10-22  8-47  9-10 

Sulphuric  acid  -  -  I'So  1'77  2'69 

Silica      -  -  -         62-40  62-54  50-11 

Oxide  of  magnesium      -  2-18  —  — 

Eoger  observes  that  the  ash  of  the  fsces  of  herbivora 
contams  scarcely  any  alkaline  carbonates. 

The  amount  of  faeces  produced  m  24  hours  varies  with  the 
quantity  and  nature  of  the  food  given.  We  have  observed 
that  on  a  diet  consisting  of  12  lbs.  of  hay,  6  lbs.  of  oats, 
and  3  lbs.  of  bran,  the  average  amount  of  fseces  passed  by 
fifteen  horses  during  an  experiment  lasting  seven  days 
amounted  to  29  lbs.  13  ozs.  in  24  hours,  the  f?eces  being 
weighed  in  their  natural  condition,  viz.,  containing  76  per 
cent,  water  ;  the  dry  material  of  this  bulk  of  fences  is  about 
71  lbs.  More  ft^ces  are  passed  during  the  night  than 
during  the  day ;  in  the  above  experiment,  during  the 
12  hours  (6  p.m.  to  6  a.m.),  the  average  amount  of  faces 
per  horse  was  18  lbs.  3  ozs.,  whilst  from  6  a.m.  to  6  p.m. 
the  amount  was  11  lbs.  10  ozs.  The  largest  amount  of 
faeces  we  have  known  a  horse  produce  was  an  average  of 
73'3  lbs.  (weighed  in  their  natural  state)  in  24  hours ;  the 
diet  consisted  of  12  lbs.  of  oats,  3  lbs.  of  bran,  and  28  lbs. 
of  hay.  In  an  experiment  carried  on  for  several  months 
with  different  horses  all  receiving  12  lbs.  hay  and  varying 
proportions  of  bran  and  oats,  the  average  daily  amount  of 
faeces  was  24  lbs.  A  horse  will  evacuate  the  contents  of 
the  bowels  about  ten  or  twelve  times  in  the  24  hours,  and 
the  food  he  consumes  takes  on  an  average  four  days  to 
pass  through  the  body. 

In  the  ox  the  amount  of  faeces  is  between  70  lbs.  and 
80  lbs.  in  the  24  hours.  In  the  sheep  it  varies  from 
2  lbs.  to  6  lbs.  daily ;  in  swine  3  lbs.  to  6  lbs.,  depending 
on  the  nature  of  the  diet. 

The  odour  of  faeces  is  distinctly  unpleasant,  due  to  the 
presence  of  indol  and  skatol ;  in  disease  they  are  often 
extremely  fatid,  and  occasionally  horrible. 

The  act  of  defaecation  is  performed  by  a  contraction  of 

14—2 


212     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  rectum  assisted  ])v  the  ahdominal  muscles,  the  glottis 
being  closed.  In  the  horse  the  contraction  of  the  rectum 
alone  is  sufficient  to  expel  its  contents ;  this  is  proved  by 
the  fact  that  this  animal  can  defnecate  while  trotting,  show- 
ing there  is  no  necessity  to  fix  the  diaphragm  and  hold  the 
breath,  though  at  rest  this  does  occur.  In  consequence  the 
rectum  of  the  horse  can  exercise  extraordinary  power ;  the 
hand  and  arm  may  be  rendered  almost  numb  by  the  pressure 
it  can  exert.  The  mass  driven  backwards  under  this  force 
causes  the  sphincters  to  dilate,  sometimes  to  an  astonishing 
degree,  and  as  the  last  trace  of  material  is  exuded,  the 
contraction  of  the  rectum  is  so  great  that  it  forces  some  of 
the  mucous  membrane  externally,  which  may  be  temporarily 
imprisoned  by  the  contracting  sphincters.  The  muscle  of 
the  rectum  receives  both  motor  and  inhibitory  fibres,  as 
previously  described.  Its  extraordinary  power  in  the 
rectum  in  the  horse  may  partly  be  due  to  the  horizontal 
position  of  the  body ;  no  crouching  of  the  body  occurs 
during  the  act  of  defalcation,  such  as  occurs,  more  or  less, 
with  all  other  domesticated  animals.  The  rectum  has  the 
whole  work  to  perform  single-handed,  even  as  we  have 
shown  above,  without  the  assistance  of  the  diaphragm  or 
abdominal  muscles. 

Two  sphincters  close  the  rectum  in  all  animals,  an 
external  of  voluntary  and  an  internal  of  involuntary 
muscle ;  they  are  presided  over  by  a  centre  in  the  cord. 
If  this  is  destroyed  the  rectum  remains  uncontracted,  and 
the  sphincter  flabby ;  in  the  dog  the  cord  may  be  destroyed 
■in  the  lumbar  region  without  interfering  with  the  act  of 
deffecation,  which  is  then  carried  on  l)y  a  reflex  mechanism. 

Meconium  is  the  dark-green  material  found  in  the  intes- 
tines of  the  foetus.  It  consists  of  biliary  acids  and  pig- 
ments, fatty  acids  and  cholesterin,  while  salts  of  mag- 
nesium and  calcium,  phosphates  and  sulphates,  sodium 
chloride,  soda,  and  potash  are  also  found  in  it.  Meconium 
is  the  product  of  liver  excretion. 


DIGESTION  213 

Pathological. 

The  diseases  of  early  Hfe  in  the  horse  are  mainly  situated  in  the 
chest,  while  those  of  the  adult  period  are  practically  confined  to  the 
abdominal  viscera,  principally  the  intestines.  The  term  colic  appears  to 
be  indissolubly  associated  with  the  horse,  and  it  becomes  a  question  of 
the  greatest  practical  and  physiological  interest  to  ascertain  the  reason 
why  digestive  disturbances  are  so  common  and  so  frequently  mortal. 
There  are  certain  obvious  explanations  of  the  fact,  but  neither  singly 
nor  combined  are  the  accepted  ideas  capable  of  explaining  some  of  the 
mysteries  surrovmding  the  origin  of  these  diseases. 

When  muscular  spasms  of  the  intestines  occur  the  disease  is  spoken 
of  as  colic  ;  in  many  cases  the  pain  which  is  exhibited  is  in  no 
respect  due  to  muscular  spasm,  and  is  only  a  symptom.  Still,  by  far 
the  majorit}'  of  intestinal  cases  are  of  this  kind,  viz.,  simple  muscular 
spasms  of  some  part  of  the  digestive  tract,  but  of  which  part  we  are 
certainly  ignorant.  It  is  obvious  that  either  the  stomach,  the  small  or 
the  large  bowels  may  be  so  affected,  but  there  are  no  definite  symptoms 
which  enable  a  positive  diagnosis  of  location  to  be  established.  It  is 
important  to  bear  in  mind  the  possibility  of  spasm  of  the  muscular 
walls  of  the  stomach,  for  there  can  be  no  doubt  it  is  generally  over- 
looked, and  the  intestines  almost  universally  blamed.  The  evidence 
supporting  the  view  we  take  of  the  liability  of  the  stomach  to  disorder 
is  afforded  by  the  frequency  of  rupture  of  the  organ,  not  that  the 
rupture  is  due  to  spasm  of  the  walls,  but  that  the  spasm  is  caused  by 
stomach  trouble,  the  rupture  following  as  a  sequel,  as  detailed  on 
p.  153.  It  is,  however,  admitted  that  stomach  spasm  is  far  less 
common  than  spasm  of  the  intestinal  portion  of  the  tract.  We  would 
here  emphasize  the  facts  set  forth  on  p.  178,  of  the  general  inability 
of  the  horse  to  vomit,  and  the  serious  bar  this  proves  to  relief,  so 
much  so  that  it  is  hardly  going  too  far  to  say  that  if  the  animal  could 
vomit  ruptured  stomach  would  practically  be  unknown,  and  stomach 
trouble  generally  a  matter  of  comparatively  slight  importance. 

In  connection  with  intestinal  trouble,  we  are  unable  to  say  what 
proportion  the  cases  affecting  the  small  intestines  bears  to  those  affect- 
ing the  large.  AYe  cannot  during  life  distinguish  colic  of  the  one  from 
colic  of  the  other.  Still,  there  are  good  grounds  for  thinking  that  the 
large  bowels  are  more  frequently  affected  than  the  small,  and  for  the 
following  reasons  : 

1.  Ingesta  pass  rapidly  through  the  small  intestines — so  rapidly 
indeed  that,  as  mentioned  at  p.  190,  these  bowels  are  nearly  always 
found  empty  at  ordinary  post-mortem  examinations,  or  the  contents 
in  such  a  fluid  condition  that  it  is  not  reasonable  to  suppose  that  they 
remain  there  long,  from  what  we  know  of  the  behaviour  of  fluids 
generally  in  the  anterior  part  of  the  digestive  tract. 


214     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

2.  On  the  other  hand,  the  large  intestines  always  contain  ingesta, 
for  the  material  passes  along  it  very  slowly,  so  that  of  the  three  or 
four  days  occupied  in  accomplishing  the  journey  from  mouth  to  anus, 
all  but  a  few  hours  is  spent  in  the  large  intestines.  It  is  reasonable, 
therefore,  to  assume  that  in  cases  of  pure  uncomplicated  disordered 
muscular  action  of  the  bowels,  the  large  intestine  in  the  majority  of 
cases  is  at  fault. 

Colic  is  not  fatal,  though  Percival  described  such  a  case.  Our 
experience  leads  us  to  believe  that  death  from  pure  spasm  of  the 
bowels  is  unknown,  and  we  would  emphasize  the  point  not  only  for 
the  sake  of  accuracy,  but  as  of  value  in  prognosis.  We  believe  that  in 
any  case  returned  as  djing  from  colic,  a  more  extensive  search  would 
have  revealed  some  fatal  lesion.  There  is  no  reason  for  believing 
that  the  pain  of  colic  ^jer  sc  is  capable  of  causing  death. 

If  this  be  accepted,  and  it  is  fortunately  capable  of  proof,  it  con- 
siderably narrows  the  causes  of  death  from  intestinal  affections,  and 
groups  them  mainly  under  two  heads  :  {a)  Inflammation  of  the  bowels, 
and  (b)  displacement  of  the  bowels. 

Enteritis,  by  which  name  inflammation  of  the  bowels  is  known, 
is  spoken  of  as  a  common  disease  of  the  horse,  but  here  again  we  join 
issue  with  accepted  doctrines  and  urge  that  it  is  an  uncommon 
disease.  Further,  that  in  the  large  majority  of  so-called  cases  of 
enteritis,  some  displacement  of  the  bowels  with  interference  to  the 
circulation  has  occurred.  That  uncomplicated  enteritis  may  exist  is 
not  disputed,  but  we  urge  its  relative  infrequency,  and  press  the  point 
that  what  looks  like  inflammation  is  more  often  strangulation.  When 
a  deep  purple  thickened  coil  of  intestine  is  found  on  opening  the 
abdomen,  such  a  case  is  not  enteritis.  The  colour  indicates  that  the 
blood-supply  has  been  imprisoned  as  the  result  of  strangulation,  and 
an  identical  appearance  would  have  been  obtained  by  ligaturing  the 
bowel.  When  half  the  double  colon  is  found  purple,  thickened,  filled 
with  blood-stained  fluid  ingesta,  the  wall  of  the  bowel  being  friable 
and  its  mucous  membrane  purple,  then  however  much  we  may  be 
tempted  to  speak  of  it  as  enteritis,  it  certainly  is  not  this  disease,  but 
strangulation.  Enteritis  must  be  reserved  for  that  condition  of  bowel 
in  which  the  mucous  membrane  alone  is  inflamed.  Such  a  bowel 
may  give  no  external  indication  of  trouble ;  the  general  vascular 
supply  is  not  interfered  with  ;  the  full  intensity  of  the  trouble  falls  on 
the  mucous  membrane,  and  such  a  condition  may  be  experimentally 
produced  by  the  administration  of  an  irritant  poison.  It  is  probable 
that  in  the  horse  the  majority,  if  not  all  the  cases,  of  pure  enteritis 
met  with,  are  due  to  a  poison  produced  during  the  process  of  digestive 
metabohsm  (see  p.  231).  That  the  presence  of  an  irritant  without 
a  poison  has  no  such  effect,  is  abundantly  proved  by  the  pounds 
of  sand  and  gravel  horses  may  carry  in  their  intestines  for  months, 


DIGESTION  215 

perhaps  j'ears,  without  producing  anj-  apparent  ill  effect,  certainly  with- 
out producing  enteritis.  Similarly,  gastritis  excepting  as  the  result  of 
poison  is  practically  unknown. 

Our  object  in  the  above  remarks  is  to  focus  attention  on  the  defects 
in  clinical  observation,  and  to  attempt  a  physiological  analysis  of  the 
most  frequent,  the  most  fatal,  and  by  far  the  most  acutely  painful  and 
distressing  group  of  diseases  that  any  animal  is  exposed  to.  There  is 
nothing  in  the  whole  range  of  comparative  pathology,  including  the 
diseases  of  man,  which  compares  in  violence,  suddenness,  and  mortality 
with  digestive  diseases  of  the  horse.  We  have  attempted  to  show  how 
physiology  is  capable  of  enabling  us  to  steer  along  a  moderately  exact 
course,  for  it  is  certain  that  unless  we  are  agreed  regarding  the  nature 
of  the  lesions  found  nt  post-mortem  examination,  we  cannot  reach  that 
goal  which  is  the  object  of  our  existence  as  a  profession,  and  of  which 
physiology  is  only  the  humble  handmaid. 

What  is  the  most  common  cause  of  death  among  horses  from 
intestinal  affections,  whether  affecting  the  large  or  small  bowels  ? 
There  is  only  one  answer  to  this,  and  time  and  careful  enquiry  will 
prove  its  accuracy.  The  answer  is  Strangulation  of  the  bowels, 
partial  or  complete.  This  strangulation  is  capable  of  physiological 
analysis.  The  most  unobservant  person  cannot  overlook  a  bunch  of 
small  intestines  so  tied  together  as  to  defy  all  attempts  at  unravelling, 
even  when  out  of  the  body,  but  it  takes  a  little  careful  observation  to 
detect  displacements  of  the  large  intestine.*  The  size,  weight,  and 
peculiar  disposition  of  the  double  colon  should  have  secured  it  immunity 
from  any  form  of  displacement ;  looked  at  in  the  abdomen,  it  appears 
impossible  for  any  force  short  of  some  mysterious  power  to  be  able  to 
influence  the  position  of  the  bowels,  yet  we  know  they  are  capable  of 
being  twisted  as  easily  as  if  they  were  made  of  cotton.  We  know  also 
that  one  portion  may  be  thrust  into  another,  in  just  the  same  way 
as  a  telescope  collapses,  and  that  a  voluminous  bowel  like  the  caecum 
may  become  completely  inverted,  and  found  within  the  colon,  though 
to  get  there  it  has  to  pass  through  an  opening  only  an  inch  or  two 
wide.  So  remarkable  indeed  are  these  lesions  that  they  cannot  always 
be  imitated  after  death,  and,  as  mentioned  above,  it  is  impossible  to 
untie  many  complicated  knots  in  the  small  bowels,  even  when  the 
organs  have  been  removed  from  the  abdomen. 

The  actual  mechanism  which  brings  about  twists  of  the  large  and 
small  intestines  is  disordered  muscular  action ;  the  factor  responsible 


*  From  the  point  of  view  of  equine  patholog\',  one  of  the  most 
valuable  contributions  made  to  veterinar}'  literature  by  the  late  Pro- 
fessor Walley  was  his  account  of  displacements  of  the  colon  in  the 
horse  (Veterinary  Journal,  vol.  ix.).  It  was  the  first  time  in  this 
country  that  the  possibility  of  these  immense  bowels  being  twisted  and 
displaced  was  ever  desci'ibed. 


21G     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

for  telescoping  intestines  is  disordered  muscular  action,  and  disordered 
muscular  action  is  the  result  of  disordei'ed  nervous  action.  For 
telescoping  to  occur,  one  portion  of  bowel  must  first  contract  until  it 
becomes  but  a  mere  shadow  of  its  former  self;  the  contracted  part 
must  then  be  drawn  within  the  dilated.  A  different  cause  is  at  work 
to  produce  a  twist  of  the  small  intestine ;  this  as  we  previously 
indicated  is  tympany  of  the  bowel,  while  in  the  case  of  the  large 
intestines  the  muscular  action  must  be  capable  of  causing  the  bowel 
to  perform  a  revolution  more  or  less  complete,  and  in  this  way 
reversing  its  position.  We  cannot  attempt  to  indicate  the  exact  dis- 
ordered action  which  occurs;  this  question  would  require  to  be  worked 
out  on  the  living  subject.  The  colon  and  csecum  are  most  liberally 
supplied  with  bands  (Figs.  48,  49,  51,  52,  and  54),  and  it  does  not  appear 
to  us  to  be  beyond  the  bounds  of  reasonable  probabilit}'  that  these  plaj- 
a  most  important  part  in  the  production  of  displacements  of  the  large 
intestines.  The  cause  of  the  disordered  nervous  action  which  leads  to 
this  may,  from  its  physiological  interest,  be  briefly  dealt  with.  Apart 
from  such  obvious  explanations  as  errors  in  feeding  (see  in  this  con- 
nection pp.  170,  237),  the  most  common  cause  of  derangement  of  the 
muscular  action  of  the  digestive  canal  is  work.  It  is  this  which 
accounts  for  the  majority  of  colic  cases  occurring  towards  the  end  of 
the  day,  the  frequency  with  which  the  seizure  occurs  at  or  shortly 
after  work,  especially  that  of  an  exhausting  nature,  and  the  practical 
absence  of  colic  among  non-working  horses.  We  have  even  known  a 
horse  in  a  cavalry  charge  rupture  the  ileum  as  completely  as  if  the 
parts  had  been  torn  asunder  by  hand ;  and  this,  it  will  be  remembered, 
is  the  thickest  and  stoutest  portion  of  the  small  intestine,  and  the 
least  likely  to  suffer  laceration.  The  connection  between  such  a  lesion 
and  an  exhausting  gallop  is  at  present  not  very  apparent,  but  the  fact 
is  undoubted. 

The  whole  subject  is  of  profound  practical  interest,  and  more  has 
been  said  on  the  matter  than  commonly  falls  to  physiology  to  deal 
with,  but  the  basis  of  exact  clinical  knowledge  is  sound  anatomy  and 
physiology,  and  we  consider  the  physiological  aspect  of  digestive 
disorders  has  not  yet  received  adequate  attention.  We  must  bear 
in  mind  that  the  whole  length  of  the  digestive  tract  is  a  chemical 
laboratory  concerned  in  the  analysis  of  food-stuffs,  isolating  and 
retaining  those  which  are  of  use,  getting  rid  of  those  which  are  useless, 
and  rendering  harmless  those  substances  capable  of  acting  injuriously. 
Not  only  is  it  a  laboratory  where  the  above  analytical  operations 
are  carried  out,  but  it  is  also  a  factory  where  the  chemical  reagents 
necessary  for  this  process  are  prepared  beforehand.  So  thoroughly 
is  the  analysis  performed,  that  the  most  complex  bodies  are  broken 
down  into  the  simplest  products.  Can  it  be  wondered  at,  that  the 
chemical  processes  may  sometimes  fail,  and  disorder  result '? 


DIGESTION  217 

.  We  see  a  faithful  reflex  of  the  laboratory  processes  in  the  disorders 
of  the  canal,  the  diarrhoea  which  is  full  of  beneficence,  the  impaction 
which  indicates  a  loss  of  muscular  power  and  physical  alteration  of  the 
contents,  the  acute  tympany  which  announces  active  fermentation, 
the  rupture  which  indicates  the  strain  on  the  walls  of  the  apparatus ; 
these  and  others  too  numerous  to  be  dealt  with,  and  which  no  mere 
mention  explains,  give  some  idea  of  the  penalty  paid  by  horses  for 
the  doubtful  privilege  of  domestication.  The  term  '  digestion  of  a 
horse  '  has  been  framed  in  absolute  ignorance  of  the  real  facts.  There 
is  no  animal  in  whicli  these  organs  are  more  readily  disturbed,  and 
none  in  which  they  are  the  subject  of  such  acutely  painful  and  mortal 
lesions. 

The  ruminant  from  the  peculiarity  of  its  physiological  arrangement 
is  far  more  liable  to  stomach  than  intestinal  trouble ;  tympany,  im- 
paction, paralysis,  and  inflammation  of  one  or  more  of  the  com- 
partments are  common.  In  spite  of  the  size  of  the  oesophagus 
impaction  is  frequent,  in  marked  contrast  to  the  horse,  in  which  it  is 
uncommon,  while  calculi,  a  special  feature  in  the  intestine  of  the 
horse,  are  found  in  the  stomach  of  the  ox,  though  brought  about  by 
very  different  causes.  Strangulation  of  the  bowels  in  the  ox  is  not 
unknown,  but  limited  to  a  special  variety  due  to  anatomical  condi- 
tions. Parasitic  trouble  in  all  animals  is  a  prominent  pathological 
feature,  the  digestive  canal  from  the  mouth  to  the  anus  being  liable  to 
infection  with  numerous  varieties  of  parasites,  and  it  also  forms  the 
main  channel  of  parasitic  entry  for  other  parts  of  the  body. 


CHAPTER   YI 

THE   LIVER   AND   PANCREAS 

Section    1. 

The  Liver. 

In  considering  the  function  of  the  Hver  it  is  necessary  to 
bear  in  mind  its  peculiar  blood-supply.  Most  glands  of 
the  body  which  are  called  upon  to  produce  a  secretion  are 
furnished  only  with  arterial  blood  for  the  purpose,  but  the 
liver  is  an  exception  to  this  rule ;  the  entire  venous  blood 
returning  from  the  splanchnic  area,  viz.,  the  bowels, 
stomach,  spleen,  pancreas,  etc.,  constitutes  the  material 
with  which  the  liver  is  flooded.  Such  a  mixture  of  blood 
derived  from  a  peculiar  and  considerable  area  must  be 
charged  with  many  products,  some  the  result  of  secretory 
activity,  others  the  soluble  constituents  of  the  elements 
of  food  ;  or  again,  substances  absorbed  from  the  intestinal 
canal,  which  are  l)ye-products  produced  during  the  gradual 
breaking-down  of  the  food  substances.  It  is  from  this 
blood  that  the  liver  performs  its  various  functions,  and 
one  of  the  most  evident,  viz.,  the  secretion  of  bile,  will  be 
dealt  with  first. 

Bile. 

The  bile  is  a  fluid  of  an  alkaline  reaction,  ))itter  taste, 
a  specific  gravity  in  the  ox  of  10?,'2  to  1025,  in  the  sheep 
from  1025  to  1031,  and  in  the  horse  1005.  The  colour  is 
yellowish-green  or  dark-green  in  herbivora,  reddish-brown 
in  the  pig,  and  golden-red  in  carnivora.  These  differences 
in  colour  depend  upon  the  character  of  the  pigment  present. 
Bile  taken    direct  from    the   liver   is  relatively  watery  in 

218 


THE  LIVER  AND  PANCREAS  219 

consistence,  that  taken  from  the  gall-bladder  is  viscid, 
due  to  admixture  with  nucleo-albumin  during  its  stay  in 
the  latter  receptacle.  The  secretion  contains  no  proteid 
which  is  somewhat  remarkable  ;  biliary  pigments,  bile 
acids,  fats,  soaps,  lecithin,  cholesterin,  and  inorganic  salts 
are  found  in  varying  quantities.  By  standing  in  the  gall- 
bladder the  solids  are  considerably  increased,  owing  to  an 
absorption  of  part  of  the  water  of  the  bile.  The  secretion 
in  the  horse  contains  no  mucin,  and,  according  to  Ellen- 
berger,  there  is  very  little  mucin  in  the  bile  of  sheep  ;  what 
was  believed  to  be  mucin  in  ox  bile,  which  conferred  on  the 
latter  its  ropy  character,  is  now  known  to  be  nucleo-albumin. 

The  dried  alcoholic  extract  of  bile  contains  in  the  ox 
3'58  per  cent,  of  sulphur,  sheep  5*71  per  cent.,  and  pig 
•33  per  cent.  The  gases  found  in  bile  are  COo,  and  traces 
of  0  and  N.  The  chief  inorganic  salts  are  sodium  chloride 
and  phosphate,  besides  which  are  found  salts  of  calcium, 
magnesium,  potassium,  iron,  with  phosphoric  and  sulphuric 
acids ;  the  sodium  salts  always  exist  in  the  largest  pro- 
portion. The  iron,  which  is  found  as  phosphate,  is  probably 
derived  from  the  haBmoglobin  of  the  blood  during  the 
formation  of  the  bile  pigments. 

The  following  table,  showing  the  percentage  composition 
of  various  biles,  is  mainly  compiled  from  EUenberger : 


Ho  rse 
Bile. 

0.1- 
Bile. 

Dog 
Bile. 

Pig 
Bile. 

Water 

95 

92-91 

95-3 

88-8 

Solids 

5 

9-6 

4-7 

11^2 

Bile  acids  - 

Bile  pigments 

Fat 

Mucin         -      J 

— 

8-3 

4^1 

10-1 

Salts 

— 

1-3 

•6 

1-1 

Percentage 

Composition 

of  the  Asli  0 

/  Ox  Bile. 

Sodium  chloride 

-     Ill 

Manganese 

peroxide     - 

•12 

Potassium 

■       4-8 

Phosphoric 

acid 

10-45 

Sodium 

-     36-7 

Sulphuric 

11 

6-39 

Calcium  carbonate 

1-4 

Carbonic 

)> 

11-26 

Magnesium 

■53 

Silica     - 

- 

•36 

Iron  oxide 

•23 

2'20     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

The  differences  found  in  the  composition  of  liile  probal)ly 
depend  upon  whether  it  ))e  taken  from  the  gall  bladder  or 
from  a  fistula,  the  former  l)eing  the  more  concentrated. 

The  Cholesterine  found  in  hile  must  l)e  regarded  in  the 
light  of  an  excretion  ;  the  liver  is  merely  a  convenient 
channel  for  getting  rid  of  this  waste  product,  which  is 
collected  from  the  many  tissues  of  which  it  forms  a  part, 
brought  to  the  liver,  and  eliminated  through  the  bile  by 
the  bowels.  It  is  found  in  very  regular  quantities,  and 
forms  the  principal  constituent  of  certain  gall-stones ;  it 
is  kept  in  solution  in  the  bile  by  means  of  the  bile  salts. 

Lecithin  is  another  waste  product  of  the  l)ody  excreted 
from  the  system  through  the  medium  of  the  bile. 

The  Bile  Pigments  are  two  in  number,  bilirubin  and  bili- 
verdin;  the  latter  is  produced  l)y  oxidation  from  the  former. 
Bilirubin  is  the  colouring  matter  of  human  bile  and  that 
of  carnivora,  whilst  biliverdin  is  the  pigment  of  the  bile  of 
herbivora.  It  is  not  uncommon  to  find  both  pigments  in 
the  same  specimen  of  bile.  These  pigments  are  insoluble 
in  water  but  soluble  in  alkalies ;  in  the  bile  they  are  held 
in  solution  by  the  bile  acids  and  alkalies.  Bilirubin  may 
be  obtained  from  the  gall-stones  of  the  ox  in  the  form  of  an 
orange-coloured  powder,  which  can  be  made  to  crystallize 
in  rhomlnc  tablets  and  prisms.  If  an  alkaline  solution  of 
bilirubin  be  exposed  to  the  air  it  becomes  biliverdin  by 
oxidation,  and  this  latter  pigment  by  appropriate  treat- 
ment may  be  obtained  as  a  green  powder.  Both  colouring 
matters  of  the  bile  l)ehave  like  acids,  forming  soluble  com- 
pounds with  metals  of  the  potassium  group,  insoluble  ones 
with  those  of  the  calcium  group  (Bunge). 

On  the  addition  of  nitric  acid  (containing  nitrous  acid) 
to  the  bile  pigments  a  play  of  colour  is  observed ;  this  is 
known  as  Gmelin's  test.  In  the  case  of  bilirubin  the 
colours  pass  from  yellowish-red  to  green,  then  to  blue, 
violet,  red,  and  yellow ;  each  of  these  colours  is  indicative 
of  a  different  degree  of  oxidation  of  the  original  bilirubin. 
Biliverdin  gives  the  same  play  of  colours  excepting  the 
initial  yellowish-red,  which  is  absent. 


THE  LIYER  AND  PANCREAS  221 

Although  bihrubm  has  not  been  obtained  from  hfemo- 
globin,  there  is  no  doubt  that  this  is  the  source  of  the  pig- 
ment, for  if  ha='moglobin  be  liberated  in  the  blood  and  enters 
the  plasma,  bile  pigments  appear  in  the  urine  ;  further, 
hemoglobin  may  be  readily  decomposed,  yielding  a  proteid 
and  hfematin  ;  and  if  this  hfematin  be  deprived  of  iron,  the 
residue  thus  obtained  is  not  very  dissimilar  in  composition 
to  bilirubin.  We  have  previously  mentioned  (p,  11)  that 
old  blood-clots  contain  an  iron  free  substance  known  as 
hifmatoidin,  and  this  is  practically  identical  in  composition 
with  bilirubin.  When  red  blood  cells  disintegrate  in  the 
ordinary  course  of  their  wear  and  tear,  the  liberated  haemo- 
globin is  brought  to  the  liver,  and  under  the  influence  of 
the  liver  cells  converted  into  the  iron  free  substance  bili- 
rubin or  biliverdin.  Part  of  the  iron  so  liberated  escapes 
from  the  body  through  the  bile,  but  the  bulk  of  it  is  re- 
tained and  again  used  in  the  formation  of  hemoglobin  by 
the  organs  which  discharge  this  function. 

Though  biliverdin  is  the  colouring  matter  of  the  bile  of 
herbivora,  yet  the  gall-stones  found  in  the  ox  consist  very 
largely  of  bilirubin  combined  with  chalk ;  in  the  pig  the 
same  combination  is  observed.  Bilirubin  is  said  by 
Hammarsten  to  be  constantly  present  in  the  serum  from 
horse's  blood  though  not  in  that  of  the  ox,  and  Salkowski 
states  that  it  is  a  normal  constituent  of  the  urine  of  the 
dog  during  the  summer.  In  the  large  intestines  both  bili- 
rubin and  biliverdin  undergo  reduction  resulting  in  the 
formation  of  stercobihn,  the  colouring  matter  of  the  faeces 
in  some  animals.  It  is  possible  also  that  some  of  the  pig- 
ment is  reabsorbed  from  the  intestinal  canal,  carried  to  the 
liver,  and  again  eliminated. 

The  Bile  Salts  are  two  in  number,  glycocholate  and  tauro- 
cholate  of  soda ;  they  are  formed  in  the  liver  by  the  union 
of  cholalic  acid  with  glycine  or  taurine,  and  exist  in  combina- 
tion with  soda.  These  salts  are  found  in  varying  propor- 
tions in  different  animals ;  thus,  glycocholate  of  soda  is 
largely  found  in  herbivora,  taurocholate  principally  in 
carnivora,  while  in  the  pig  hyoglycocholic  and  hyotauro- 


222     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

cholic  acids  are  found.  Both  salts  are  soluble  in  water, 
have  a  markedly  alkaline  reaction,  rotate  the  plane  of 
polarized  light  to  the  right,  and  may  be  obtained  in  a 
crystalline  form  as  highly  deliquescent  acicular  needles. 
Glycocholic  acid  is  the  chief  bile  acid  in  herbivora,  it  is 
produced  by  the  union  of  glycine  with  cholalic  acid ;  it  is 
diminished  by  an  animal  and  increased  by  a  vegetable 
diet.  Taurocholic  acid  is  produced  from  taurine  and  cho- 
lalic acid  and  exists  principally  in  carnivora,  though  small 
quantities  may  be  found  in  the  ox.  This  acid  differs  from 
the  first  characteristically  by  containing  sulphur,  by  which 
it  shows  its  proteid  origin.  Glycine  or  glycocoU  also  owes 
its  origin  to  the  proteids  of  the  food,  and  if  administered  it 
reappears  externally  as  urea.  It  cannot  be  traced  in  the 
free  state  in  the  body,  but  occurs  in  the  urine  combined 
with  benzoic  acid,  in  the  form  of  hippuric  acid.  Petten- 
kofer's  test  for  bile  acids  is  performed  as  follows :  A  drop 
of  the  fluid  is  placed  on  a  white  earthenware  surface,  and 
to  it  is  added  a  drop  of  a  strong  (10  to  20  per  cent.)  solution 
of  cane-sugar,  and  a  similar  quantity  of  strong  sulphuric 
acid ;  a  beautiful  purple-red  colour  forms.  The  colour  is 
due  to  furfurol,  and  is  produced  by  the  action  of  the  acids 
on  the  sugar  and  the  subsequent  reaction  with  cholalic 
acid. 

The  origin  of  the  bile  acids  is  involved  in  obscurity ; 
taurine  and  glycine  are  probably  formed  from  the  disinte- 
gration of  proteid,  the  precursors  of  cholalic  acid  are 
unknown.  Nor  do  we  know  why  glycine  should  predominate 
in  some  animals  and  taurine  in  others,  but  it  appears  clear 
that  the  bile  salts  are  formed  in  the  liver  cells.  In  the 
intestines  a  portion  of  the  bile  salts  is  reabsorbed,  carried 
to  the  liver,  and  again  excreted  ;  or  they  may  be  split  up  in 
the  intestines  into  their  constituents,  the  glycine  and  taurine 
being  carried  to  the  liver  to  be  reutilized,  while  the  cholalic 
acid  is  excreted.  This  economical  measure  has  a  twofold 
advantage,  for  not  only  can  the  glycine  and  taurine  be  used 
over  and  over  again,  but  the  bile  acids  are  the  best  of 
cholagogues,  and  stimulate  the  production  of  bile. 


THE  LIVER  AND  PANCREAS  223 

Bile  is  secreted  under  a  very  low  pressure,  which  is  the 
reverse  of  what  occurs  in  the  saliva ;  low  as  the  pressure 
is  ('58  inch  of  mercury),  it  is  higher  than  that  of  the  blood 
in  the  portal  vein.  If  the  pressure  in  the  bile  duct  be 
raised  the  bile  is  reabsorbed,  being  taken  up  by  the 
lymphatics  of  the  liver  and  so  conveyed  to  the  blood 
stream.  It  is  probable  that  the  majority  of  cases  of 
jaundice  are  due  to  obstructive  causes,  though  exceptions 
to  this  rule  occur.  The  secretion  of  bile  is  a  continuous 
one ;  whether  the  animal  be  in  full  digestion  or  fasting, 
the  flow  is  not  intermittent  as  in  the  case  of  the  saliva. 
Though  continuous,  it  is  not  uniform  ;  it  reaches  its 
maximum  in  the  dog  between  the  second  and  fourth  hours 
after  a  meal ;  this  is  followed  l)y  a  fall,  and  again  about 
the  seventh  hour  by  a  rise.  A  similar  curve  is  given  by 
the  pancreatic  secretion,  and  it  can  be  shown  that  a  specific 
substance,  secretin,  which  stimulates  the  production  of  pan- 
creatic juice,  also  hastens  the  secretion  of  bile. 

In  those  animals  possessing  a  gall-bladder  this  receptacle 
is  filled  with  bile  during  abstinence,  or  if  it  be  empty  it  is 
filled  even  during  digestion.  The  reflux  of  bile  from  the 
biliary  duct  to  the  gall-bladder  is  caused  by  a  sphincter- 
like contraction  of  that  portion  of  the  duct  penetrating  the 
wall  of  the  intestine,  by  which  means  the  bile  is  driven 
back  through  the  cystic  duct  to  the  gall-bladder.  The  bile 
as  formed  is  propelled  along  the  bile  ducts  by  a  contrac- 
tion of  the  muscular  coat  of  the  tubes,  but  doubtless  both 
the  forcing  onward  of  the  bile  and  the  circulation  through 
the  liver  are  largely  assisted  by  the  respiratory  move- 
ments, during  which  the  liver  is  compressed  between  the 
abdominal  viscera  and  the  diaphragm. 

By  some  it  is  considered  that  no  bile  enters  the  bowel 
while  the  stomach  is  empty,  but  that  the  passage  of  acid 
chyme  along  the  duodenum  causes  a  reflex  contraction  of 
the  gall-bladder,  and  an  injection  of  bile  into  the  intestine. 

The  amount  of  bile  secreted  varies,  but  is  greater  in 
herbivora  than  carnivora.  Colin's  experiments  gave  him 
the  following  amounts  a^  hourly  secretions  : 


224     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Horse  -  8    ozs.  to  10    ozs.  per  hour  (250  to  310  grammes). 

Ox-  -  8    ozs.  to    4    ozs.    ,,       ,,       (93  to  120  grammes). 

Sheep  -  \  oz.   to    .5    ozs.    ,,       .,         (8  to  150  grammes). 

Pig  -  2    ozs.  to    5    ozs.    .,       „       (62  to  150  grammes). 

Dog  -  I  oz.    to      ^  oz.      „       ,,         (8  to    16  grammes). 

The  Use  of  the  Bile  from  a  digestive  point  of  view  is 
disappointing,  inasmuch  as  it  does  not  digest  in  the  sense 
that  pepsin  and  trypsin  do.  It  is  intimately  connected 
with  the  function  of  the  pancreas,  with  which  object  the 
secretions  are  poured  out  either  close  together  in  the  bowel, 
or,  as  in  some  animals,  by  a  duct  practically  common  to 
the  two  glands.  As  the  horse  possesses  no  gall-bladder 
the  secretion  is  poured  into  the  intestine  as  fast  as  it  is 
prepared ;  not  so  with  the  ox,  sheep,  pig  and  dog,  where 
the  bulk  of  it  is  stored  up  in  a  capacious  receptacle  until 
required.  The  reason  offered  for  the  horse  having  no  gall- 
bladder is  that  as  digestion,  under  ordinary  circumstances, 
never  ceases  the  bile  is  poured  into  the  bowel  as  fast  as  it 
is  secreted,  but  that  in  the  case  of  other  animals  it  is  only 
poured  out  when  the  contents  of  the  stomach  are  passing 
out  into  the  intestine.  This  explanation,  however,  does  not 
meet  all  the  difficulties  of  the  case.  The  following  animals, 
like  the  horse,  have  no  gall-bladder — the  camel,  elephant, 
rhinoceros,  tapir,  and  deer. 

The  bile  being  alkaline  its  jfirst  action  on  the  chyme  is  to 
neutralize  the  gastric  juice  and  precipitate  the  albumoses 
and  peptones.  One  effect  of  this  is  probably  to  delay  the 
progress  of  the  chyme  along  the  bowel,  by  which  means 
absorption  is  assisted. 

Bile  has  a  solvent  and  emulsifying  effect  on  fats,  being 
more  active  in  the  presence  than  in  the  absence  of  pan- 
creatic juice.  Bile  cannot  split  up  fats  into  fatty  acids  and 
glycerine  as  the  pancreas  does,  but  if  free  fatty  acids  are 
present  the  bile  salts  are  decomposed,  their  soda  set  free, 
and  soluble  soaps  formed  ;  the  soaps  so  formed  assist  in 
rendering  the  emulsifying  effect  of  the  bile  permanent  and 
the  absorption  of  fat  much  easier.  Fat  will  not  readily 
pass    through   a    membrane,    but    if    the    latter   be   first 


THE  LIVER  AND  PANCREAS  225 

moistened  with  bile  the  passage  is  greatly  facilitated.  In 
Voit's  experiments  on  dogs  it  was  found  that  by  cutting 
off  the  flow  of  bile  to  the  intestine  the  absorption  of  fat 
fell  from  99  per  cent,  to  40  per  cent.  The  solvent  action 
of  bile  on  fat  is  the  chief  digestive  function  of  this  fluid, 
the  working  constituents  being  the  bile  salts.  Bile  has  no 
action  on  proteid.  According  to  Hofmeister  the  bile  of 
the  ox,  sheep,  and  horse  converts  starch  into  sugar,  whilst 
the  bile  of  the  pig  and  dog  possesses  no  such  or  only  to  a 
limited  extent.  It  has  been  said  that  bile  has  an  antiseptic 
effect  on  the  intestinal  contents,  keeping  them  from  putre- 
faction and  promoting  peristalsis,  for  it  has  been  found 
that  when  it  is  prevented  from  entering  the  bowel,  con- 
stipation and  extreme  fcetor  of  the  intestinal  contents 
result.  Bile,  however,  is  not  a  true  antiseptic.  The  clay- 
coloured  faeces  obtained  in  jaundice  are  probably  due  to  the 
presence  of  unacted-on  fat ;  the  fat  encloses  the  proteids 
which  putrefy,  hence  the  odour.  The  bile  acts  as  a  natural 
purgative  and  keeps  up  intestinal  peristalsis ;  by  so  doing 
it  hurries  the  food  out  of  the  system  before  it  undergoes 
putrefactive  decomposition. 

Grlycogen. 

It  is  quite  certain  that  the  largest  gland  in  the  body 
must  have  some  other  function  than  that  of  the  secretion 
of  a  fluid  of  comparatively  unimportant  digestive  power, 
and  such  is  the  case  ;  the  liver  manufactures  and  stores 
up  in  its  cells  a  peculiar  substance  known  as  glycogen  or 
animal  starch.  Glycogen  is  spoken  of  as  starch,  though  it 
diflers  from  vegetable  starch  in  many  important  character- 
istics ;  thus,  it  is  soluble  instead  of  insoluble  in  cold  water, 
and  it  is  stained  reddish-brown  instead  of  blue  by  iodine. 

The  literature  of  the  formation  and  use  of  glycogen  is 
extensive,  perhaps  no  substance  has  given  rise  to  greater 
controversy ;  yet  the  glycogen  story  which  is  accepted  to- 
day is  the  one  originally  related  by  Claude  Bernard,  who 
was  the  discoverer  of  this  singular  substance. 

The   sugar  in  the  food,  and  that  derived  from  starch- 

15 


226     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

conversion,  finds  its  way  by  means  of  the  intestinal  vessels 
into  the  portal  vein,  from  here  it  passes  into  the  liver  ; 
under  ordinary  circumstances  it  is  stored  up  in  the  liver  as 
glycogen,  being,  in  fact,  reconverted  into  a  kind  of  starch, 
and  gradually  doled  out  to  the  system  as  sugar  when 
required.  The  liver  regulates  the  amount  of  sugar  which 
should  pass  into  the  blood ;  so  much  and  no  more  is  ad- 
mitted to  the  circulating  fluid,  the  amount  varying  between 
•05  and  '15  per  cent.  The  sugar  in  the  blood  of  the  ox 
was  estimated  by  C.  Bernard  at  '17  per  cent.,  in  the  calf 
•1  per  cent.,  and  in  the  horse  '09  per  cent.  When  the 
liver  fails  to  regulate  the  amount  of  sugar  in  the  blood 

^^#V@\^       (     (^ 


AFTER   FOOD. 


Fig.  56. — Liver  Cells  from  the  Dog  during  Fasting  and  after 
Food  (Waller,  after  Heidenhain). 

During  fasting  the  cells  contain  no  glycogen  ;  after  receiving  food  they 
become  swollen  with  this  substance. 

diabetes  is  produced,  and  this  occurs  when  the  amount  of 
sugar  rises  to  more  than  "2  per  cent. 

The  glycogen  which  is  thus  stored  up  in  the  liver  for 
future  use  may  in  two  days  be  made  to  disappear  by 
starving  and  working  the  animal,  the  material  in  this  way 
escaping  from  the  liver  as  sugar,  and  passing  into  the 
general  circulation  through  the  hepatic  veins.  If  food, 
particularly  carbo-hydrate,  be  now  given  the  store  of 
glycogen  is  rapidly  replenished,  and  the  sugar-liberating 
functions  once  more  established  (Fig.  56). 

The  storing  up  of  glycogen  by  the  liver  and  its  subsequent 
utilization  is  very  closely  allied  to  a  similar  process  in  the 
vegetable  kingdom ;  the  starch  in  the  leaves  of  plants  may 
pass  down  the  stem  as  sugar  for  the  purpose  of  nourish- 


THE  LIVER  AND  PANCREAS  227 

ment  and  be  again  formed  into  starch.  Similarly  in  the 
animal  the  starch  must  be  first  converted  into  sugar  before 
the  bloodvessels  of  the  bowel  can  take  it  up,  then  in  the 
liver  once  more  converted  into  glycogen,  and  lastly  again  into 
sugar  before  being  finally  used  by  the  tissues.  The  sugar 
formed  from  starch  in  the  bowel  is  maltose,  while  that 
formed  in  the  liver  from  glycogen  is  glucose.  This  con- 
version of  glycogen  into  glucose  is  due  to  the  presence  of 
a  ferment  in  the  liver  cells. 

The  total  amount  of  glycogen  obtained  from  a  given 
quantity  of  food  is  not  wholly  stored  in  the  liver  ;  the  latter 
organ  can  only  hold  a  limited  amount,  which  in  the  dog 
does  not  exceed  17  per  cent,  of  its  weight,  and  in  other 
animals  is  less.  We  know  as  a  fact,  that  the  liver  having 
taken  up  all  the  sugar  it  can  from  the  portal  vessels  and 
converted  it  into  stored-up  glycogen,  allows  the  balance  to 
pass  through  the  hepatic  veins  into  the  general  circulation 
as  sugar,  and  that  it  is  deposited  in  other  organs,  princi- 
pally the  muscles,  as  glycogen  for  future  use.  The  muscles 
of  well-fed  animals  contain  in  this  way  a  considerable 
quantity  of  glycogen ;  even  after  nine  days'  starvation  in 
the  horse  from  1  per  cent,  to  2*4  per  cent,  has  been  found. 
Ordinarily  it  may  be  stated  that  the  muscles  hold  as  much 
glycogen  as  the  liver,  but  it  takes  longer  by  means  of  work 
and  starvation  to  free  the  muscles  from  glycogen  than  to 
clear  the  liver. 

The  presence  of  glycogen  in  muscle  is  not  essential  to 
contraction,  for  there  are  muscles  in  which  no  glycogen  is 
found  and  yet  in  which  active  contraction  takes  place.  In 
the  muscles  of  the  embryo,  before  striation  has  occurred, 
the  amount  of  glycogen  existing  is  something  considerable  ; 
as  much  as  40  per  cent,  of  the  dry  material  of  the  embryo 
muscle  may  consist  of  this  substance.  As  striation  appears 
the  glycogen  leaves  the  muscles  to  a  great  extent,  and  the 
liver  takes  on  the  process  of  production. 

The  Use  of  Glycogen. — The  existence  of  glycogen  in  the 
embryonic  muscle  points  to  its  use  in  active  nutrition  and 
rapid  growth ;  further,  it  is  found  in  the  placenta,  where  it 

15—2 


228     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

is  used  for  the  nourishment  of  the  foetus,  and  also  in 
rapidly  developing  cells,  such  as  some  found  in  cartilage 
and  the  white  cells  of  the  blood ;  in  all  these  and  other 
places  it  is  simply  stored  for  future  requirements.  In  the 
adult  the  chief  use  of  glycogen  is  to  facilitate  the  metabolic 
production  of  muscular  energy  and  animal  heat,  and  this  it 
does  in  its  glucose  form  as  the  result  of  oxidation. 

The  sources  of  glycogen  have  been  a  fertile  cause  of 
discussion  and  object  of  experimental  inquiry.  It  was 
natural  to  consider,  as  we  have  so  far  done,  carbo-hydrate 
material  as  the  chief  contributing  agent ;  it  was  less  certain 
whether  proteids  contributed,  while  the  consensus  of  opinion 
was  against  fat  taking  any  share  in  the  process.  We  must 
examine  each  of  these  in  a  little  more  detail. 

We  have  learnt  that  starch  is  not  absorbed  as  starch, 
but,  depending  upon  the  nature  of  the  diastatic  ferment,  is 
converted  into  maltose,  or  maltose  and  some  dextrin,  and 
subsequently  dextrose.  These  sugars  are  readily  converted 
into  glycogen  by  the  liver  cells  by  the  process  of  dehydra- 
tion. Cane-sugar  and  milk-sugar  are  not  readily  converted 
into  glycogen,  but  since  these  double  sugars  undergo  inver- 
sion in  the  intestinal  canal  before  absorption — cane-sugar 
into  dextrose  and  levulose,  and  milk-sugar  into  dextrose 
and  galactose — they  may  in  this  form  be  readily  converted 
into  glycogen.  The  effect  of  proteid  on  glycogen  formation 
is  not  so  easily  settled.  It  is  observed  that  in  diabetes,  though 
all  carbohydrate  food  be  withheld,  yet  sugar  may  appear 
in  the  urine  on  an  exclusively  proteid  diet ;  the  same  thing 
is  observed  in  the  experimental  glycosuria  which  may  be 
produced  by  the  administration  of  phloridzin,  and,  further- 
more, that  sugar  may  be  produced  even  when  the  animal  is 
starved.  The  conclusion  appears  irresistible  that  proteid 
can  produce  sugar,  and  this  is  explained  by  saying  that 
certain  proteids  split  into  a  nitrogenous  and  non-nitrogenous 
portion,  the  former  being  converted  into  urea,  while  the  non- 
nitrogenous  residue  is  converted  into  sugar  and  may  thus 
give  rise  to  glycogen.  Proteids,  such  as  casein,  which  do 
not  contain  a  carbo-hydrate  group,  may  take  no  share  in  the 


THE  LIVEK  AND  PANCREAS  229 

production  of  glycogen.  There  are  a  few  observers  who 
regard  fat  as  a  source  of  glycogen,  and  there  is  some 
evidence  to  show  that  it  may  contribute,  for  it  has  been 
said  that  glycerin  acts  as  a  sugar  former.  If  this  is  so 
the  conversion  of  fat  into  glycogen  through  its  splitting  up 
in  the  intestinal  canal  into  fatty  acid  and  glycerin  would 
not  be  a  difficult  matter.  On  the  other  hand,  experiment 
shows  that  when  an  animal  is  fed  solely  on  fat,  the 
glycogen  disappears  from  the  liver  as  quickly  as  it  does  in 
starvation.  The  question  is,  therefore,  very  far  from  being 
settled. 

The  Liver  Ferment. — When  a  liver  is  rapidly  removed 
from  the  body  of  a  recently  killed  animal  which  has  been 
appropriately  fed,  it  contains  a  quantity  of  glycogen ;  if  it 
is  allowed  to  stand  the  glycogen  gradually  becomes  reduced 
in  amount  and  sugar  takes  its  place  ;  finally  all  the  glycogen 
disappears.  This  change  is  brought  about  by  a  diastatic 
ferment  in  the  liver  cells  which  changes  the  glycogen  into 
sugar.  If  the  liver  on  removal  from  the  body  be  rapidly 
minced  and  boiled,  the  ferment  is  destroyed  and  dextrose  is 
not  formed. 

How  the  Supply  of  Sugar  is  Regulated. — Glycogen  is  a 
temporarj^  reserve  of  carbo-hydrate  material,  which  is  issued 
as  required  to  the  system  in  the  form  of  glucose,  and  by  the 
process  of  oxidation  yields  heat  and  energy.  It  is  readily 
used  up  in  the  interval  between  meals  and  readily  renewed. 
The  sugar  in  the  blood  maintains  a  remarkably  regular 
percentage,  -1  to  "2  per  cent.,  and  no  doubt  this  is  effected 
by  the  gradual  supply  of  this  material  from  the  temporary 
reserve  stored  in  the  form  of  glycogen.  Should  the  per- 
centage of  sugar  rise  in  the  blood,  the  excess  is  got  rid  of 
through  the  kidneys  (diabetes)  and  lost  to  the  body.  The 
liver  itself  does  not  appear  to  be  able  to  regulate  its  sugar 
output  to  the  blood ;  this  would  seem  to  be  one  function  of 
the  pancreas,  the  '  internal  secretion  '  of  which,  in  some  way 
which  is  not  clearly  understood,  prevents  the  liver  giving  off 
its  glycogen  as  sugar  too  rapidly.  Removal  of  the  pancreas, 
as  we  shall  show  later,  is  followed  by  diabetes.     If  expressed 


230    A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

pancreatic  and  expressed  muscle  juice  be  mixed  together 
an  active  glycolytic  substance  results,  and  it  is  considered 
that  as  neither  of  the  above  are  capable  of  acting  alone, 
the  internal  ferment  of  the  pancreas  acts  upon  a  ferment 
in  the  muscles  and  makes  the  decomposition  of  sugar 
possible. 

Diabetic  Puncture. — Bernard  discovered  that  if  the  floor 
of  the  fourth  ventricle  be  punctured  at  a  certain  definite  spot, 
temporary  diabetes  resulted,  the  urine  contained  sugar,  and 
the  liver  possessed  no  glycogen.  This  spot  is  known  as  the 
diabetic  centre,  and  the  effect  of  the  puncture  is  to  stimulate 
it  and  temporarily  destroy  the  glycogen-holding  capacity  of 
the  liver,  in  consequence  of  which  the  material  is  liberated 
as  sugar.  The  evidence  of  this  is  that  if  the  animal  be 
starved  before  the  puncture  is  made  no  sugar  appears  in 
the  urine.  Stimulation  of  the  central  end  of  the  vagus 
or  of  the  depressor  nerve  produces  glycosuria,  though 
stimulation  of  the  first  causes  the  abdominal  blood-pressure 
to  rise,  and  of  the  second  causes  it  to  fall.  From  this  circum- 
stance it  is  considered  that  the  effect  of  the  puncture  is  not 
to  produce  mere  vascular  dilatation,  but  rather  that  it 
stimulates  some  secretory  nerve.  The  diabetic  centre  is  a 
reflex  one,  the  afferent  or  ingoing  nerves  being  most  of 
the  sensory  nerves,  the  efferent  being  the  spinal  cord, 
sympathetic  and  splanchnics.  It  has  been  suggested  that 
the  muscles  at  the  moment  of  contraction  set  up  afferent 
impulses  which  are  carried  to  the  diabetic  centre  and  sugar 
thus  liberated.  This  would  place  the  muscle  in  the  position 
of  not  only  using  up  sugar  but  of  being  able  to  call  forth 
its  production  as  required.  If  this  be  proved  to  be  true, 
it  is  easy  to  understand  how  the  heart,  the  most  active 
muscle  in  the  body,  is  able  to  regulate  the  production  of  its 
energy  yielding  substance. 

Further  Uses  of  the  Liver. 

fi  We  have  studied  two  uses  of  the  liver,  viz.,  the  formation 
of  bile  and  the  storing  up  of  glycogen,  but  there  are  other 
functions  of  this  gland  to  consider. 


THE  LIVER  AND  PANCREAS  231 

Another  important  use  of  the  liver  is  the  formation  of 
urea.  The  source  of  urea  is  the  proteid  constituent  of 
the  food,  which  in  the  process  of  disintegration  yields 
certain  amido-acids  such  as  leucine  and  tyrosine.  These 
substances  may  be  formed  in  the  intestinal  canal  as  the 
result  of  pancreatic  digestion,  or  they  may  be  formed  in 
the  living  cell  as  the  result  of  the  breaking  down  of 
proteid.  Under  any  circumstances  the  leucine  undergoes 
a  series  of  oxidative  changes,  mainly  in  the  liver,  resulting 
in  the  formation  of  urea  which  is  passed  on  to  the  kidneys 
for  excretion. 

The  further  facts  regarding  the  formation  of  urea  are 
best  dealt  with  in  the  section  devoted  to  the  kidneys. 

As  the  result  of  proteid  decomposition  in  the  intestinal 
canal  certain  aromatic  compounds  are  formed ;  these  are 
united  with  sulphuric  acid  and  got  rid  of  by  the  kidneys 
as  conjugated  sulphuric  acids.  In  this  combination  the 
originally  jmsoiious  inoteid  products  are  converted  into 
non-poisonous  ones,  and  this  change  is  effected  in  the 
liver  (Bunge).  In  this  we  have  a  very  important  function 
of  the  liver  demonstrated,  viz.,  as  a  neutraliser  of  poisons 
introduced  into  the  blood  by  the  intestines.  It  is  a  note- 
worthy fact  that  many  metallic  poisons  are  also  arrested  in 
the  liver,  for  example  mercury  and  arsenic. 

The  numerous  and  complicated  changes  produced  by  the 
liver  may  thus  be  summarized :  It  forms  bile,  regulates 
the  supply  of  sugar  to  the  system,  and  stores  up  as 
glycogen  what  is  not  required.  It  guards  the  systemic 
circulation  against  the  introduction  of  certain  nitrogenous 
poisons,  such  as  ammonia,  by  transforming  them  into  urea, 
and  against  other  poisons  of  proteid  origin  by  converting 
them  into  harmless  products,  by  conjugation  with  alkaline 
sulphates. 


232     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Section  2. 
The    Pancreas. 

The  fluid  secreted  l)y  the  pancreas  performs  certain 
important  functions  in  digestion.  It  has  been  remarked 
that  there  is  scarcely  any  animal  which  does  not  possess  a 
secretion  allied  to  the  pancreatic ;  even  those  invertebrates 
without  a  peptic  or  biliary  apparatus  are  in  possession  of 
one.  From  the  resemblance  of  the  pancreas  to  the  salivary 
glands,  it  has  been  termed  the  abdominal  salivary  gland. 

The  pancreatic  fluid  from  herbivora  can  only  be  obtained 
with  extreme  difficulty ;  to  establish  a  i^ancreatic  fistula 
in  the  horse  is  a  formidable  operation,  necessitating  an 
incision  from  the  sternum  to  the  pubis  and  the  turning 
back  of  the  bowels.  Colin  has  established  these  fistulse 
both  in  the  horse  and  ox,  but  the  profound  impression  on 
the  nervous  system  ])roduced  by  such  extensive  inter- 
ference must  considerably  affect  the  character  of  the 
secretion  and  the  amount  manufactured. 

Pancreatic  fluid  is  an  alkaline,  clear,  colourless  fluid  like 
water,  and  though  viscid  in  some  animals  is  not  so  in  the 
horse.  It  has  a  saltish  unpleasant  taste,  and  a  specific 
gravity  of  about  1010  ;  the  viscid  secretion  of  the  dog  has 
a  specific  gravity  of  1030.  The  following  analysis  of  the 
fluid  in  the  horse  is  given  l)y  Hoppe-Seyler : 

Water     -     98-25 

r  Organic  matter  -       'SS,  containing  "86  of  fer- 
J  ments. 

Solids     -       1-74  <  g^j^g         _  _       .gg^         ^^      ^^^-^  sodium 

{  phosphate. 

100-00 

Schmidt  found  the  fluid  of  the  dog  to  have  the  following 
composition : 

Water  -     90-00 

(  Organic  matter  -     9-04 

Solids    -       9-92  -|    Salts  -  -  -     '88     containing    much 

[  sodium  chloride. 


THE  LIVER  AND  PANCREAS  233 

The  salts  present  are  sodium  chloride  in  abundance, 
potassium  chloride  in  traces,  sodium  carbonate  and 
phosphate,  calcium  and  magnesium  phosphates  in  small 
quantities.  The  organic  solids  are  remarkable  for  the 
amount  of  proteid  present  in  them ;  they  vary  in  amount 
in  different  animals,  for  example  9  per  cent,  in  the  dog  and 
■9  per  cent,  in  the  horse. 

Mechanism  of  Pancreatic  Secretion. — The  pancreatic  secre- 
tion is  influenced  by  special  secretory  nerves ;  stimulation 
of  the  vagus  or  splanchnic  may,  after  a  long  latent  period, 
give  rise  to  a  secretion,  though  it  is  not  yet  settled  whether 
these  fibres  produce  it  during  the  act  of  digestion.  The 
outpouring  of  the  acid  chyme  from  the  stomach  into  the 
duodenum  at  once  gives  rise  to  a  secretion  of  pancreatic 
juice,  and  it  was  supposed  that  the  acid  acted  on  the 
secretory  nerves  and  produced  a  secretion  reflexly.  Bayliss 
and  Starling,  however,  demonstrated  the  remarkable  fact 
that  if  an  extract  of  the  mucous  membrane  of  the 
duodenum  or  jejunum  be  made  by  scraping  the  bowel,  and 
acting  on  it  by  weak  hydrochloric  acid,  a  substance  may  be 
obtained  which  when  injected  into  the  blood  produces  a 
profuse  pancreatic  secretion.  To  this  internal  secretion  of 
the  intestinal  cells  they  gave  the  name  Secretin,  the  nature 
of  which  has  not  been  determined.  Two  facts  are  clearly 
established,  first,  that  it  is  not  a  ferment  as  it  is  not 
destroyed  by  boiling,  and  secondly  that  acid  is  an  essential 
part  of  the  process,  for  if  the  mucous  membrane  of  the 
bowel  be  extracted  with  either  water  or  saline  solution 
secretin  is  not  obtained.  It  is  the  acid  chyme,  therefore, 
acting  on  the  mucous  membrane  of  the  intestine  which 
produces  secretin  ;  this  is  absorbed  by  the  blood,  and  thus 
produces  its  specific  action  on  the  pancreas. 

Uses  of  the  Secretion. — The  pancreatic  juice  is  poured 
into  the  bowel  in  the  horse  and  sheep  by  an  opening 
common  to  the  pancreas  and  liver,  while  in  the  ox,  pig, 
and  dog,  the  ducts  of  the  liver  and  pancreas  are  separate, 
and  open  within  a  short  distance  of  each  other. 

It  is  essentially  a  digestive  fluid,  and  acts  on  the  three 


234     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

classes  of  food  stuffs,  viz.,  proteids,  fats  and  carbo- 
hydrates ;  to  enable  this  to  be  effected,  it  contains  three 
ferments  or  their  precursors,  viz. : 

A  Proteolytic  Enzyme  which  acts  on  proteids  (Trypsin). 

A  Diastatic  Enz^-me  which  acts  on  carbo-hydrates  (Amylopsin). 

A  Lipolytic  Enzyme  which  acts  on  fats  {Lipase  or  Steajjsin). 

Observations  appear  to  show  that  the  proportion  of  each 
of  these  ferments  in  the  secretion  depends  on  the  character 
of  the  food  ;  if,  for  example,  the  food  is  rich  in  fat  the 
secretion  would  be  rich  in  lipase.  It  is  also  probable  that 
not  only  does  the  nature  of  the  food  determine  the  pre- 
dominance of  each  enzyme,  but  also  the  amount  of  fluid 
to  be  secreted.  This,  as  a  rule,  reaches  its  maximum  in 
the  dog  between  the  second  and  fourth  hour  after  taking 
food,  and  corresponds  to  the  greatest  activity  of  the  liver. 
In  dogs  which  have  been  starved  active  secretion  of  bile, 
pancreatic  juice,  and  intestinal  fluid,  takes  place,  it  is  said, 
every  two  hours,  and  lasts  for  twenty  minutes.  The  cause 
of  this  is  by  no  means  clear.  All  the  fluid  thus  poured  out 
is  reabsorbed. 

Trypsin. — It  has  been  observed  that  pancreatic  juice 
taken  direct  from  a  fistula  in  the  duct  may  have  little  or  no 
action  on  the  proteids  of  food,  but  if  the  same  fluid  be 
allowed  to  become  contaminated  by  the  intestinal  contents 
it  at  once  becomes  active.  Evidently  the  addition  of  a 
something  from  the  bowel  has  brought  about  a  marked 
change  in  the  proteolytic  character  of  the  secretion. 
Investigation  shows  that  though  the  secretion  taken  direct 
from  the  pancreas  contains  the  precursor  of  trypsin,  viz., 
trypsinogen,  yet  in  the  latter  form  the  ferment  is  unable  to 
act  on  the  proteid  of  food  until  it  has  itself  been  acted 
upon  by  another  ferment.  This  ferment  is  derived  from 
the  mucous  membrane  of  the  intestinal  canal.  A  ferment 
acting  on  a  ferment  has  been  described  as  a  kinase,  and  as 
this  one  is  derived  from  the  bowel  it  is  called  enterokinase, 
a  very  small  amount  of  which  is  capable  of  converting 
inactive   trypsinogen   into   active   trypsin.     It  is  remark- 


THE  LIYEE  AND  PANCEEAS  235 

able  that  of  the  three  ferments  secreted  by  the  pancreas, 
trypsin  is  the  only  one  which  is  secreted  in  an  inactive 
condition.  Pawlow  considers  this  to  be  due  to  the  fact 
that  if  trypsin  were  active  i»  the  pancreatic  juice,  it  would 
destroy  its  fellow-ferments,  but  that  in  the  bowel  these 
ferments  are  protected. 

The  fact  that  extracts  of  pancreas,  as  obtained  usually 
from  a  slaughter-house,  may  be  made  more  tryptically  active 
by  the  addition  of  a  little  dilute  acetic  acid,  does  not  now 
imply  that  the  acid  has  converted  the  trypsinogen  into 
trypsin,  as  has  usually  been  supposed.  The  pancreas  used 
in  the  preparation  of  the  extracts  is  already  contaminated 
with  minute  quantities  of  enterokinase,  whose  activity  is 
greatly  increased  by  neutralizing  the  alkalinity  of  the 
extracts.  If  a  pancreas  be  obtained  under  conditions 
which  ensure  the  absence  of  any  admixture  with  even 
traces  of  enterokinase,  extracts  of  such  a  pancreas  cannot 
be  rendered  more  tryptically  active  by  the  addition  of 
dilute  acid  (Starling). 

It  is  here  desirable  to  draw  attention  to  the  fact  that 
secretin  and  enterokinase  are  both  derived  from  the 
mucous  membrane  of  the  intestinal  canal,  and  care  must 
be  taken  to  avoid  confusing  them :  the  former  is  not  a 
ferment,  the  latter  is.  The  function  of  secretin  is  to  cause 
the  production  of  pancreatic  juice,  that  of  enterokinase  is 
to  endow  one  of  the  ferments  of  the  pancreatic  juice  with 
its  remarkable  proteolytic  properties. 

The  action  of  trypsin  on  proteids  is  most  interesting. 
The  proteid  molecule  is  very  complex ;  the  use  of  trypsin  is 
to  split  it  up  into  simpler  products,  with  the  object  of 
facilitating  its  absorption.  As  we  shall  point  out  later,  no 
food  substance  is  taken  up  excepting  in  its  simpler  form, 
and  the  proteids  of  oats,  barley,  hay,  or  flesh,  have  to  be 
reconstructed  in  order  to  form  part  of  the  tissues  of  the 
living  animal.  To  enable  this  to  be  done  trypsin  acts  on 
the  large  proteid  molecule  and  breaks  it  down  in  the  pro- 
duction of  a  number  of  simpler  bodies  of  smaller  molecular 
weight ;    on  these  the  tissue  cells  set  to  work,  and  by  a 


236     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

process  of  synthesis  construct  the  form  of  proteid  needed 
by  the  body.  It  can  be  easily  shown  that  the  action  of 
trypsin  on  proteid  is  much  more  satisfactory  and  thorough 
if  the  latter  has  previously  been  acted  upon  by  pepsin. 
Trypsin  like  pepsin  produces  albumose  and  peptones  ;  but 
the  process  does  not  stop  at  peptone,  no  peptone  can  be 
found  in  the  blood,  and  none  remains  after  a  prolonged 
pancreatic  digestion.  The  action  of  the  trypsin  is,  in  fact, 
to  produce  a  large  number  of  simpler  end-products,  of 
which  the  amido-acids  leucine  and  tyrosine  are  the  best 
known  and  most  easily  obtained :  besides  these  aspartic 
and  glutaminic  acids,  tryptophan,  and  the  hexone  bases 
lysine,  arginine,  and  histidine.  Should  any  proteid  or 
peptone  have  escaped  the  action  of  pepsin  and  trypsin,  it 
may  be  attacked  by  another  enzyme  found  in  the  intes- 
tinal mucous  membrane,  known  as  erepsin,  which  also  has 
the  power  of  breaking  down  albumoses  and  peptones  into 
leucine  and  tyrosine.  Erepsin  is  found  in  most  of  the 
tissues  of  the  body,  so  is  not  specific  to  the  intestine. 

Under  the  influence  of  bacterial  action  aromatic  bodies 
are  formed — phenol,  indol,  and  skatol,  the  latter  being 
responsible  for  the  faecal  odour  of  a  pancreatic  digestion 
mixture.  These  substances  are  produced  from  tryptophan, 
one  of  the  end  products  of  the  primary  decomposition  of 
proteids. 

Amylopsin,  the  diastatic  ferment,  has  an  action  on  starchy 
food  similar  to  that  of  ptyalin,  but  more  rapid,  the  final 
products  being  maltose  and  achroodextrin.  The  hydrolytic 
action  of  amylopsin  stops  at  maltose  and  achroodextrine, 
but  these  are  in  turn  attacked  by  the  maltase  of  the  succus 
entericus  and  converted  into  dextrose. 

Lipase  or  steapsin,  the  fat-splitting  ferment,  acts  upon 
neutral  fats,  splitting  them  into  free  fatty  acid  and 
glycerin.  The  splitting  process  is  followed  by  saponifica- 
tion, viz.,  the  liberated  fatty  acid  combines  with  the 
alkaline  salts  to  form  soaps ;  as  the  result  of  this  the 
production  of  an  emulsion  becomes  possible.  In  emulsifi- 
cation  the  oil  globules  are  rendered  extremely  small  without 


THE  LIYEK  AND  PANCREAS  237 

the  power  of  coalescing,  and  at  one  time  it  was  considered 
that  fat  in  this  finely  divided  state  was  capable  of  entering 
the  villi,  but  there  can  be  no  doubt  that  the  minute  fat 
globules  are  further  split  into  fatty  acid  and  glycerin,  and 
that  only  the  products  of  this  splitting  enter  the  villi. 
Once,  however,  within  the  epithelial  cells  of  the  villi, 
the  synthesis  of  fatty  acid  and  glycerin  into  fat  becomes 
possible,  and  recent  work  indicates  that  this  may  be 
efiected  by  lipase ;  in  other  words,  the  same  ferment  which 
does  the  splitting  is  possessed  of  a  reversible  action. 
Lipase  is  readily  destroyed,  so  that  unless  quite  fresh  it 
does  not  work  in  artificial  digestions.  Under  natural  con- 
ditions it  is  greatly  aided,  and  the  process  rendered  much 
quicker  by  the  action  of  the  bile. 

On  p.  170  we  have  alluded  to  Pawlow's  work  on  the 
quantity  and  quality  of  the  gastric  juice,  being  regulated 
by  a  specific  action  on  the  part  of  the  food  itself.  Similarly, 
the  same  observer  has  shown  that  the  ferment  contents  of 
the  pancreatic  juice  are  adapted  to  the  character  of  the 
diet ;  a  definite  and  constant  diet  leads  to  the  formation  of  a 
pancreatic  juice  which  is  unable  to  deal  eftectively  with  a 
sudden  change  in  diet.  The  practical  bearing  of  this  in 
the  feeding  of  animals  is  far-reaching.  As  a  profession  we 
have  recognised  for  years  the  disastrous  effects  of  sudden 
changes  in  diet ;  modern  science  offers  the  explanation  of 
its  action,  which  in  all  probability  is  brought  about  as  the 
result  of  an  internal  secretion. 

The  Changes  occurring  in  the  Cells  of  the  gland  correspond 
very  closely  with  those  described  for  the  salivary  secretion. 

When  a  pancreas  or  lobe  of  a  pancreas  has  been  at  rest 
for  some  time  the  cells  forming  it  are  rendered  very  in- 
distinct ;  the  lumen  of  the  alveolus  is  nearly  obliterated  by 
their  swollen  condition,  and  the  cells  are  seen  crowded 
with  granules ;  these  are  so  arranged  that  the  margin 
presents  a  clear  or  fairly  clear  zone,  while  within  this  there 
is  an  intensely  granular  zone  (Fig.  57,  A).  The  minute 
granules  filling  the  cell  are  the  mother  substance  of  the 
secretion.     When  activity  commences  the  granules  appear 


238     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 


to  pass  centrally  towards  the  alveolus,  leaving  the  cell 
comparatively  clear  excepting  that  portion  immediately 
abutting  on  the  alveolus,  which  even  in  the  exhausted 
condition  remains  granular.  These  changes  result  in  the 
cells  becoming  distinct  and  clearly  defined,  and  moreover, 
as  they  have  emptied  their  granular  contents  into  the 
alveolus  as  pancreatic  secretion,  they  have  consequently 
become  much  smaller.  The  narrow  clear  zone  seen  in  the 
resting  gland  has  now  become  broad,  the  previously  choked 


Fig.  57. — A  Portion  of  the  Pancreas  of  the  Eabbit  (Kuhne 
AND  Sheridan  Lea).  A,  at  rest  ;  B,  in  a  state  of  activity 
(Foster). 

a,  The  inner  granular  zone  in  A  is  larger  and  more  closely  studded 
with  fine  granules  than  in  B,  in  which  the  granules  are  fewer  and 
coarser,  b,  The  outer  transparent  zone  is  small  in  A,  larger  in  B, 
and  in  the  latter  marked  with  faint  striae,  c,  The  lumen  is  very 
obvious  in  B,  but  indistinct  in  A.  d,  An  indentation  of  the 
junctions  of  the  cells  seen  in  the  active  but  not  in  the  resting 
glands. 

alveolus  is  readily  defined,  whilst  the  nucleus  of  the  cell, 
which  was  hidden  in  the  charged  condition,  can  easily  be 
seen  in  the  exhausted  gland  (Fig.  57,  B).  These  changes 
have  been  worked  out  on  the  pancreas  of  the  living  rabbit 
by  Kuhne  and  Sheridan  Lea. 

Amount  of  Secretion. — From  the  investigations  of  Colin 
and  others  we  know  that  in  most  animals  the  secretion  of 
pancreatic  juice  is  continuous  though  not  uniform.  In 
ruminants  the  largest  secretion  is  towards  the  end  of 
rumination  ;  in  the  dog  the  maximum  is  reached  between 
the  second  and  fourth  hours  after  feeding,  this  maximum 


THE  LIVEE  AND  PANCEEAS  239 

being  followed  by  a  fall,  and  about  the  seventh  hour  by  a  rise. 
It  will  be  remembered  that  the  bile  gives  a  similar  curve. 
In  the  dog  it  is  generally  considered  there  is  no  secretion 
during  starvation,  but  immediately  food  begins  to  pass  out 
of  the  stomach  the  pancreas  becomes  active.  In  this  con- 
nection, however,  it  is  desirable  to  remember  that  according 
to  some  observers  a  starved  dog  will  actively  secrete  pan- 
creatic juice  for  twenty  minutes  every  two  hours.  The 
continuous  secretion  of  the  gland  in  herbivora  is  provided 
for  by  all  the  lobes  not  being  active  at  the  same  time.  In 
the  ox  the  amount  of  juice  secreted  is  between  7  and  9  ozs. 
(265  grammes)  per  hour,  in  the  horse  it  is  much  the  same, 
in  the  sheep  i  to  j  oz.  (7  to  8  grammes),  pig  about  ^  to 
^  oz.  (5  to  15  grammes)  per  hour,  and  in  the  dog  still  less 
(2  to  8  grammes).  There  is  no  necessary  ratio  between 
the  size  of  the  animal,  the  weight  of  the  gland,  and  the 
amount  of  pancreatic  fluid  secreted ;  carnivora  secrete  rela- 
tively more  than  herbivora. 

The  pressure  under  which  the  pancreatic  juice  is  secreted 
is  low ;  it  is  said  to  be  equal  to  '67  inch  of  mercury,  which 
is  very  little  greater  than  that  of  the  bile. 

Pancreatic  Diabetes.* — If  the  pancreas  of  a  dog  be  com- 
pletely removed  there  is  a  disappearance  of  all  glycogen 
from  the  tissues,  and  the  animal  dies  in  the  course  of  a 
month  or  less  with  diabetes,  since  the  power  of  oxidizing 
glucose  is  lost.  The  glucose  consequently  accumulates  in 
the  blood,  and  is  separated  by  the  kidneys.  In  addition  to 
there  being  sugar  in  the  urine,  there  is  also  an  increase  in 
the  amount  of  fluid  produced  and  an  excess  of  urea.  If 
the  depancreated  animal  be  placed  on  a  purely  proteid 
diet,  no  difference  occurs  in  the  amount  of  sugar  excreted  ; 
even  if  no  food  be  given  sugar  is  still  formed.  If  the 
removal  of  the  gland  is  incomplete  glycosuria  still  occurs, 
but  it  will  vary  in  intensity  from  fatal  to  transient  effects, 
depending  upon  the  amount  of  pancreas  left  behind,  and 
this   is   explained  by  the  fact  that  sugar  may  be  formed 

*  To  avoid  repetition,  this  matter  should  be  read  in  conjunction  with 
the  remarks  on  Glycogen,  p.  229. 


240    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

from  proteid.  In  fact,  it  is  possible  by  experience  to  leave 
behind  just  sufficient  of  the  gland  to  prevent  diabetes 
arising.  In  any  case  fatal  results  may  be  avoided  by  graft- 
ing portions  of  pancreas  beneath  the  skin,  the  presence  of 
these  preventing  diabetes. 

Evidently,  therefore,  in  some  way  pancreatic  tissue  is 
intimately  mixed  up  with  the  sugar  question,  and  it  has 
been  assumed  that  the  pancreas  produces  an  '  internal 
secretion'  (see  p.  229),  the  use  of  which  is  devoted  to  pre- 
serving the  organism  from  an  excess  of  sugar,  either  by 
regulating  the  amount  which  is  to  be  liberated  into  the 
blood  from  the  seats  of  sugar  production  (liver  and  muscles), 
or  by  stimulating  the  sugar- splitting  action  of  the  tissue 
cells  (p.  230).  Very  little  is  known  of  the  subject ;  it 
certainly  appears  that  the  acting  agent  is  not  an  enzyme, 
for  its  property  is  not  lost  in  a  pancreas  which  has  been 
boiled ;  further,  the  internal  secretion  cannot  act  alone,  it 
requires  the  presence  of  a  ferment  formed  in  the  muscle, 
and  the  combination  is  then  capable  of  rapidly  decomposing 
dextrose. 

The  blood,  as  pointed  out  previously  (p.  226),  will  not 
tolerate  more  than  '2  per  cent,  of  sugar  in  circulation,  any- 
thing over  this  is  rejected  and  got  rid  of  through  the 
kidneys ;  in  pancreatic  diabetes  there  may  be  double  this 
amount  of  sugar  in  the  blood. 

Histologically,  the  pancreas  is  a  compound  tubular 
gland  like  the  salivary  glands,  but  there  are  certain  groups 
of  cells  peculiar  to  it  which  form  spherical  or  oval  bodies 
capable  of  being  seen  with  the  unaided  eye.  These  are 
known  as  the  Inlands  of  Langerhans ;  each  is  surrounded 
by  a  rich  capillary  network  of  bloodvessels,  and  the  view 
has  been  advanced  that  these  islands  are  the  seat  of  the 
internal  secretion  of  the  pancreas. 

Pathological. 

The  most  common  pathological  condition  of  the  liver  is  Jaundice, 
and  the  majority,  if  not  all,  cases  of  jaundice  are  obstructive,  viz., 
there  is  some  obstruction  to  the  free  pouring  out  of  bile ;  in  con- 
sequence there  is  a  backward  pressure,  which  being  greater  than  the 


THE  LIVER  AND  PANCREAS  241 

low  blood-pressure  under  which  bile  is  secreted,  the  bile  is  reabsorbed, 
and  stains  the  tissues  yellow.  There  is  also  a  form  of  jaundice 
affecting  the  horse  and  dog  in  South  Africa,  due  to  a  parasite  in  the 
blood,  but  in  these  cases  the  yellow  tint  is  derived  from  the  destruction 
of  red  corpuscles  and  the  liberation  of  their  colouring  matter.  Biliary 
Calculi,  consisting  largely  of  cholesterin,  are  not  uncommon  in 
ruminants,  but  rare  in  the  horse.  Fatty  Liver  is  common  in  all 
animals  over-fed  and  under-worked.  In  the  horse  it  may  lead  to 
Rupture  of  the  liver  during  work.  Enlargements  of  the  liver  are 
very  common  as  the  result  of  vascular  disturbance  elsewhere  ;  it  is 
not  uncommon  as  a  sequel  to  pneumonia,  strangles,  and  other 
prolonged  febrile  changes.  Abscess  of  the  liver  is  rare,  but  not 
unknown.  Parasitic  disease  of  the  liver  is  one  of  the  epizootic 
diseases  of  sheep,  and  common  in  the  ox,  but  rare  in  the  horse.  The 
parasite  occupies  the  bile  ducts,  which  become  practically  occluded. 

In  India,  calcareous  degeneration  of  the  liver  is  one  of  the  most 
common  affections  of  this  organ,  and  throughout  the  tropics  generally 
liver  disorders  are  very  frequent. 

The  pancreas  is  seldom  the  seat  of  pathological  disturbances  ;  it  may 
be  affected  with  abscess  in  strangles  or  in  septic  diseases,  but  such 
conditions  are  unrecognisable  during  life. 


16 


CHAPTER  VII 
ABSORPTION 

Section  1. 
Lymph. 

Lymph  may  be  regarded  as  the  material  by  which  the 
tissues  are  directly  nourished,  and  by  which  effete  material 
is  collected  from  them  and  taken  back  to  the  blood ;  there 
are  certain  non-vascular  structures,  such  as  the  cornea, 
cartilage,  etc.,  where  the  lymph  circulation  is  the  only 
means  by  which  the  part  is  supplied  with  nourishment. 
Speaking  generally,  however,  the  lymphatic  system  may  be 
described  as  the  drainage  system  of  the  body,  in  contra- 
distinction to  the  blood  or  irrigating  system. 

The  Lymph  Spaces. — The  tissues  are  bathed  in  lymph, 
which  is  contained  in  the  lymphatic  spaces  existing  between 
the  capillary  blood-vessels  and  capillary  lymph-vessels. 
There  is  a  constant  passage  of  material  from  the  blood  into 
the  tissues,  and  from  the  tissues  into  the  blood. 

The  lymph  spaces  are  irregular  passages  in  the  connec- 
tive tissue,  the  larger  ones  being  lined  by  epithelioid  plates 
of  a  peculiar  irregular  outline ;  these  spaces  exist  outside 
the  bloodvessels,  and  the  lymph  finds  its  way  from  the 
bloodvessels  into  the  lymph  spaces.  From  the  lymph 
spaces  the  fluid  reaches  the  lymph  capillaries,  but  the 
means  by  which  it  gets  there  is  not  clear,  for  it  appears 
certain  that  excepting  in  a  few  cases  there  is  no  direct 
communication  between  the  space  and  the  capillary.  In 
the  vessels  of  the  brain  a  peculiar  arrangement  is  present, 
the  lymphatic  vessel  surrounds  the  artery  and  obtains  its 

242 


ABSORPTION  243 

lymph  direct ;  such  are  known  as  peri- vascular  lymphatics. 
The  lining  of  the  Lymph  Capillary  is  composed  of  the  same 
epithelioid  plates  with  irregular  outline  which  are  found  in 
the  spaces,  and  it  is  believed  that  at  the  junction  of  the 
plates,  crevices  or  intervals  may  exist  through  which  fluid 
may  find  its  way  by  the  simple  process  of  transudation. 
From  the  lymph  capillary  begins  the  Lymphatic  Vessel, 
which  in  addition  to  an  epithelioid  lining  has  also  a  muscular 
coat,  more  marked  in  the  large  than  in  the  small  vessels, 
and  also  a  connective-tissue  covering.  In  the  interior  of 
these  vessels  valves  are  found  which  are  essentially  similar 
in  structure,  arrangement,  and  mode  of  action  to  those  in 
the  veins.  Immediately  beyond  each  valve  there  is  a  dila- 
tation of  the  vessels  which  gives  them  a  beaded  appearance 
when  the  lymphatic  is  distended. 

The  whole  of  the  lymphatics  of  the  body  converge  to- 
wards a  central  vessel,  the  thoracic  duct ;  those  from  the 
left  side  of  the  head  and  neck,  the  left  fore  limb,  the  chest, 
abdominal  cavity,  and  hind  limbs,  unite  with  the  duct  at 
different  points,  and  this  in  turn  opens  into  the  anterior  vena 
cava ;  from  the  right  side  of  the  head  and  neck,  and  right 
fore  limb,  the  vessels  collect  and  pour  their  contents  by  a 
separate  duet  into  the  same  vein.  The  thoracic  duct  is 
nothing  more  than  a  large  lymphatic  vessel,  possessing  the 
same  structure  as  the  lymphatic  vessels  above  described, 
the  muscular  coat  being  especially  well  marked.  The 
thoracic  duct  receives  the  lymph  not  only  from  the  ordinary 
tissues  but  also  from  the  intestinal  canal.  During  starvation 
the  mesenteric  lacteal  vessels  convey  to  the  duct  a  fluid 
which  is  essentially  lymph,  but  during  digestion  this  clear 
fluid  is  replaced  by  a  turbid  white  fluid  known  as  chyle ;  at 
this  period  the  lacteal  vessels  are  carrying  not  only  lymph 
but  also  the  products  of  digestion,  the  milkiness  of  the  chyle 
being  due  to  the  presence  of  emulsified  fats. 

The  Serous  Cavities  of  the  pleura,  pericardium,  and  peri- 
toneum, have  been  looked  upon  as  large  lymphatic  spaces, 
though  even  this  is  now  by  some  considered  doubtful.  The 
fluid  they  contain  is  lymph,  and  they  are  in  direct  CQm- 

16—2 


244     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

munication  with  lymphatic  vessels,  especially  those  of  the 
diaphragm.  In  the  diaphragm  slits  or  stomata  exist,  and 
into  these  the  lymph  readily  finds  its  way,  being  aspirated 
into  the  vessels  during  the  respiratory  movements  of  this 
organ ;  so  readily  is  this  effected  that  the  diaphragm  may 


Fig,  58. — Diagrammatic  Section  of  Lymphatic  Gland. 

ad,  Adenoid  tissue  containing  lymph  corpuscles,  excepting  to  the  left  of 
the  figure  ar,  where  they  are  omitted  in  order  to  show  the  adenoid 
reticulum.  The  region  ad  is  normally  densely  packed  with  lymph 
corpuscles  and  constitutes  the  glandular  substance.  The  corpuscles 
are  here  drawn  in  scanty  numbers,  so  as  not  to  obscure  the  central 
capillary  v.  In  the  adenoid  tissue  may  be  seen  a  capillary  blood- 
vessel V.  Outside  the  core  of  adenoid  tissue  is  the  lymph  sinus  or 
space  Is,  across  which  run  branched  nucleated  corpuscles  which 
are  simply  an  open  network  of  connective  tissue.  These  corpuscles 
are  shown  on  a  black  ground  in  order  the  better  to  distinguish  the 
lymph  space.  Surrounding  the  whole  is  the  trabecular  frame- 
work t. 


be  injected  in  a  recently  dead  subject,  by  placing  some  milk 
on  its  surface  and  establishing  artificial  respiration. 

The  lymphatic  vessels  in  their  course  pass  through 
bodies  known  as  lymphatic  glands,  entering  at  one  side  and 
emerging  at   the   other.     Experience   shows    that   in   its 


ABSORPTION  245 

passage  through  these  glands  the  lymph  has  corpuscles 
added  to  it  which  ultimately  become  white  blood  corpuscles, 
and  moreover  it  acquires  the  property  of  clotting.  The 
gland  consists  of  a  capsule  within  which  is  a  mass  of  adenoid 
tissue  divisible  into  a  cortex  and  medulla.  The  capsule  sends 
in  bands  of  tissue  (traheculce)  which  divide  the  gland  into 
compartments  or  alveoli,  those  in  the  cortex  being  much 
larger  than  those  in  the  medulla.  The  alveoli  contain  a 
network  of  connective  tissue  whose  central  part  is  finely 
meshed  {adenoid  tissue),  closely  packed  with  lymph  cor- 
puscles and  constitutes  the  glandular  substance.  The 
adenoid  tissue  does  not  occupy  the  entire  alveolus,  but  fills 
up  the  centre,  and  is  maintained  in  position  by  branched, 
nucleated,  connective  tissue  corpuscles  passing  to  the  wall 
of  the  alveolus.  In  this  way  a  space  or  channel  is  formed 
between  the  central  mass  of  adenoid  tissue  and  the  wall  of 
the  alveolus ;  this  channel  is  known  as  a  lymph-sinus  (see 
Fig.  58).  It  is  through  the  lymph-sinuses  of  the  cortex 
that  the  gland  is  in  direct  communication  with  the  afferent 
lymphatic  vessels.  In  the  adenoid  tissue  of  the  alveolus 
is  found  a  network  of  bloodvessels ;  the  tissue  itself  is  filled 
with  corpuscles  known  as  leucocytes,  which  are  also  found 
in  the  more  open  network  extending  across  the  lymph  sinus. 
The  medulla  of  the  gland  presents  no  essential  difference  in 
structure  to  that  of  the  cortex,  excepting  that  the  reticular 
network  is  more  complex,  closer,  and  more  extensive.  The 
efferent  lymphatic  vessels  originate  in  the  lymph  sinuses  of 
the  medulla. 

Lymph  is  a  slightly  yellow-coloured  fluid,  alkaline  in 
reaction,  with  a  specific  gravity  of  1012  to  1022,  and 
possessing  the  power  of  spontaneous  clotting.  The  clot  it 
yields  is  not  so  firm  as  that  of  blood  and  takes  longer  to 
form  ;  moreover,  the  bulk  of  fibrin  is  much  smaller.  Lymph 
may  be  regarded  essentially  as  blood  minus  the  red  cor- 
puscles ;  it  contains,  therefore,  the  proteids  of  that  fluid, 
viz.,  fibrinogen,  paraglobulin,  and  serum  albumin  though 
in  smaller  amounts,  cells  resembling  the  white  cells  of  the 
blood,  extractives,  salts,  and  gases.      The  fluid  in  which 


246     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

these  are  contained  is  spoken  of  as  lymph  plasma.  The 
gases  consist  principally  of  carbon  dioxide,  whose  amount 
is  greater  than  in  arterial  but  less  than  in  venous  blood, 
a  small  quantity  of  nitrogen,  and  only  traces  of  oxygen. 
Amongst  the  extractives  some  observers  have  found  urea, 
a  substance  which  exists  more  largely  in  lymph  than  in 
blood,  and  which  is  said  to  be  alvv'ays  present  in  the  cow. 
The  salts  are  distributed  much  as  are  those  in  l)lood,  viz., 
potash  and  phosphates  in  the  corpuscles,  and  soda  in 
the  plasma.  It  is  evident  that  the  composition  of  the 
lymph  cannot  be  uniform  but  must  depend,  among  other 
causes,  upon  the  nature  of  the  food  supply,  and  the  source 
of  the  lymph. 

The  lymph  cells  or  leucocytes  exhibit  amceboid  move- 
ments and  are  identical  with  white  blood-cells  ;  they  are 
more  numerous  in  those  vessels  which  have  passed  through 
lymphatic  glands,  for  it  is  in  the  gland  that  the  leucocytes 
are  manufactured  and  added  to  the  lymph.  The  cells 
consist  of  proteids,  lecithin,  cholesterin  and  fat,  and  their 
nuclei  contain  nuclein.  Owing  to  their  power  of  move- 
ment they  are  able  to  pass  through  the  bloodvessels  into 
the  tissues  and  vice  versa.  The  proportion  of  lymph 
corpuscles  to  fluid  is  about  the  same  as  the  proportion  of 
white  corpuscles  to  blood. 

The  Quantity  of  Lymph  in  the  body  is  difficult  to  arrive  at, 
and  varies  considerably.  Colin  obtained  from  a  lymphatic 
in  the  neck  of  horses  between  1  to  4  lbs.  (|  to  2  kilos)  in 
24  hours ;  the  mean  amount  was  2  lbs.  6  ozs.  (2  kilos) 
for  the  same  period,  but  he  notes  that  the  variations  are 
very  wide,  and  that  herbivora  secrete  more  than  carnivora, 
and  young  animals  more  than  adults.  The  amount  of 
material  collected  from  the  thoracic  duct  of  a  cow  in 
24  hours  has  been  found  to  be  209  lbs.  (91  kilos),  but  this 
is  no  guide  to  the  quantity  of  lymph  in  the  body,  as  the 
material  in  the  thoracic  duct  is  mixed  with  the  chyle  from 
the  intestines.  It  is  usual,  however,  in  this  vessel  to 
consider  two-thirds  of  the  contents  to  represent  chyle  and 
one-third  lymph.     The  quantity  of  mixed  chyle  and  lymph 


ABSORPTION  247 

obtained   by  Colin   from   the   thoracic   duct,   some   hours 
after  the  animals  had  been  fed,  was  as  follows : 

Horse,  30  to  90  lbs.  (14  to  40  kilos)  in  24  hours. 
Ox,  46  to  209  lbs.  (20  to  91  kilos)        „         „ 
Sheep,  6h  to  10  lbs.  (3  to  9-5  kilos)     „         „ 
Dog,  3  to  6  lbs.  (1-3  to  2-6  kilos)  „         „ 

The  Formation  of  Lymph. — The  theory  of  lymph  formation 
is  by  no  means  settled  ;  the  rival  views  may  be  classified 
as  physical  and  secretory.  The  physical  theory  is  based 
upon  the  laws  of  filtration,  diffusion,  and  osmosis,  while 
the  secretory  or  vital  theory  is  based  upon  the  activity  of 
the  living  cells  of  the  body.  The  physical  theory  will  first 
claim  our  attention,  and  it  is  the  one  which  at  the  present 
time  finds,  perhaps,  more  general  acceptance.  According 
to  this  theory  lymph  is  formed  as  the  result  of  the  operation 
of  the  three  following  factors. 

Filtration  through  the  walls  of  the  capillaries  from  the 
blood  to  the  tissues  is  always  possible  when  the  pressure 
of  blood  in  the  capillaries  is  higher  than  that  of  the  fluid 
in  the  tissues.  It  can  easily  be  shown  that  an  increase  or 
decrease  in  capillary  pressure  increases  or  decreases  the 
amount  of  filtration. 

Difusioi  and  Osmosis. — The  diflerence  existing  between 
the  composition  of  the  blood  plasma  and  the  liquid  in  the 
tissues  outside  the  capillary  vessel  is  a  cause  of  diffusion 
and  osmosis.  Such  differences  are  frequent,  as  for  example, 
after  every  period  of  digestion,  and  the  equilibrium  of  com- 
position can  easily  be  restored  by  the  setting  in  of  diffusion 
and  osmotic  currents.  Differences  in  composition  may 
arise  not  only  between  the  blood  and  the  tissues  but  the 
tissues  and  the  blood,  and  in  both  cases  are  adjusted  in  the 
same  way.  Diffusion  in  liquids  seems,  as  in  gases,  to  be 
the  result  of  the  continual  movement  of  its  molecules,  so 
that  two  liquids  miscible,  but  utterly  unlike,  if  brought 
into  contact  will  gradually  form  a  homogeneous  mixture ; 
or  if  they  be  separated  liy  a  membrane  permeable  to  the 
molecules,  diffusion  will  occur  through  this  and  a  mixture 
of  uniform  composition  result.     Diffusion  through  a  mem- 


248    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

l)rane  is  known  as  osmosis.  Substances  which  are  diffusible 
are  known  as  crystalloids,  those  which  are  non-diffusible 
are  called  colloids.  Sugar  or  salt  are  good  examples  of 
diffusible  bodies,  proteid  and  starch  are  examples  of  colloids, 
the  large  size  of  the  molecules  of  the  latter  preventing  their 
passage  through  an  animal-  or  other  membrane.  This 
difference  in  the  behaviour  of  these  two  classes  of  substances 
as  regards  their  osmotic  properties  affords  a  useful  and 
ready  means  known  as  dialysis  of  separating  the  crystalloids 
from  the  colloids. 

If  two  masses  of  water  be  separated  by  a  membrane  the 
diffusibility  of  each  being  equal,  as  many  molecules  will 
pass  into  one  chamber  as  enter  into  the  opposite,  though 
to  all  appearances  no  change  in  the  fluid  is  taking  place. 
If  one  chamber  contains  salt  solution  and  the  other  plain 
water,  it  will  l)e  found  that  much  more  water  passes  into 
the  salt  solution  than  salt  solution  into  the  water,  the  rate 
of  transference  of  the  salt  depending  upon  the  concentra- 
tion of  the  salt  solution ;  the  force  which  brings  this  about 
is  known  as  the  osmotic  pressure.  It  can  be  shown  that  the 
osmotic  pressure  is  proportional  to  the  number  of  molecules 
of  the  crystalloid  in  solution. 

If  a  strong  solution  of  common  salt  be  injected  into  the 
blood,  an  osmotic  current  is  created  proceeding  at  first  from 
the  tissues  into  the  blood ;  in  course  of  time  the  salt  will 
diffuse  out  of  the  vessels  into  the  tissues  and  an  osmotic 
current  will  then  be  set  up  in  an  opposite  direction,  viz., 
from  blood  to  tissues.  The  diffusibility  of  proteid  substances 
is  very  slight  or  entirely  absent,  their  osmotic  properties 
are  corresj)ondingly  small  and  by  some  have  been  denied. 
It  is  therefore  difficult  to  explain  the  passage  of  the  pro- 
teids  from  the  blood  to  the  tissues,  excepting  on  the  ground 
of  filtration. 

A  picture  of  what  is  occurring  in  the  tissues  under 
ordinary  functional  activity  is  probably  as  follows :  The 
tissue  elements  are  nourished  by  the  lymph  thereby  effecting 
an  alteration  in  the  composition  of  the  latter,  which  is 
made  good  by  diffusion  and  osmosis  from  the  blood.     As  the 


ABSORPTION  249 

result  of  tissue  activity  the  proteid  molecule  gets  broken 
down  with  the  production  of  simpler  and  crystalloid  bodies. 
These  pass  into  the  lymph  from  the  tissues,  and  raise  its 
concentration  ;  by  so  doing  they  draw  water  from  the  blood- 
vessels to  the  lymph,  or  if  the  concentration  in  the  latter 
is  less  than  that  of  the  blood,  diffusion  occurs  from  lymph 
to  blood. 

The  permeability  of  the  capillary  wall  as  a  factor  in 
lymph  production  has  been  urged  by  Starling.  He  finds 
that  whereas  the  capillaries  in  the  limbs  and  connective 
tissues  generally  present  a  very  considerable  resistance  to 
the  filtration  of  lymph  through  them,  and  keep  back  a 
large  portion  of  the  proteids  of  the  blood-plasma,  the 
intestinal  capillaries,  on  the  other  hand,  are  much  more 
permeable,  while  those  of  the  liver  are  of  the  greatest 
j)ermeability,  a  very  small  capillary  pressure  producing  a 
large  transudation  of  lymph  containing  as  much  proteid 
as  the  plasma  itself.  So  slight  is  the  effect  of  capillary 
pressure  on  lymph  production  in  the  limbs  and  connective 
tissues,  that  in  other  parts  of  the  body  it  has  been  necessary 
to  explain  the  production  as  mainly  due  to  diffusion  and 
osmosis.  Constant  osmotic  interchange  between  blood  and 
tissue-cell  occurs  through  the  medium  of  the  lymph,  and 
with  remarkable  rapidity ;  thus,  if  the  osmotic  equilibrium 
be  disturbed  by  injecting  a  large  dose  of  dextrose  into  the 
circulation,  within  half  a  minute  it  is  readjusted.  The 
slight  influence  of  capillary  pressure  in  lymph  production, 
mentioned  above,  only  holds  good  for  the  normal  capillary ; 
any  impaired  nutrition  of  the  vascular  wall,  which  may 
easily  arise,  increases  its  permeability,  and  the  slightest 
increase  of  capillary  pressure  then  produces  an  increase  in 
lymph  production. 

Starling  records  the  remarkable  fact  that  no  lymph  can 
be  obtained  from  a  resting  limb,  though  active  or  passive 
movements  of  it  at  once  cause  a  flow  of  lymph.  The  only 
part  of  the  body  which  ]3roduces  a  continuous  flow  of 
lymph  during  rest  is  the  alimentary  canal.  Though  no 
lymph   is   yielded   by  a   resting   limb,   yet   the   chemical 


250     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

changes  in  the  tissue  are  still  occurring,  oxygen  is  being 
absorbed,  carbonic  acid  and  other  waste  products  got  rid 
of,  but  their  channel  of  excretion  is  effected  by  the  blood- 
vessels. 

The  secretory  theory  of  lymph  production  is  based  on  the 
knowledge  of  the  secretory  activity  of  epithelium  generally. 
It  was  natural,  therefore,  to  regard  the  endothelial  lining 
of  the  capillary  vessels  as  the  possible  seat  of  secretory 
activity.  Heidenhain,  the  exponent  of  this  theory,  showed 
that  certain  bodies  {Lymphagogues)  when  injected  into  the 
blood-stream  caused  an  increased  flow  of  lymph,  and  he 
regarded  these  as  direct  excitants  of  secretion.  The  oppo- 
nents of  this  theory  show  that  these  substances  act  by  their 
deleterious  effect  on  the  capillary  wall,  or  by  their  causing 
water  to  be  taken  up  from  the  tissues ;  the  effect  of  taking 
up  water  is  to  raise  the  total  volume  of  blood  in  the  vessels, 
and  so  cause  a  general  rise  in  blood-pressure,  and  in 
consequence  a  transudation  of  lymph. 

At  present  it  is  not  possible  to  decide  between  the  rival 
theories  of  lymph  formation  ;  it  may  be  proved  that 
under  given  conditions  both  play  a  part  in  the  process.  It 
seems  impossible  to  exclude  the  living  activity  of  the  cell- 
body,  so  strongly  urged  in  the  matter  of  other  secretions, 
while  it  is  equally  certain  that  there  are  other  conditions 
which  are  only  possible  of  explanation  on  a  physical  basis. 
We  cannot  suppose  that  the  condition  of  the  cells  forming 
the  capillary  wall  is  invariably  the  same.  This  wall  is  the 
membrane  across  which  the  physical  factors  have  to  play 
their  part.  Even  if  the  latter  are  the  chief  agents  in  the 
whole  process  they  must  still  be  more  or  less  subject  to 
the  changing  states  of  the  cellular  capillary  wall.  And  in 
connection  with  both  these  views,  it  is  well  to  bear  in  mind 
that  no  lymph-secretory  fibres  have  been  discovered, 
though  their  existence  is  possible,  and  further  we  do  not 
know  positively  in  which  way  the  tissue  spaces  communicate 
with  the  lymphatic  vessels,  or  whether,  like  the  blood- 
vessels, the  latter  form  a  closed  system. 

As  fast  as  the  lymph  finds  its  way  into  the  spaces  it  is 


ABSOKPTION  251 

normally  passed  on  to  the  lymphatic  capillaries,  so  that  the 
rate  of  output  is  equivalent  to  the  rate  of  removal ;  when 
however  the  output  is  greater  than  the  rate  of  removal  the 
lymph  accumulates  in  the  tissues  and  CEdema  results.  It 
is  conceivable  tliat  the  rate  of  removal  need  not  neces- 
sarily always  be  at  fault,  but  that  the  rate  of  secretion 
may  be  so  greatly  inci'eased  that  the  outgoing  channels 
are  unequal  to  the  demands  made  upon  them.  Such  an 
increased  secretion  of  lymph  lies  on  the  shoulders  of 
the  vascular  system,  and  experience  shows  that  in  the 
majority  of  cases  increased  formation  of  lymph  is  a  more 
common  cause  of  oedema  than  defective  drainage.  It 
is  well  known  that  interference  with  the  venous  circula- 
tion is  productive  of  a?dema  ;  disease  of  the  right  side  of 
the  heart  or  portal  obstruction  is  a  fruitful  source  of 
trouble,  the  explanation  being  that  there  is  not  only  an 
increase  of  pressure  in  the  capillaries  as  the  result  of 
the  venous  obstruction,  but  also  a  back  flow  of  venous 
blood  which  is  kept  in  contact  with  the  wall  of  the 
capillary,  and  induces  changes  in  the  epithelioid  cells  result- 
ing in  increased  lymph  formation.  The  swollen  legs  so 
common  in  horses  kept  idle  in  the  stable  are  practically  due 
to  the  same  cause.  The  venous  blood  ascends  the  limbs 
against  gravity  and  exerts  on  the  capillaries  of  the  legs 
below  the  knees  and  hock  a  pressure  which  is  nearly  equiva- 
lent to  the  height  of  the  vein  ;  as  a  result  the  cells  of  the 
capillary  wall  are  the  seat  of  an  increased  exudation,  and 
the  legs  accordingly  '  fill,'  a  condition  removable  by  exercise. 
The  Movement  of  Lymph  is  largely  brought  about  by 
muscular  contractions  in  the  neighbourhood  of  the  vessels, 
by  which  means  they  are  compressed  and  their  contents 
forced  onwards,  since  the  valves  which  the  vessels  contain 
prevent  a  back  flow.  The  obstruction  caused  by  the 
lymphatics  passing  through  glands  is  not  serious,  while  the 
involuntary  muscle  fibres  in  the  capsule  of  the  gland  more 
than  compensate  by  their  contraction  for  any  resistance  in 
the  gland  itself.  The  pressure  of  the  lymph  in  the  lymph 
spaces  is  higher  than  that  in  the  jugular  vein,  so  the  flow  of 


252    A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

lymph  from  the  tissues  to  the  vein  is  assisted  by  the  fact 
that  the  fluid  is  passing  from  a  region  of  higher  to  one  of 
lower  pressure.  The  movements  of  the  diaphragm,  tendons, 
and  fasciae  produce  an  aspirating  effect  on  the  lymph 
circulating  through  them.  In  the  case  of  the  diaphragm 
the  lymphatic  vessels  drain  the  two  large  lymphatic  sacs 
the  pleura  and  peritoneum.  Owing  to  the  direction  taken 
by  the  fibres  of  the  diaphragm,  compression  is  exerted  on 
the  lymph  spaces  during  its  contraction,  while  a  sucking 
action  is  produced  when  it  relaxes.  This  pumping  arrange- 
ment exists  in  tendons,  fasciae  of  muscles,  etc.,  and  is  a 
valuable  aid  in  lymph  circulation. 

Once  the  lymph  from  the  abdominal  viscera  and  hind 
quarters  has  found  its  way  into  the  thoracic  duct,  its 
passage  into  the  general  circulation  is  favoured  by  gravity, 
by  the  muscular  contraction  of  the  coat  of  the  duct  and 
by  the  negative  pressure  produced  in  the  anterior  vena  cava 
vein  by  the  process  of  inspiration,  while  the  aspiration  of 
the  thorax  keeps  the  duct  filled ;  the  combined  result  of 
these  forces  is  that  the  lymph  is  aspirated  out  of  the  duct 
into  the  vessel.  This  aspirating  influence  has  been  proved 
on  the  horse  by  experimental  inquiry,  a  negative  pressure 
in  the  thoracic  duct  having  been  observed  during  inspira- 
tion, and  a  positive  pressure  during  expiration.  In  a 
manometer  placed  in  the  thoracic  duct  of  the  ox,  Colin 
states  that  mixed  chyle  and  lymph  rose  in  five  minutes  to 
a  height  of  over  three  feet ;  this  pressure  is  one  third  of 
the  blood  pressure  in  the  aorta,  and  appears  excessive. 
The  lateral  pressure  in  a  lymph  vessel  in  the  neck  of  the 
horse  was  from  one  half  to  three  quarters  of  an  inch  of  a 
weak  solution  of  soda ;  in  the  dog  the  lateral  pressure  was 
half  that  found  in  the  horse. 

The  thoracic  duct  terminates  in  the  anterior  vena  cava 
at  the  jugular  confluent  in  a  variety  of  ways  ;  its  most 
usual  method  is  that  it  dilates  before  entering  the  vein, 
and  from  the  dilatation  either  one  or  two  very  short 
vessels  are  formed  which  enter  the  anterior  cava,  the 
entrance  being  guarded  by  a  valvular  arrangement.     The 


ABSOEPTION  253 

right  lymphatic  channel  also  opens  into  the  anterior  cava 
at  the  jugular  confluent,  the  entrance  being  furnished 
with  a  double  semilunar  valve.  The  blood  in  the  vena 
cava  vein  is  prevented  from  passing  into  the  thoracic  duct 
by  the  presence  of  these  valves,  which  normally  only  allow 
fluid  to  pass  in  one  direction,  viz.,  from  the  duct  into  the 
vein.  Colin  has  observed  that  it  is  not  uncommon  in  the 
horse  to  find  the  lymph  in  the  thoracic  duct  slightly  blood- 
stained, a  slight  leakage  from  the  vein  into  the  duct  being 
liable  to  occur  in  this  animal,  though  such  has  never  been 
seen  in  the  ox. 

The  lymph  moves  slowly  in  its  vessels.  Weiss  has  observed 
a  rate  of  from  9  to  11  inches  (23  to  28  cm.)  per  minute  in 
a  large  lymphatic  in  the  neck  of  the  horse,  but  the  velocity 
in  the  small  vessels  is  very  much  less. 

Section  2. 
Chyle. 

In  the  thoracic  duct  the  lymph  from  the  body  meets  with 
the  lymph  coming  from  the  intestines,  termed  chyle.  This 
chyle  is  derived  from  the  villi  and  passes  up  the  mesentery 
by  many  vessels,  which  in  the  horse  are  said  by  Colin  to 
number  1,200.  Each  of  these  passes  through  a  lymphatic 
gland  before  entering  the  receptaculum  chyli.  Chyle  is 
closely  allied  to  lymph  in  its  chemical  composition,  but  it 
diflers  from  it  in  containing  during  digestion  a  quantity  of 
neutral  fat,  which  gives  it  a  milky  appearance.  The 
amount  of  this  fat  in  dogs  may  vary  from  2  per  cent,  to 
15  per  cent,  or  even  more.  The  fat  is  partly  in  the  con- 
dition of  measurably  large  droplets,  such  as  are  seen  in 
milk,  but  the  bulk  of  it  exists  as  extraordinarily  minute 
particles ;  hence  the  name  '  molecular  basis,'  which  is 
applied  to  the  fat  particles  in  chyle  collectively. 

The  Villi. — We  have  mentioned  that  in  the  ordinary 
tissues  the  radicles  of  the  lymph-vessels  are  the  lymph 
spaces,  but  in  the  wall  of  the  small  intestines  the  origins  of 
the    lymph-vessels    are    highly   differentiated    structures, 


254     A  MANUAL  OF  VETEKINAEY  PHYSIOLOGY 

known  as  villi  and  solitary  glands.  The  villi  (Fig.  59) 
are  innumerable  projections  from  the  inner  surface  of 
the  mucous  membrane  shaped  like  minute  fingers ;  they 
are  only  found  in  the  small  intestines,  and  have  been 
calculated  by  Colin  to  amount  to  forty  or  fifty  millions  in 
the  horse  and  ox.  In  the  interior  and  central  part  of  the 
villus  is  a  vessel  termed  the  lacteal;  it  may  be  single 
or  multiple,  straight  or  branched,  and  at  the  base  of 
the  villus  it  opens  by  a  valvular  arrangement  into  the 
lymphatic  system.     Surrounding  the  lacteal  is  a  network 


Fig.  59. — Vertical  Section  of  a  Villus  :  Cat.     x  300  (Stewaet). 

a,  Layer  of  columnar  epitheliura  covering  the  villus — the  outer  edge 
of  the  cells  is  striated ;  b,  central  lacteal  of  villus ;  c,  unstriped 
muscular  fibres  ;  d,  mucin-forming  goblet-cells. 

of  capillary  bloodvessels,  while  filling  up  the  finger  of  the 
villus  not  otherwise  occupied  by  vessels  is  a  peculiar 
structure  found  especially  in  lymphatic  glands  and  known 
as  adenoid  tissue  (p.  245) ;  this  tissue  is  relatively  larger  in 
amount  in  the  villi  of  carnivora  than  of  herbivora  (Fig.  60). 
Covering  the  entire  villus  is  a  basement  membrane  on  which 
is  set  a  layer  of  columnar  cells,  placed  so  that  their  narrowest 
end  is  next  the  basement  membrane  and  their  broadest  next 
the  interior  of  the  intestine.  The  cells  at  their  narrowest 
part  are  in  touch  with  the  adenoid  tissue  of  the  villus. 
Each  cell  contains  a  nucleus,  and  on  that  edge  next  the 


ABSOEPTION 


255 


interior  of  the  bowel  is  a  clear  band  bearing  fine  striations- 
Lying  between  the  columnar  cells  are  others  which  from 
their  shape  are  spoken  of  as  '  goblet  cells '  (Fig.  59)  ;  by- 
means  of  a  pore  they  extrude  their  contents,  consisting  of 
a  transparent  material  known  as  mucin,  into  the  intestine. 
Within  the  villus  are  bands  of  involuntary  muscle-fibre 
arranged  parallel  to  the  axis  of  the  villus,  by  the  con- 
traction of  which,  combined  with  the  peristaltic  move- 
ments of  the  intestine,  the  capacity  of  the  lacteal  vessel  is 
altered  in  such  a  way  that  it  is  alternately  filled  with 
lymph  from  the  reticular  adenoid  tissue,  and  emptied  of 


Lymph 

vessel.     -JL 


Epithelimn 


Fig.  60. — Transverse  Section  of  Villi  of  Carnivorous  and 
Herbivorous  Animals  (^YALLER,  after  Heidenhain). 

The  large  cells  in  the  epithelial  zone  of  the  dog  are  the  goblet  cells. 

lymph  into  the  lymphatic  vessel  at  the  base  of  the  villus. 
This  is  known  as  the  pumping  action  of  the  villus,  and 
provides  an  important  factor  in  the  furtherance  of  the  chyle 
(lymph)  towards  the  thoracic  duct. 

The  other  lymph  radicles  found  in  the  intestine  are  the 
Solitary  Follicles,  which  are  found  studding  the  whole  of 
the  mucous  membrane  of  the  small  intestines  ;  these  solitary 
follicles  are  at  certain  places  in  the  ileum  collected  into 
masses  where  they  are  known  as  Pcyer's  Patches. 

The  Solitary  Follicle  is  essentially  a  lymphatic  structure 
and  is  not  concerned  like  the  villus  in  absorbing  anything 
from  the  food.     It  consists  of  a  mass  of  adenoid  tissue,  the 


256     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

network  of  which  is  filled  with  leucocytes ;  within  the  net- 
work are  capillary  bloodvessels,  and  surrounding  the  whole 
is  a  space  across  which  branches  of  the  adenoid  network  pass. 
This  space  is  known  as  a  lymph  space  or  sinus  ,•  it  is  lined, 
like  those  previously  described,  with  epithelioid  plates, 
and  opens  into  a  lymphatic  vessel.  As  the  lymph  passes 
through  the  adenoid  tissue,  some  of  the  corpuscles  found 
in  the  meshes  of  the  network  are  added  to  it  and  become 
lymph  corpuscles. 

Chyle  is  a  turbid  fluid  of  alkaline  reaction  and  a  specific 
gravity  of  1007  to  1022.  In  starving  animals  it  is  trans- 
parent owing  to  the  absence  of  fat,  and  it  is,  in  fact,  at 
this  time  practically  pure  lymph.  Colin  observed  that 
the  chyle  of  herbivora  was  yellowish  or  yellowish  green  ; 
it  is  possible  that  this  colour  may  be  due  to  chlorophyll 
taken  up  from  the  food.  In  the  horse,  as  collected  from 
the  thoracic  duct,  it  is  often  reddish,  due,  no  doubt,  to  a 
slight  leakage  from  the  vena  cava,  such  as  has  been 
previously  noted  (p.  253). 

In  the  small  intestines  of  the  horse,  it  has  been  observed 
by  Colin  that  almost  immediately  after  food  has  been  given, 
waves  of  chyme  are  passed  into  the  duodenum  from  the 
stomach ;  in  consequence  the  lacteals  in  the  mesentery  in 
connection  with  this  portion  of  intestine  become  opaque, 
though  previously  they  were  filled  with  a  colourless  fluid. 
As  the  chyme  passes  along  the  bowel  the  other  lacteals  in 
their  turn  become  opaque,  until  at  last  the  whole  of  them 
are  filled  with  this  milky  fluid.  Colin  draws  especial  attention 
to  this  regular  invasion  of  the  lacteals  from  the  duodenum 
to  the  ileum. 

The  movement  of  chyle  is  due  to  the  muscular  contrac- 
tion of  the  intestinal  villi  forcing  it  onwards,  while  the 
valves  in  the  lacteals  prevent  its  return. 


ABSORPTION  257 

Section  3. 
Absorption  in  General. 

The  activity  of  absorption,  especially  in  the  horse,  has 
been  made  known  to  us  by  the  experiments  of  Colin. 

Absorption  from  the  Respiratory  Passages  is  remarkably 
rapid.  Colin  showed  that  potassium  ferrocyanide  could  be 
detected  in  the  blood  two  minutes  after  being  injected  into 
the  trachea,  and  that  it  appeared  in  the  blood  before  it  was 
found  in  the  chyle  ;  the  same  salt  was  also  found  in  the 
urine  eight  minutes  after  being  introduced  into  the  trachea. 
A  solution  of  nux  vomica  injected  into  the  trachea  produced 
tetanic  symptoms  in  three  minutes ;  turpentine,  alcohol, 
and  ether  were  also  rapidly  absorbed,  but  oil  could  not  be 
taken  up,  and  was  rejected  by  the  nostrils. 

Such  drugs  as  morphia,  pilocarpin,  physostigmin,  etc., 
are  all  rapidly  absorbed  from  the  air-passages,*  and  accord- 
ing to  our  observations  produce  their  physiological  effect 
in  a  shorter  time  than  when  simply  injected  under  the 
skin.  The  lungs  also  have  the  power  of  absorbing  certain 
poisons  like  curare,  which  are  not  absorbed  when  introduced 
into  the  digestive  canal.  The  absorption  of  water  from  the 
bronchial  passages  is  very  rapid.  Colin  introduced  six 
quarts  of  water  per  hour  into  the  trachea  of  a  horse ;  the 
animal  was  destroyed  at  the  end  of  3|  hours  and  no 
fluid  was  found  in  the  bronchi.  He  also  poured  into 
the  air-passages  one  pint  of  water  at  a  time  ;  repeating 
this  without  intermission,  he  poured  in  74  pints  of  water 
before  he  caused  death.  So  rapid  is  absorption  from  the 
bronchi,  that  a  horse  may  be  placed  under  chloroform 
almost  instantaneously  by  an  intra-tracheal  injection  of  the 
drug.t  The  rapidity  of  absorption  is  therefore  very  great,  but 

*  It  is  interesting  to  observe  that  the  injection  of  Uquids  into  the 
trachea  (either  high  up,  or  as  low  as  its  bifurcation)  excites  the  reflex 
act  of  swallowing,  probably  due  to  stimulation  of  the  recurrent  nerve. 

t  It  is  not  intended  here  to  recommend  the  mtra-tracheal  adminis- 
tration of  chloroform,  which  is  not  only  dangerous  but  produces  the 
greatest  excitement  in  the  patient. 

17 


258    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

in  spite  of  the  facility  with  which  drugs  are  taken  up,  the 
Hning  membrane  of  the  bronchial  tubes  is  remarkably 
tolerant  of  such  irritating  agents  as  turpentine,  strong 
liquid  ammonia,  acetic  acid,  etc.,  and  offers  in  a  state  of 
health  an  almost  impassable  barrier  to  putrid  organic 
infusions,  or  at  any  rate  these  do  not  appear  to  produce 
any  local  irritation  when  injected. 

Absorption  from  the  Cellular  Tissue  is  very  active,  and 
both  the  bloodvessels  and  lymphatics  take  part  in  the 
process  ;  ferrocyanide  of  potassium  injected  into  the  face 
has  been  detected  in  a  carotid  lymphatic  in  seven  minutes. 
The  rapidity  of  cellular  tissue  absorption  is  hastened  by 
muscular  movement. 

Absorption  from  the  Conjunctiva  is  very  pronounced  for 
some  drugs  such  as  atropin  and  certain  organic  poisons, 
but  there  are  others  which  are  not  absorbed  so  readily. 
Curare  is  not  absorbed  through  the  conjunctiva,  and  Colin 
could  not  infect  horses  with  anthrax  b}'  placing  anthrax 
blood  and  fluids  in  the  conjunctival  sac. 

Absorption  by  the  Skin,  if  the  surface  be  unbroken,  is  slow 
even  for  those  drugs  which  will  pass  through  it,  while 
there  are  many  organic  and  inorganic  substances  which 
refuse  to  pass  through  the  unbroken  epidermis.  Colin 
kept  the  lumbar  region  of  a  horse  wet  for  5  hours  with 
a  solution  of  ferrocyanide  of  potassium  ;  the  salt  was 
detected  in  the  urine  in  4h  hours,  although  the  skin 
was  quite  unbroken.  From  a  wound  or  abraded  surface, 
absorption  will  occur  rapidly  with  some  agents,  slowly  with 
others.  Colin  placed  a  horse's  foot  with  a  vvound  on  the 
coronet  in  a  solution  of  ferrocyanide  of  potassium ;  in  20 
minutes  he  detected  the  salt  in  a  lymphatic  of  the  thigh. 
In  connection  with  absorption  from  a  wounded  surface,  he 
found  that  the  poison  was  taken  up  quite  as  readily  by  the 
lymphatics  as  by  the  bloodvessels. 

The  mucous  membrane  of  the  vagina  is  found  by  experi- 
ment to  absorb  very  slowly. 

Experiments  made  on  Absorption  from  the  Pleural  and 
Peritoneal  Cavities  showed  that    such    drugs  as  strychnin 


ABSORPTION  259 

rapidl}'  produce  fatal  symptoms  when  injected  into  these 
sacs ;  even  in  such  a  short  time  as  from  3  to  7  minutes 
tetanic  S3'mptoms  supervene.  Potassium  iodide  injected 
into  the  peritoneal  cavity  of  a  sheep  may  be  detected  in 
the  thoracic  duct  5  to  8  minutes  after  the  operation. 

Starling  and  Tubby  have  shown,  however,  that  the 
active  agents  in  absorption  from  these  sacs  are  the  blood- 
vessels, and  that  the  share  taken  by  the  lymphatics  is 
insignificant.  If  methylene  blue  be  injected  into  the 
pleural  cavity  the  dye  api^ears  in  the  urine  long  before  any 
trace  of  colour  can  be  perceived  in  the  lymph  flowing  from 
the  thoracic  duct. 

Stomach  absorption,  or  rather  its  absence  in  herbivora, 
has  been  dealt  with  at  p.  176.  Even  in  the  dog  it  is  now 
admitted  that  absorption  is  by  no  means  so  certain  as  was 
at  one  time  supposed.  Water  for  instance  passes  through 
the  stomach  and  undergoes  no  absorption  ;  salts  are  only 
absorbed  with  difficulty ;  sugars  and  peptones  are  taken 
up,  but  only  if  in  sufficient  concentration ;  ordinarily  they 
are  absorbed  with  difficulty. 

Intestinal  Absorption. — The  absence  of  stomach  absorp- 
tion in  the  horse  and  ox  points  to  intestinal  absorption  as 
being  of  considerable  importance  in  herbivora.  That  this 
absorption  is  very  rapid  is  proved  by  Colin's  experiments. 
Hydrocyanic  acid  injected  into  the  small  intestine  of  a 
horse  caused  death  in  1  to  1^  minutes,  and  potassium 
ferrocyanide  injected  into  the  bowel,  after  tying  the 
mesenteric  lymphatics,  was  detected  in  the  blood  6  minutes 
afterwards. 

Tlte  Paths  of  Alsorption. — The  paths  by  which  intestinal 
absorption  occurs  are  (1)  through  the  villi  into  the  lacteals, 
and  (2)  through  the  bloodvessels  into  the  venous  system. 
This  points  to  the  possibility  that  some  substances 
taken  up  from  the  bowel  may  at  once  pass  into  the  blood 
via  the  thoracic  duct  (Fig.  61),  while  others  must  first 
proceed  to  the  liver  by  the  portal  vessels  for  further 
elaboration  before  entering  the  blood. 

It  will  be  remembered  that  the  villi  are  found  only  in 

17—2 


260     A  MANUAL  OF  VETEKINARY  PHYSIOLOGY 

the  small  intestines  ;  in  the  large  intestines  there  are  no 
villi.  It  must  not,  however,  be  supposed  that  absorption 
in  the  latter  is  exclusively  carried  on  by  the  bloodvessels, 
for  remembering  the  large  chain  of  glands,  along  the  colon 
in  particular,  it  is  probable  that  the  material  absorbed 
passes  through  these  glands  to  a  greater  or  less  extent,  as 
in  the  mesentery,  before  entering  the  circulation.  There 
is,  at  any  rate,  a  well-developed  lymphatic  system  in  the 


V^HiSyc'^^ 


■^^, 


.A 


-%.- 

'''^>^ 


Fig.  61. — Loop  of  Small  Intestine  of  the  Horse  during 
Active  Absorption,  with  Distended  Lacteals. 


walls  of  the  large  intestine,  and  it  is  certain  that  material 
is  taken  up  from  this  bowel  both  by  the  bloodvessels  and 
Ij^mphatics.  The  amount  of  this  must  be  considerable, 
when  the  size  of  these  bowels  is  borne  in  mind  and  the 
character  of  their  contents. 

Substances  can  be  taken  up  with  extreme  rapidity  from 
the  large  bowels.  Colin  observed  that  18  minutes  after 
injecting  a  solution  of  nux  vomica  into  the  caecum  of  the 
horse  convulsions  began,  and  8  minutes  later  the  animal 


ABSORPTION  261 

Tvas  dead.  Anesthetics,  such  as  ether,  may  also  be 
administered  per  rectum  and  produce  narcosis.  Finally, 
and  from  some  points  of  view  most  important  of  all, 
proteids  may  be  absorbed  from  the  rectum  and  single 
colon,  in  spite  of  the  fact  that  there  is  no  proteolytic  ferment 
to  render  them  soluble. 

Absorption  of  Fat. — If  a  cannula  be  placed  in  the  thoracic 
duct  of  a  starving  dog,  the  lymph  which  escapes  is  identical 
with  that  from  any  other  part  of  the  body.  If  the  animal 
be  now  fed  on  a  diet  rich  in  fat,  the  lymph  becomes  milky, 
and  even  the  blood  plasma  becomes  turbid  from  fat,  if  the 
contents  of  the  duct  are  permitted  to  enter  the  general 
circulation.  It  is  evident  that  the  lymphatics  are  the  path 
by  which  the  fat  enters  the  body,  for  comparative  analysis 
of  the  blood  of  the  portal  vein  and  carotid  artery  shows  that 
the  amount  of  fat  in  the  two  is  the  same.  The  blood- 
vessels, therefore,  have  nothing  to  do  with  the  absorption 
of  fat,  yet  from  an  open  thoracic  duct  not  more  than 
60  per  cent,  of  the  total  fat  given  in  an  experimental  diet 
can  be  recovered ;  after  deducting  that  excreted  unabsorbed 
with  the  ffeces,  there  still  remains  a  balance  unaccounted 
for.  The  fate  of  this  missing  portion  of  fat  is  still  a  matter 
of  conjecture. 

It  has  been  shown  (p.  236)  that  fat  in  the  small  intestine 
is  both  saponified  and  emulsified,  the  former  being  a 
chemical,  the  latter  a  physical  change.  These  processes 
result  from  the  separate  and  combined  action  of  the 
pancreatic  juice  and  bile,  and  they  lead  to  two  possible 
views  as  to  the  mechanism  of  fat  absorption.  Emulsification 
reduces  the  fat  (and  fatty-acids)  to  a  state  of  subdivision 
into  particles  so  minute  that  they  might  conceivably  be 
simply  passed  as  such,  through  the  epithelial  cells  of  the 
villi  to  the  lacteals,  by  an  activity  of  these  cells  comparable 
to  the  ingestive  powers  of  a  white  blood-corpuscle.  This 
would  readily  account  for  the  appearance  characteristic  of 
chyle  (p.  253),  the  minuteness  of  the  fat  particles  it  contains 
being  probably  intended  to  prevent  embolism  by  plugging 
of  the  capillaries.     The  view  thus  indicated  was  the  one 


262  A  MANUAL  OF  VETEPJNAEY  PHYSIOLOGY 

formerly  most  prevalent.  On  the  other  hand,  bile  has,  in 
virtue  of  its  bile-salts,  an  extremely  active  solvent  action  on 
both  fatty  acids  and  soaps :  hence  the  possibility  that  fat  is 
split  up  so  as  to  give  rise  to  variable  relative  amounts  of 
fatty-acid  and  soap,  which  then  pass  in  solution,  into  the 
cells  of  the  villi,  as  do  the  })roteids  and  carbohydrates. 

If  the  intestinal  mucous  membrane  of  an  animal  in  full 
fat  absorption  is  stained  with  osmic  acid  the  epithelial  cells 
are  found  to  be  crowded  with  minute  particles  of  varying 
size,  whose  blackness  shows  them  to  be  fat  (Fig.  62).  This 
fact  provided  the  chief  support  for  the  view  that  fat  reaches 


Fig.  62.— Mucous  Membrane  of  Frog's  Intestine  duiung 
Absorption  of  Fat  (Schafer). 

ej),   Epithelial  cells ;  str,  striated  border  ;   c,  lymph  corpuscles  ; 
I,  lacteal. 

the  lacteals  in  a  state  of  minute  mechanical  subdivision  not 
necessarily  involving  much  chemical  change.  If  this  were 
so  we  should  expect  to  see  some  of  the  fat-particlea  in 
transit  through  the  striated  border  of  the  epithelial  cells, 
and  this  is  never  observed.  But  if  we  hence  discard  the 
first  possibility  and  accept  the  more  current  view  that  fats 
are  absorbed  in  solution,  we  must  assume  that  there  is  a 
rapid  reconstruction  of  neutral  fats  inside  the  epithelial 
cells  after  the  absorption  of  the  dissolved  soaps  or  fatty- 
acids,  inasmuch  as  the  cells  still  alicai/s  give  with  osmic 
acid  the  appearance  characteristic  of  the  presence  within 
them  of  minute  fat-particles.  This  synthetic  reconstruction 
of  fat  may  possibly  be  brought  about  by  the  reversible 
activity  of  the  lipase  ferment  to  which  we  referred  on  p.  237, 


ABSOEPTION  263 

though  it  is  more  probably  due  to  the  constructive  activity 
of  the  cell-protoplasm.  This  second  view  of  the  mechanism 
of  fat-absorption  further  enables  us  to  understand  the  all- 
important  role  of  bile  in  the  absorption  of  fat. 

Absorption  of  Carbohydrates.  —  The  digestive  changes 
undergone  by  starch  are  described  on  p.  142.  The  sugar 
formed  from  starch  by  the  saliva  is  maltose,  the  maltose 
by  the  aid  of  the  succus  entericus  and  epithelial  cells  of 
the  intestine  is  converted  into  glucose,  this  and  its  allies 
being  the  only  form  in  -which  sugar  can  be  utilized  by  the 
sj'stem ;  both  cane  and  milk  sugar  must  be  thus  converted 
or  else  they  are  excreted  in  the  urine.  It  is  clear  from  what 
has  been  said  that  the  path  of  absorption  for  carbohydrates 
is  the  bloodvessels. 

Absorption  of  Proteid, — If  the  thoracic  duct  of  a  dog 
be  ligatured  and  a  large  proteid  meal  given  it  is  perfectly 
absorbed,  as  shown  by  the  increase  in  urea,  while  there  is  no 
increase  in  the  amount  of  lymph  or  of  its  proteid  contents. 
This  clearly  shows  that  the  absorbed  products  pass  into 
the  bloodvessels.  Proteids  before  absorption  are  rendered 
soluble  by  conversion  into  peptones  and  proteoses,  yet  there 
is  no  blood  in  the  body,  including  that  of  the  portal  area, 
which  is  found  to  contain  even  a  trace  of  peptone  or 
proteose ;  in  fact,  the  presence  of  these  substances  in  the 
blood  acts  as  a  poison,  giving  rise  to  peptonuria.  The 
peptones  and  proteoses  enter  the  blood  as  ordinary 
proteid,  so  that  during  their  passage  through  the  epithelial 
wall  of  the  intestines  they  become  regenerated.  Beyond 
the  above  facts,  very  little  is  known  of  the  absorption  of 
proteid. 

Absorption  of  Water  and  Salts. — These  are  taken  up  by 
the  bloodvessels  and  with  remarkable  rapidity.  The 
amount  of  water  capable  of  absorption  is  very  considerable. 
The  material  passes  into  the  bloodvessels  either  through 
the  epithelium  or  between  the  epithelial  cells. 


CHAPTEE  YIII 

DUCTLESS  GLANDS  AND  INTERNAL  SECRETIONS 

The  ductless  glands  of  the  bod}'  are  represented  by  the 
spleen,  thyroid,  thymus,  adrenals,  pituitary,  and  pineal 
bodies.  The  function  of  these  is  either  imperfectly  known 
or  entirely  unknown,  but  within  recent  years  experimental 
enquiry  has  thrown  some  light  on  their  use  as  glands 
producing  an  internal  secretion,  viz.,  a  something  carried 
away  by  the  blood  or  lymph  stream  and  utilized  elsewhere 
by  the  body. 

Internal  secretions  are  not  limited  to  ductless  glands.  It 
is  now  known  that  the  pancreas,  liver,  and  other  glands 
produce,  in  addition  to  the  visible  secretion  passing  away 
by  their  duct,  another  or  internal  secretion  passing  away  by 
lymph  or  blood  channels,  and  quite  distinct  from  the 
ordinary  fluid  secreted  by  the  gland  (see  also  p.  285). 

The  discovery  of  secretin  (p.  233)  by  Starling  and  Bay- 
liss  opened  up  a  field  of  the  highest  imjDortance,  possessing 
possibilities  the  extent  of  which  cannot  be  forecast.  In 
secretion  we  have  a  specific  chemical  excitant,  or  hormone, 
and  it  may  yet  be  shown  that  secretions  which  have  been 
regarded  as  due  to  the  influence  of  the  nervous  system  are 
in  reality  produced  by  a  chemical  stimulant  furnished  by 
the  body  itself.  Edkins,  indeed,  considers  this  is  so  of 
the  gastric  juice ;  while  Starling  and  Bayliss  point  to 
the  specific  chemical  excitant  theory  as  offering  some 
explanation  of  the  sympathy  between  the  uterus  and 
the  mammary  gland,  the  occurrence  of  menstruation,  and 
periodic    sexual   excitement  in  the   lower   animals.      The 

264 


THE  INTERNAL  SECRETIONS  265 

ovary  has  been  suggested  as  the  seat  of  production  of 
such  chemical  excitant.  The  corpus  kiteum  is  regarded 
as  a  ductless  gland,  its  internal  secretion  being  connected 
with  the  fertilization  and  implantation  of  the  ovum.  The 
influence  of  the  ovaries  on  the  development  of  the  external 
genital  organs  may  also  in  this  way  be  explained,  for  the 
arrested  development  which  occurs  as  the  result  of 
removing  the  ovaries  in  the  young  animal  is  prevented 
by  implanting  them  in  a  distant  part  of  the  body.  The 
sympathy  between  ovaries  and  mammary  glands  is  further 
shown  by  the  remarkable  fact  that  a  cow  ovariotomised 
when  in  full  milk  remains  in  milk  for  two  or  three  years. 
The  influence  of  the  ovaries  on  psychic  conditions  is  well 
recognized  :  some  forms  of  vice  in  the  mare  are  cured  or 
improved  by  removal  of  the  ovaries.  It  is  to  be  noted  that 
apparently  the  complete  removal  of  all  trace  of  ovarian 
tissue  in  the  cat  and  dog  may  not  invariably  prevent  periodic 
sexual  excitement  (Leeney).  It  has  been  stated  that  the 
removal  of  the  ovaries  from  the  dog  affects  metabolism, 
especially  the  consumption  of  oxygen,  which  falls  off,  and 
that  this  may  be  neutralized  by  the  administration  of  an 
extract  of  ovary ;  this  causes  the  metabolism  to  rise  above 
the  normal,  but  does  not  affect  the  un-operated  animal. 

Similarly,  there  can  be  no  doubt  as  to  the  testicles  forming 
an  internal  secretion.  It  is  fair  to  assume  that  among  other 
functions  the  implantation  of  the  characteristics  of  the 
male,  especially  the  aggressive  characteristics,  must  be 
regarded  as  part  of  its  duty.  Otherwise  it  is  difficult  to 
account  for  the  alteration  in  character  which  occurs  as  the 
result  of  complete  castration,  and  the  modifying  change 
which  follows  from  leaving  some  of  the  epididymis  attached 
to  the  cord.  The  influence  of  the  testicles  on  the  growth  of 
bone  is  recognized  in  man  ;  the  long  bones  continue  to 
grow,  due  to  the  delay  in  the  ossification  of  the  epiphyses  ; 
the  same  is  said  to  have  been  observed  in  animals.  The 
effect  of  castration  on  the  eating  properties  of  flesh  is  well 
known.  The  influence  on  the  thymus  gland  is  very 
marked ;    instead   of  disappearing  at   puberty,  castration 


266     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

causes  the  gland  to  become  larger  and  more  persistent. 
The  effect  of  removal  of  the  testicles  and  ovaries,  on  the 
dog,  cat,  deer  and  birds,  may  be  conveniently  considered 
in  the  chapter  on  '  Generation  and  Development." 

The  adaptation  of  the  digestive  fluids  to  the  nature  of 
the  food  has  been  referred  to  (p.  170).  This  and  the  influence 
of  a  fixed  diet  in  producing  a  more  effective  digestive  secre- 
tion, and  the  harm  resulting  from  sudden  changes  in  diet 
(p.  237),  may  possibly  be  regulated  by  a  specific  chemical 
excitant.  These  are  matters  of  the  highest  practical  im- 
portance in  the  feeding  and  management  of  animals. 

The  chief  lesson  that  the  present  work  on  internal  secre- 
tions teaches  is  that  an  organ  apparently  functionless  may 
l)e  performing  some  ofiice  of  the  highest  importance,  while 
even  those  actively  employed  in  the  preparation  of  an 
obvious  secretion  may,  in  addition,  l)e  carrying  out  im- 
portant chemical  activities — the  liver,  for  example,  with 
its  external  secretion  of  bile  and  its  internal  secretion  of 
urea  and  glycogen  ;  the  pancreas,  with  its  digestive  fluid, 
and  its  invaluable  internal  secretion,  which  regulates  the 
destruction  of  sugar.  Even  the  kidney,  in  all  probability, 
possesses  an  internal  secretion  affecting  metabolism.  The 
spleen,  on  the  other  hand,  would  appear  to  possess  neither 
an  internal  nor  an  external  secretion,  for  it  has  frequently 
been  removed  without  ill  effects ;  but  the  question  must  be 
dealt  with  in  a  little  more  detail. 

The  Spleen,  in  spite  of  the  numerous  observations  to 
which  it  has  been  subjected,  is  still  a  physiological  enigma. 
Its  vascular  arrangement  is  peculiar  in  that  it  is  capable 
of  holding  a  considerable  quantity  of  blood,  and  for  this 
purpose  readily  lends  itself  to  change  of  size.  Further,  it 
is  the  only  tissue  in  the  body  where  the  cell  elements  are 
directly  bathed  in  blood  without  the  intervention  of  even  a 
capillary  wall.  The  spleen  contains  a  considerable  amount 
of  involuntary  muscular  fibre  and  is  capable  of  movement. 
Th'Bse  movements  have  been  carefully  studied,  and  it  is 
established  that  they  are  of  two  kinds,  a  slow  expan- 
sion which  occurs  after  a  meal  followed  by  contraction,  and 


THE  INTERNAL  SECEETIONS  267 

a  rhj'tlimical  expansion  and  contraction  occurring  in  certain 
animals,  such  as  dogs  and  cats,  at  intervals  of  about  one 
minute.  It  is  believed  that  the  latter  movement  is  for  the 
purpose  of  assisting  the  circulation  through  the  organ, 
to  which  the  splenic  pulp  offers  considerable  resistance. 
That  the  movement  is  brought  about  by  the  bands  of 
involuntary  muscular  fibre  is  undoubted ;  the  spleen  is 
liberally  supplied  with  motor  nerves,  and  stimulation  of 
these  leads  to  a  reduction  in  the  volume  of  the  organ.  It 
is  even  believed  that  there  may  be  nerves  to  the  spleen, 
which  produce  dilatation. 

The  use  of  the  gland  is  largely  based  on  conjecture.  By 
some  it  has  been  considered  the  seat  of  formation  of  red 
blood-corpuscles,  and  that  this  function  is  present  during 
intra-uterine  life  and  shortly  after  birth  is  undoubted ;  but 
there  is  no  evidence  of  this  function  in  the  adult.  It  has 
been  claimed  to  be  the  seat  of  destruction  of  the  red  cells 
and  of  phagocytosis,  and  on  this  point  there  are  some  telling 
facts ;  for  instance,  certain  large  amceboid  cells  found  in  the 
sj)leen  are  capable  of  ingesting  and  destroying  worn-out 
blood-cells  and  other  solid  matter  such  as  micro-organisms, 
while  the  richness  of  the  splenic  pulp  in  iron  is  regarded 
as  due  to  the  haemoglobin  of  the  destroyed  red  blood-cells. 
The  theory  is  very  plausible  though  by  no  means  definitely 
proved ;  at  the  same  time  there  is  great  difficulty  in  getting 
away  from  the  fact  that  the  spleen  appears  in  every  way 
to  be  admirably  suited  to  act  the  part  of  a  blood  filter. 

The  lymphoid  tissue  of  the  spleen,  like  that  of  lymphoid 
tissues  in  general,  is  capable  of  forming  a  substance  from 
which  uric  acid  may  be  readily  produced,  and  the  spleen 
has  in  consequence  been  regarded  by  many  as  the  seat  of 
active  metabolic  changes  with  the  formation  of  uric  acid.  The 
evidence,  however,  is  not  sufficiently  conclusive  to  warrant 
our  regarding  uric  acid  as  a  special  product  of  the  spleen. 

Some  physiologists  have  suggested  that  the  spleen 
produces  an  enzyme  which  converts  trypsinogen  into 
trypsin.  There  is  no  reason  why  the  spleen  might  not  do 
so,  but  it  by  no  means  follows  that  this  is  normally  its 


268  A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

function,  nor  would  there  appear  to  be  any  necessity  for 
this  action  in  face  of  the  fact  that  it  is  one  of  the  special 
functions  of  the  intestinal  juice. 

In  connection  with  all  these  theories  it  is  well  to  remem- 
ber that  the  spleen  may  be  removed  completely  and  no  ill 
effects  follow. 

Thyroid. — Some  of  the  most  interesting  work  on  the 
ductless  glands  has  been  carried  out  on  the  thyroid,  and  it 
is  largely  to  this  body  that  such  little  knowledge  as  we 
as  yet  possess  of  internal  secretion  is  mainly  due. 

For  years  it  had  been  observed  that  atrophy  or  absence 
of  this  gland  in  the  human  subject  was  associated  with 
arrested  development  both  mental  and  physical ;  the  man 
so  affected  remained  a  child  both  in  intelligence  and 
appearance.  This  stimulated  experimental  enquiry,  and 
the  thyroids  were  removed  in  many  animals,  the  majority 
of  carnivora  dying  as  the  result,  while  half  of  the  herbivora 
recovered  from  the  operation.  So  contradictory  were  the 
results  obtained  by  different  observers  on  the  gland  and  its 
uses,  that  the  whole  question  was  submitted  to  very  close 
enquiry,  which  revealed  the  fact  that  the  ordinary  thyroid 
consists  of  two  distinct  portions,  one  part  the  thijroid 
proj)er,  the  other  the  parathyroids.  In  most  animals  much 
the  same  results  are  obtained  when  both  parts  are 
removed,  but  when  the  parathyroids  alone  are  excised, 
death  rapidly  ensues,  preceded  by  convulsions.  The 
removal  of  the  thyroid  only  gives  rise  to  a  train  of  S3'mp- 
toms  accompanied  by  chronic  wasting,  much  slower  in 
development  than  in  the  case  of  the  parathyroids.  Such 
are  the  broad  lines  of  distinction  between  the  two  portions 
of  the  combined  thyroid  body.  The  colloid  substance  con- 
stitutes the  internal  secretion  of  the  thj^roid,  but  forms  no 
part  of  the  secretion  of  the  parathyroids  ;  and  histologically 
while  the  former  consists  of  vesicles  lined  by  a  single  laj-er 
of  cubical  epithelium,  the  parathyroid  is  composed  of  columns 
of  epithelium-like  cells.  The  gland  contains  a  nucleo- 
proteid  and  colloid  substance  ;  the  latter  is  not  a  nucleo- 
proteid,  and  is  remarkable  for  containing  iodine  in  organic 


THE  INTERNAL  SECEETIONS  269 

combination  with  the  proteid.  The  iodine- containing  sub- 
stance is  termed  iodothyrin  ;  it  is  a  brown  amorphous 
material,  containing  phosphorus  and  10  per  cent,  of  iodine. 

As  to  the  uses  of  these  bodies  little  is  known.  That  they 
produce  an  internal  secretion  which  finds  its  way  into 
the  system  by  the  bloodvessels  or  lymphatics  is  certain ; 
it  is  probable  that  this  secretion  is  mainly  directed  to  the 
nutrition  of  the  body,  especially  of  the  central  nervous 
system.  Some  have  considered  that  the  gland  produces  a 
substance  which  neutralizes  poisons  formed  during  meta- 
bolism. The  consensus  of  evidence  is  that  an  internal 
secretion  is  produced  which  is  essential  to  the  body.  It 
appears  beyond  all  doubt  that  when  from  any  cause  the 
gland  fails  to  supply  the  secretion,  the  symptoms  may  be 
relieved  by  the  administration  of  an  extract  of  the  thyroid 
body,  or  even  by  feeding  the  patient  on  the  prepared  gland. 

Thymus. — This  body,  composed  of  modified  lymphoid 
tissue,  is  mainly  of  use  in  foetal  and  very  early  life ;  later 
on  it  atrophies.  Nothing  is  known  of  its  function,  though 
it  is  observed  that  castration  appears  to  have  an  effect  on 
its  disappearance,  as  the  process  of  atrophy  is  much  slower 
in  the  castrated  as  compared  with  the  uncastrated  animal, 
while  its  early  removal  has  been  observed  to  be  associated 
with  a  rapid  growth  of  the  testicles. 

The  experimental  removal  of  the  Adrenals  in  any  animal 
is  rapidly  followed  by  death,  preceded  by  symptoms  of 
great  muscular  prostration  and  diminution  of  vascular  tone. 
In  Addison's  disease  in  man  these  bodies  are  affected,  and 
give  rise  to  much  the  same  symptoms  as  above,  and  in 
addition  bronzing  of  the  skin  is  present.  Like  the  thyroids 
the  adrenals  consist  of  two  distinct  tissues,  a  medulla 
which  can  be  shown  to  be  derived  during  the  process  of 
development  from  the  sympathetic  nervous  system,  while 
the  cortex  is  formed  from  the  mesoblast.  While  nothing 
is  known  of  the  function  of  the  cortex,  the  medulla  yields 
under  experimental  enquiry  some  remarkable  and  charac- 
teristic results. 

An  extract  of  the  medulla  of  the  gland  when  injected 


270     A  MANUAL  OF  VETEEINAEY  PHYSIOLOGY 

into  the  blood  causes  a  marked  increase  in  blood  pressure  ; 
even  extremely  small  doses  produce  this  effect.  If  the  vagi 
are  intact  the  heart-beat  is  simultaneously  slowed,  if  cut  the 
beat  is  quiclvened.  The  active  agent  is  known  as  adrenalin, 
and  its  effect  on  the  circulation  in  causing  constriction  of 
the  small  vessels  is  so  marked  as  to  be  turned  to  account 
in  minor  surgery.  The  result  of  the  constriction  of  the 
vessels  is  a  rise  in  blood  pressure,  and  this  is  not  necessarily 
central  in  origin,  as  it  may  be  obtained  after  the  constrictor 
centre  in  the  spinal  cord  has  been  destroyed.  Adrenalin 
acts  upon  all  plain  muscle  and  gland  cells  which  receive 
sympathetic  fibres,  and  it  is  distinctly  noteworthy  that  the 
effects,  whether  they  be  augmentory  or  inhibitory,  are 
identical  with  those  produced  by  stimulation  of  the  sympa- 
thetic fibres  (Langley),  of  which  system  the  medulla  of  the 
gland  is,  as  pointed  out  above,  merely  an  outgrowth. 

It  is  probable  that  the  function  of  this  gland  is  con- 
cerned in  the  provision  of  a  substance  intimately  connected 
with  muscular  metabolism,  especially  '  tone,'  not  only  of 
the  skeletal  muscles,  but  also  of  the  muscular  fibres  of 
the  circulatory  system.  There  is  also  considered  to  be  some 
connection  between  the  adrenals  and  the  sexual  system.  In 
rabbits  the  cortex  of  the  gland  becomes  twice  the  normal 
thickness  during  pregnancy ;  and  it  is  believed  that  in  man 
a  connection  exists  between  the  adrenals  and  the  growth  of 
the  body,  the  development  of  puberty,  and  sexual  maturity. 

Very  little  is  known  of  the  function  of  the  Pituitary  Body. 
The  part  has  been  experimentally  removed,  and  in  such 
cases  muscular  weakness,  twitchings,  and  a  lowered  tem- 
perature have  been  observed.  The  pituitary  is  closely 
allied  to  the  adrenals  in  the  effects  on  the  circulation  of 
extracts  made  from  it,  while  in  its  general  metabolic 
functions  it  is  considered  to  be  related  to  the  thyroid. 
In  man  the  singular  disease  acromegaly,  characterized  by 
an  overgrowth  of  the  bones  of  the  face  and  extremities,  is 
associated  with  disease  of  the  pituitary  body. 

Nothing  is  known  of  the  uses  of  the  Pineal  Body.  It  is 
regarded  as  the  dorsal  eye  of  a  remote  ancestor. 


CHAl^TEE  IX 

THE  SKIN 

It  is  obvious  that  one  important  function  the  skin  performs 
is  that  of  affording  cover  to  the  dehcate  parts  beneath  ; 
wherever  the  chance  of  injury  is  the  greatest  the  skin  is 
the  thickest,  while  in  those  parts  where  sensibiUty  is  most 
required  it  is  thinnest.  The  skin  of  the  back,  quarters, 
and  limbs  are  good  examples  of  the  first  type ;  on  the  back 
especially  a  protective  covering  is  found  which,  in  some 
horses,  is  as  much  as  a  quarter  of  an  inch  in  thickness : 
the  face  and  muzzle  are  a  good  example  of  the  latter  variety, 
the  skin  in  some  parts  being  as  thin  as  paper.  In  those 
regions  not  exposed  to  violence  it  is  also  thin,  as  on  the 
inside  of  the  arms  and  thighs.  In  spite  of  the  thinness  of 
the  skin  its  strength  is  remarkable  ;  a  horse's  body  may  be 
dragged  along  by  the  thin  skin  of  the  head. 

The  skin  as  an  organ  of  touch  is  of  great  importance.  All 
animals  appear  most  sensitive  to  even  slight  skin  irrita- 
tion ;  liies  will  cause  horses  considerable  suffering,  and  the 
elephant,with  its  thick  hide,  is  quite  as  intolerant  of  these  tor- 
mentors as  is  a  well-bred  horse.  The  skin  is  highly  endowed 
with  sensory  nerves,  especially  that  part  connected  with  the 
organs  of  prehension;  the  long  hairs,  'feelers,'  growing 
from  the  muzzle  of  the  horse  end  in  special  tactile  structures 
in  the  skin  (Fig.  63). 

The  skin  is  a  bad  conductor  of  heat,  and  this  is  consider- 
ably assisted  by  the  layers  of  fat  found  beneath  or  at  no 
great  distance  from  it,  as  in  the  abdominal  region ;  it  is 
the  subperitoneal  fat  which  protects  the  viscera  of  animals 
living  in  the  open  and  lying  in  wet  places.     The  epidermal 

271 


272    A  MANUAL  OF  VETEEINAKY  PHYSIOLOGY 


covering  of  the  skin  relieves  the  parts  beneath  from  exces- 
sive sensitiveness ;  through  the  sebaceous  secretion  it  assists 
in  preventing  loss  of  heat,  while  the  greasy  covering  helps 
the  hair  to  throw  off  rain,  prevents  the  penetration  of 
water,  and  so  saves  the  epidermis  from  disintegration. 
Horn  is  skin  which  has  undergone  a  modification. 

Hair. — Not  all  parts  of  the  body  are  covered  by  hair. 
There  is  very  little  on  the  muzzle  and  lips,  and  it  is  very 
scanty  on  the  inside  of  the  thighs,  inside  the  cartilage  of 
the  ears,  and  on  the  mammary  gland  and  genitals.     By 


Fig.  63. — Section  of  Mucous  Membrane  of  the  Horse's  Lip, 

SHOWING    THE    NeRVE    EnDINGS    IN    THE    ToUCH    PaPILL.E. 

means  of  the  hair  the  heat  of  the  body  is  maintained  and 
prevented  from  passing  off  too  rapidly.  The  thickness  of 
the  hairy  covering  varies  considerably  with  the  class  of 
horse :  the  better  bred  the  animal  the  finer  the  coat. 
Draught  horses  yield  between  7  lbs.  and  8  lbs.  of  mixed 
hair,  dirt,  and  dandruff  by  clipping ;  in  a  well-bred  horse 
this  would  be  reduced  to  10  oz.,  or  even  less ;  the  amount 
of  hair  of  the  mane  and  tail  is  about  li  lbs.  It  is  a  well- 
known  fact  that,  excepting  the  hair  of  the  mane  and  tail, 
that  of  every  other  part  of  the  body  has  only  a  temporary 


THE  SKIN 


273 


existence,  and  is  changed  twice  a  year,  once  for  a  thick, 
and  once  for  a  fine  coat.  During  this  period  horses  are 
generally  regarded  as  not  being  at  their  best,  and  changing 
the  coat  is  always  urged  as  a  cause  of  loss  of  condition  or 
stamina.  The  permanent  hair  is  not  entirely  represented 
by  that  of  the  mane  and  tail,  the  eyelashes  are  permanent, 
ftlso  the  long  tactile  hairs  on  the  muzzle.  The  temporary 
hairs  on  the  horse  are  of  two  kinds  which  can  only  be  dis- 
tinguished by  their  rate  of  growth.  If  a  part  be  clipped,  or, 
preferably,  shaved  and  the  growth  watched,  in  a  short  time  it 


Fig.  64.— Section  of  Horse's  Skin,  showing  the  Casting  Off  of 
THE  Old  Hair  and  Growth  of  the  New. 

It  will  be' observed  that  both  are  emerging  from  the  same  follicle. 

will  receive  a  scanty  covering  of  long  rapidly  growing  hair, 
followed  by  a  slow  growth  of  ordinary  hair.  There  is  no 
difference  in  the  two  hairs  excepting  the  length.  The  long 
rapidly  growing  hairs  are  known  as  'cat  hairs';  they  are 
not  numerous,  being  about  27  to  the  square  inch,  while 
the  ordinary  hairs  are  about  4,300  to  the  square  inch.* 
The  growth  of  the  hair  is  regulated  by  the  surrounding 

*  I  am^indebted  to  Major  Newsom,  Army  Veterinary  Corps,  for  the 
trouble  he  has  taken  in  making  this  tedious  calculation. 

18 


274     A  MANUAL  OF  YETEEINARY  PHYSIOLOGY 

temperature  ;  if  horses  in  the  depth  of  winter  are  placed  in 
a  heated  atmosphere,  such  as  a  horse  deck  on  board  ship, 
the  majority  commence  to  shed  their  winter  coat  in  a  few 
days,  though  the  temperature  of  the  outside  air  may  be  at 
freezing-point ;  similarly,  if  taken  from  a  warm  to  a  cold 
locality  the  hair  responds  by  becoming  longer.  Speaking 
generally  the  above  statements  are  correct,  but  there  are 
exceptions  and  modifications.  Some  horses  do  not  shed  their 
coat  after  passing  into  a  warmer  latitude  ;  the  mechanism 
which  regulates  the  periodical  shedding  of  hair  refuses  to 
respond  to  the  changed  condition  of  affairs,  so  that  in 
passing  from  north  to  south  of  the  Equator  with  its  reversal 
of  seasons,  the  animal  may  grow  a  summer  coat  in  winter 
and  vice  versa  for  at  least  a  year  after  entering  the  new 
latitude.  The  permanent  hair  of  the  body,  viz.  the  mane 
and  tail,  may  grow  to  almost  any  length,  but  the  temporary 
hair  of  the  surface  of  the  body  only  grows  to  a  definite  length. 
The  full  length  having  been  attained  nothing  will  make  it 
grow  longer,  yet  if  the  horse  be  clipped  hair  at  once  grows 
rapidly,  but  only  to  its  original  length ;  in  other  words, 
everything  is  present  for  the  needful  growth  to  occur,  but 
there  is  a  restraining  influence  present  which  determines 
the  length  of  hair  according  to  the  season. 

Of  the  pigment  in  hair  which  gives  colour  to  the  coat 
our  knowledge,  until  quite  recently,  has  been  of  the  scantiest 
kind.  The  active  investigation  now  being  carried  out  of 
Mendel's  theories  of  heredity,  when  applied  to  the  special 
case  of  heredity  in  coat-colour,  made  it  essential  to  know 
more  about  the  origin,  nature,  and  behaviour  of  the  hair 
pigments,  and  so  we  now  have  some  information  which  is 
both  interesting  and  promising.* 

Using  the  name  in  its  generic  sense,  three  difl'erent 
forms  of  *  melanin  '  are  found  in  hairs — black,  chocolate, 
and  yellow.  Of  these  the  black  is  extremely  insoluble,  and 
hence  very  difficult  to  deal  with ;  as  also  is  the  chocolate 
pigment,  though  to  a  less  extent.    The  yellow,  on  the  other 

*  Florence  M.  Durham  (Proc.  Roy.  Soc,  vol.  Ixxiv.,  p.  310,  1904), 
and  further  researches  as  yet  unpublished. 


THE  SKIN  275 

hand,  dissolves  readily  in  numerous  solvents,  and  may  thus 
be  easil}'  obtained.  In  its  reactions  it  differs  entirely  from 
the  black  and  chocolate  pigments.  In  the  case  of  mice  there 
is  now  no  doubt  that  their  varying  colours  are  due  to  the 
presence  in  their  hairs  of  one  or  more  of  these  three  pig- 
ments. The  less  numerous  experiments  so  far  made  with 
horse-hairs,  which  are,  however,  to  be  carried  out  shortly 
on  a  large  scale,  suggest  no  doubt  as  to  the  different  colours 
of  horses  being  due  to  causes  essentially  the  same  as  those 
which  give  the  various  colours  to  mice.  As  to  the  origin 
of  these  pigments,  it  has  generally  been  presumed  that 
they  must  be  derivatives  of  hemoglobin,  but  there  are  no 
pathological  or  purely  chemical  facts  in  definite  support  of 
this  view.  On  the  other  hand,  it  has  been  shown  *  that  an 
extract  can  be  made  from  the  skins  of  rats,  rabbits,  and 
guinea-pigs,  which  acts  on  tyrosine  (see  p.  236)  in  such  a 
way  as  to  give  rise  to  pigment  substances.  From  the  con- 
ditions under  which  the  conversion  is  most  readily  effected, 
and  the  fact  that  the  activity  of  the  extract  is  at  once 
destroyed  by  boiling,  the  active  agent  is  regarded  as  a 
ferment,  and,  in  accordance  with  the  systematic  nomencla- 
ture now  used,  is  therefore  known  as  ti/wsinase.  A  further 
fact  of  extreme  interest  is  that  the  colour  of  the  pigment 
formed  from  tyrosine  corresponds  to  the  colour  of  the 
animal  from  whose  skin  the  active  extract  is  made.  Black 
pigments  are  produced  when  animals  are  used  whose  skin 
contains  black  pigment,  and  yellow  substances  are  obtained 
when  the  skin  contains  orange  pigment.  With  the  exception 
of  black  and  grey  horses  which  are  liable  to  turn  grey  or 
white,  all  other  colours  are  practically  permanent  even  to 
old  age.  We  do  know,  however,  that  injuries  to  the  skin  of 
horses,  even  of  a  slight  character,  are  commonly  followed 
by  a  growth  of  perfectly  white  hair,  which  never  regains 
its  pigment. 

Experience  shows  that  the  heavy  winter  coat  grown  by 
horses  is  the  cause  of  considerable  sweating  at  work,  and 
the  general  practice  of  clipping  has  hence  been  introduced. 

*  Loc.  cit. 

18—2 


276     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Of  its  value  there  can  be  no  doubt ;  it  considerably  reduces 
the  risk  of  cold  and  chest  diseases,  for  animals  on  coming 
in  from  work  may  be  readily  dried  and  thus  protected 
from  chills.  Horses  which  sweat  freely  at  work  soon  lose 
'  condition  ' ;  our  observations  have  shown  that  this  is  due 
to  the  proteid  lost  by  the  skin,  for,  as  we  shall  presently 
see,  proteids  are  regularly  found  in  the  sweat  of  the  horse. 
Clipping  largely  prevents  this  loss.  The  influence  of  clipping 
on  temperature  is  dealt  with  in  the  chapter  devoted  to 
'  Animal  Heat.' 

In  some  animals,  as  for  instance  the  dog  and  cat,  the 
hairs  are  rendered  erect  under  excitement  such  as  anger  or 
fear  ;  this  is  due  to  the  involuntary  muscle  attached  to  the 
hair  follicle,  and  the  process  is  under  the  influence  of  the 
sympathetic  nervous  system.  The  fibres  for  the  body  hair 
emerge  from  the  spinal  cord  by  the  inferior  roots,  pass  to 
the  grey  ramus  of  the  sympathetic  chain,  and  run  to  the 
skin  by  the  dorsal  cutaneous  nerves ;  the  fibres  for  the  head 
and  neck  are  in  the  cervical  sympathetic.  Under  the  in- 
fluence of  cold  the  hairs  on  the  horse's  body  may  become 
erect,  but  there  is  no  indication  of  this  under  physical 
excitement,  as  in  the  case  of  the  dog  and  cat. 

It  is  possible  that  the  prescience  of  a  coming  storm  or 
change  of  weather  exhibited  by  cattle  may  probably  be  due 
to  the  highly  hygroscopic  i^roperties  of  their  hair.  Hair  is 
one  of  the  few  organic  substances  which  elongate  instead 
of  shorten  as  they  grow  moist.  The  effect  of  movement  of 
every  hair  on  the  surface  of  the  body  may  cause  a  mechanical 
stimulation  of  the  hair-follicle  nerves,  and  so  gives  rise  to 
an  uneasiness  which  presages  the  coming  change. 

Sweat. — By  means  of  glands  in  the  skin  a  fluid  termed 
'  sweat,'  and  a  fatty  material  known  as  '  sebum,'  are 
secreted.  Sweat,  or  perspiration,  is  not  found  to  occur 
over  the  general  surface  of  the  body  in  any  other  hairy 
animal  than  the  horse.  There  are  certain  parts  of  the 
skin  which  sweat  more  readily  than  others ;  the  base  of  the 
ears  in  the  horse  is  the  first  place  where  sweating  begins, 
the  neck,  side  of  chest,  and  back  follow,  lastly  the  hind- 


THE  SKIN  277 

quarters.  No  sweating  takes  place  on  the  legs ;  the  fluid 
found  there  has  run  down  from  the  general  surface  of  the 
body.  Mules  and  donkeys  sweat  with  difficulty,  and  then 
principally  at  the  base  of  the  ears.  The  ox  sweats  freely 
on  the  muzzle,  and  sweating  even  from  the  general  surface 
of  the  body  has  occasionally  been  observed.  It  has  been 
said  that  sheep  perspire,  while  it  is  certain  that  both  the 
dog  and  cat,  especially  the  latter,  sweat  freely  on  the  foot- 
pads as  also  on  the  muzzle,  though  not  on  the  general 
surface  of  the  body.  The  sweating  of  the  pig  is  confined 
to  the  snout. 

The  secretion  of  sweat  is  continuous.  When  excreted  in 
small  amounts  it  evaporates  as  fast  as  it  is  formed,  passing 
off  as  the  insensible  vapour  which  is  always  rising  from 
the  surface  of  the  skin,  and  is  known  as  '  insensible  per- 
spiration.' When  the  secretion  is  rapid  and  copious  or  the 
surrounding  atmospheric  conditions  are  unfavourable  to  its 
evaporation,  it  collects  on  the  skin  as  that  visible  fluid 
material  which  is  ordinarily  termed  '  sweat.'  Colin  gives 
various  numerical  statements  respecting  the  insensible 
perspiration,  from  which  we  gather  that  14  lbs.  of  water 
probably  represent  this  loss  in  the  horse  for  24  hours. 
Much  depends  upon  the  humidity  and  temperature  of  the 
atmosphere ;  the  drier  and  hotter  it  is,  within  certain 
limits,  the  greater  the  insensible  perspiration. 

The  amount  of  sweat  secreted  daily  can  only  be  roughly 
determined  ;  there  are  many  conditions  which  affect  it,  such 
as  the  length  of  coat,  nature  of  the  work,  and  pace. 
Grandeau  by  estimating  the  total  water  consumed  in  the 
food  and  drink,  and  that  voided  in  the  urine  and  faeces, 
arrived  at  the  amount  of  vapour  passing  away  in  the  breath 
and  perspiration.  The  mean  amount  of  water  evaporated 
daily  by  these  two  channels  under  different  conditions  of 
work  was  as  follows  : 


At  rest 

-       6-4  lbs, 

Walking  exercise  - 

-       8-6    „ 

At  work  walking  - 

-     12-7    „ 

Trotting    - 

-     13-4    „ 

At  work  trotting  - 

-     20-6    „ 

278    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

In  each  case  the  distance  walked  and  trotted  and  the 
load  drawn  were  the  same.  It  is  unfortunate  that  we  have 
no  means  in  the  above  experiments  of  determining  the 
proportion  which  the  water  of  respiration  bears  to  that  of 
perspiration. 

Evaporation  from  the  surface  of  the  skin  is  a  most  im- 
portant source  of  loss  of  heat ;  so  marked  is  this  in  the 
horse  that  the  resulting  fall  in  temperature  may  even  carry 
it  below  the  normal,  if  the  sweating  be  very  profuse  or  the 
wetted  area  a  large  one. 

The  compensating  action  existing  between  the  kidneys 
and  skin  observed  in  men  exists  also  in  the  horse,  viz.,  when 
the  skin  is  acting  freely  less  water  passes  by  the  kidneys, 
and  rice  versa. 

Sweat  obtained  from  the  horse  is  always  strongly  alkaline  ; 
after  filtration  it  is  the  colour  of  sherry,  which  is  probably 
accidental,  and  due  to  contamination  with  dandruff,  which 
contains  a  pigment,  chlorophyll ;  it  possesses  a  peculiar 
horse-like  odour,  and  has  a  specific  gravity  of  1020.  We 
found  horse's  sweat  to  have  the  following  composition  :* 


Water        -  -     94-38 


Containing. 


r  Serum  albumin    -  -     O'lOo 

Organic  matters    -       0-52  -       ,,       globulin     -  -     0-327 

I^Fat  -  -  -     0-002 

[Consisting  principally  of  potash, 
^  T^  _  _       C.I  A    I      and  soda,  chlorides,  some  mag- 

nesia, a  little  lime,  and  traces 
of  phosphates. 


1 


The  proteids  are  thus  seen  to  be  serum-albumin  and 
globulin,  and  their  constant  presence  has  been  determined 
by  a  number  of  observations  ;  the  mineral  matter  is  very 
high  and  consists  principally  of  soda  and  potash,  especially 
the  latter.  It  will  be  observed  that  the  mineral  matter 
greatly  exceeds  the  organic  matter ;  in  horses  which  have 
sweated  freely  the  matted  hair  (which  is  due  to  albumin)  is 
often  seen  covered  with  saline  material,  looking  like  fine 

*  '  The  Sweat  of  the  Horse,'  Jourrial  of  Physiology ,  vol.  xi,,  1890. 


THE  SKIN  279 

sand.  There  appears  to  be  some  complemental  action 
between  the  skin  and  the  kidneys  in  the  elimination  of 
soda  and  potash  ;  during  rest  the  kidneys  eliminate  these 
salts,  whilst  during  work  they  are  assisted  by  the  skin. 
Urea  is  also  probably  present  in  sweat  (see  p.  283).  It  is 
difficult  to  see  why  horses  should  excrete  albumin  by  the 
skin  ;  the  loss  thus  produced  accounts  for  the  great  reduc- 
tion of  vitality  and  strength  in  animals  which  sweat  freely 
at  work,  and  for  which  clipping  is  the  only  preventive. 

Nervous  Mechanism  of  Sweating.  —  A  skin  may  sweat 
under  quite  opposite  conditions,  viz.,  both  with  a  hot 
flushed  skin  and  a  bloodless  cold  skin,  in  others  words  an 
animal  may  sweat  when  it  is  hot  or  when  it  is  cold.  The 
former  is  a  physiological  condition  and  regulates,  as  we 
shall  see,  the  body  temperature ;  the  latter  is  abnormal, 
but  it  occurs  and  disproves  at  once  any  notion  of  sweating 
necessarily  depending  upon  a  congested  condition  of  the 
vessels  of  the  skin.  Experiments  show  that  most  of  the 
features  of  sweating  can  be  accounted  for  through  the 
agency  of  the  nervous  system.  Though  we  are  ignorant 
of  the  manner  in  which  the  nerves  terminate  in  the  sweat 
glands,  still  it  is  certain  that  there  are  special  branches  of 
nerves,  whose  function  it  is  to  determine  the  secretion  of 
sweat,  and  these  are  quite  distinct  from  those  which  regulate 
the  vascular  supply.  If  the  peripheral  end  of  the  divided 
sciatic  in  the  cat  be  stimulated  the  foot-pads  sweat ;  the 
proof  that  this  reaction  is  a  specifically  nervous  one  is  easy, 
apart  from  the  fact  that  stimulation  of  the  sciatic  causes  a 
violent  constriction  of  the  bloodvessels  in  the  leg,  for  the 
sweating  occurs  when  the  leg  has  been  cut  off  or  the  aorta 
tied,  and  it  is  absent  under  the  influence  of  atropin.  The 
effect  of  atropin  on  the  sweat  glands  is  very  closely  allied 
to  its  action  on  the  salivary  glands  (p.  146)  ;  it  paralyzes 
the  secretory  nerves  which  produce  sweat. 

As  with  the  salivary  glands,  so  in  the  present  case 
secretion  is  not  due  to  any  increased  supply  of  blood.  It 
is  true  that  in  normal  sweating,  as  is  so  readily  seen  in 
man,  the  skin  is  flushed  as  the  increased  secretion  takes 


280    A  MANUAL  OF  YETEKINARY  PHYSIOLOGY 

place,  l)ut  the  increased  blood  suppl}^  which  the  flushing 
indicates  is  merely  the  necessary  adjuvant,  not  the  cause 
of  the  secretion ;  it  supplies  the  glands  with  the  extra 
material  they  now  require,  the  secretory  nerves  causing  the 
gland-cells  to  utilize  the  increased  supply. 

The  secretion  of  sweat  may  be  induced  in  man,  the  cat, 
and  the  dog,  though  not  in  the  horse,  by  the  injection  of 
l^ilocarpin.  In  this  case  the  action  is  peripheral — that  is 
to  say,  on  the  glands  themselves — since  it  occurs  when  the 
sciatic  nerves  are  cut  previously  to  the  injection. 

As  we  have  seen,  secretion  is  ordinarily  brought  about 
by  specific  efferent  nerves,  and  these  originate  in  the  central 
nervous  system,  from  which  the  necessary  secretory  im- 
pulses are  directly  supplied.  But  secretion  may  also  be 
readily  induced  by  the  stimulation  of  afferent  nerves,  as  in 
the  all-important  case  of  a  rise  in  the  surrounding  tempera- 
ture. These  facts  lead  at  once  to  the  belief  that  '  sweat 
centres  '  must  exist  in  the  central  nervous  system  com- 
parable to  those  of  the  respiratory  and  vascular  mechanisms, 
though  they  have  not  as  yet  been  so  definitely  localized. 
There  seems  to  be  no  doubt  that  the  spinal  cord  contains 
sweat  centres.  The  existence  of  a  similar  centre  in  the 
medulla  is  less  certain,  though  probable,  since  in  some  men 
pers})iration  over  the  face  and  neck  results  from  merel}' 
smelling  a  pungent  substance,  such  as  curry-powder,  and 
becomes  profuse  if  the  latter  is  introduced  into  the  mouth. 

The  sweat-nerve  supply  to  the  fore  and  hind  limbs  passes 
out  of  the  cord  by  means  of  the  rami  communicantes  of  the 
sympathetic  system,  and  so  reaches  the  brachial  and  sciatic 
plexus  respectively  ;  the  sweat  fi])res  for  the  head  and  neck 
are  in  the  cervical  sympathetic ;  those  for  the  face  in  the 
horse,  the  muzzle  in  the  ox,  the  snout  in  the  pig,  run  in 
branches  of  the  fifth  pair  of  nerves.  Division  of  the 
cervical  sympathetic  in  the  horse  produces  profuse  sweat- 
ing of  the  head  and  neck,  limited  to  the  side  operated  upon  ; 
this  may  be  due  to  vaso-motor  paralysis,  though  a  different 
interpretation  has  been  placed  on  it,  viz.,  that  the  sympa- 
thetic carries  inhibitory  impulses  to  the  sweat  glands  of  the 


THE  SKIN  281 

head,  so  that  on  division  the  secretory  fibres  act  without 
opposition.  In  the  ox  Arloing  has  shown  that  division  of 
the  cervical  sympathetic  causes  the  muzzle  on  the  same 
side  to  become  dry ;  stimulation  of  the  cut  end  of  the 
nerve  is  followed  by  secretion,  but  this  is  not  so  when  the 
nerve  degenerates,  though  even  then  the  glands  respond  to 
pilocarpin. 

As  previously  stated,  a  high  temperature  favours  the 
activity  of  the  epithelium  lining  the  sweat  glands,  for  if  the 
limb  of  a  cat  be  kept  warm  a  larger  secretion  of  sweat  is 
obtained  on  stimulating  the  sciatic  than  in  a  limb  kept 
cold,  in  which  latter  stimulation  of  the  sciatic  may  produce 
no  secretion  whatever.  Further,  if  a  cat  in  which  one 
sciatic  has  been  divided  be  placed  in  a  hot  chamber  profuse 
secretion  will  occur  on  the  foot-pads  of  the  limbs  not  sub- 
jected to  interference,  while  on  the  side  on  which  the  sciatic 
has  been  divided  no  sweating  occurs.  This  is  a  further 
proof  of  the  existence  of  a  reflex  mechanism,  to  which 
we  have  already  drawn  attention.  It  has  been  thought 
that  the  sweating  which  takes  place  at  death  is  due  to  a 
dyspnceic  condition  of  the  blood  and  in  many  cases  this 
may  be  so,  but  it  is  difficult  to  account  for  the  profuse  cold 
sweating  in  ruptures  of  such  viscera  as  the  stomach  and 
intestines,  or  the  localized  hot  sweating  which  is  often  so 
well  marked  in  horses  between  the  thighs  immediately  after 
the}'  are  destroyed.  Thrombosis  of  both  iliac  arteries  may 
occur  in  the  horse,  and  a  marked  symptom  of  this  trouble 
is  the  peculiarity  in  the  accompanying  sweating;  the 
general  surface  of  the  body  may  sweat  freely  but  not  the 
hind-quarters.  The  cause  of  this  peculiarity  has  not  been 
worked  out. 

In  comparing  the  sweat  glands  with  the  salivary  we  must 
be  careful  not  to  draw  too  close  a  parallel,  for  though  in 
certain  features  they  agree,  in  others  they  are  very  different; 
for  instance  in  the  horse  pilocarpin  produces,  as  in  other 
animals,  a  profuse  salivary  flow,  but,  unlike  its  action  on 
man,  the  dog,  and  cat,  it  has  no  effect  whatever  in  producing 
sweating. 


282     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  peculiar  breaking  out  into  sweats  which  occurs  in 
horses  after  work  has  no  j^arallel  in  man ;  some  animals 
will  l)reak  out  two  and  three  times  for  hours  afterwards,  even 
after  having  been  rubbed  quite  dry.  This  maybe  connected 
with  the  necessity  for  a  discharge  of  body  heat,  since  the 
internal  temperature  rises  above  the  normal  during  work, 
in  some  cases,  it  is  said,  as  much  as  4°  Fahr.  to  5°  Fahr., 
and  remains  so  for  some  time  afterwards.  Another 
peculiarity  in  sweating  of  the  horse  is  the  patchy  perspira- 
tion observed  occasionally,  such  as  a  wet  patch  on  the  side 
or  quarter  which  dries  slowly,  or  may  remain  for  days  or 
weeks  in  a  wet  or  damp  condition.  Finally,  there  is  no 
drug,  so  far  as  we  are  aware,  which  produces  sweating  in 
horses  ;  this  is  perhaps  an  explanation  of  the  common  use 
of  nitre  in  veterinary  practice,  the  kidneys  being  made  to 
do  the  work  of  the  skin. 

The  changes  occurring  in  the  secreting  cells  of  the  sudo- 
riferous glands  of  the  horse  have  been  described  by  Renault. 
When  charged  the  cells  are  clear  and  swollen,  the  nucleus 
being  situated  near  their  attached  ends  ;  when  discharged 
they  are  smaller,  granular,  and  their  nucleus  more  centrally 
placed. 

Sebaceous  Secretion  or  Sebum  is  a  fatty  material  formed 
in  the  sebaceous  glands  of  the  skin,  which  in  the  horse 
are  freely  distributed  over  the  whole  surface  of  the  body. 
Though  it  is  spoken  of  as  a  secretion,  yet  the  process 
involved  is  not  secretory,  inasmuch  as  the  cellular  elements 
of  the  gland  are  not  actively  employed  pouring  out  material, 
but  are  themselves  shed  after  undergoing  fatty  metamor- 
phosis. The  greasy  material  thus  produced  saves  the 
epithelium  from  the  disintegrating  influence  of  wet,  keeps 
the  skin  supple,  and  gives  the  gloss  to  the  groomed  coat ; 
from  its  greasy  nature  it  assists  in  preventing  the 
penetration  of  rain,  and  thereby  saves  to  an  extent  undue 
loss  of  heat. 

Dandi'uiF. — The  material  removed  from  horses  by  groom- 
ing consists  of  a  white  or  grey  powder  which  can  readily  be 
moulded  by  pressure  into  a  dough-like  mass.     It  consists 


THE  SKIN  283 

of  epithelial  scales,  fat,  largely  in  the  form  of  lanolin, 
colouring  matter,  salts,  and  a  considerable  amount  of  silica 
and  dirt,  the  two  latter  depending  upon  the  cleanliness  of 
the  animal.  The  amount  of  dandruff  lost  in  an  ordinary 
grooming  varies  from  20  to  60  grains  for  clean  horses,  and 
170  to  200  grains  for  very  dirty  animals.  An  analysis  of 
dandruff  from  the  horse  gave  the  following  composition  :* 

Water      -  -  -  17-96 

Fat  -  -  -  12-40 

Organic  matter  -  -  56-22   containing  I'O?  of  urea. 

Ash  -  -  -  13-42  „  2-45  of  silica. 

100-00 

The  fatty  matter  in  the  skin  proves  to  be  lanolin,  the 
same  as  that  found  in  the  fleece  of  sheep ;  it  explains  the 
reason  why  horses  living  in  the  open  should  not  be  too 
freely  groomed,  and  supports  the  prejudice  which  has 
always  existed  against  this  practice.  It  is  evident  that 
with  free  grooming  the  loss  in  fat  alone  is  something  con- 
siderable, and  the  animal  exposed  to  chill.  The  amount  of 
fat  depends  upon  the  diet ;  on  hay  alone  there  is  very  little 
in  the  dandruff,  whilst  on  oats  there  is  a  considerable 
amount.  The  urea  shown  in  the  analysis  is  no  doubt 
derived  from  the  sweat. 

Dandruff  contains  a  colouring  matter  found  to  be 
chlorophyll,  which  has  undergone  modification  by  passing 
from  the  digestive  canal  to  the  skin.  The  use  of  this  pig- 
ment is  unknown,  in  fact,  the  horse  is  the  only  vertebrate 
in  which  chlorophyll  has  so  far  been  found  as  a  constituent 
of  any  cutaneous  excretion. 

In  certain  places,  as  in  the  prepuce,  considerable 
quantities  of  sebum  are  found.  The  sebaceous  secretion 
of  the  prepuce  of  the  horse  consists  of  50  per  cent,  fat, 
and  also  contains  calcium  oxalate.  The  ear-wax  and  eyelid 
secretions  are  also  of  a  sebaceous  nature. 

In  the  sheep  a  considerable  quantity  of  fatty  substance 

*  '  Dandruff  from  the  Horse,  and  its  Pigment,'  Journal  of  Physi- 
ologij,  vol.  XV.,  1893. 


284     A  MANUAL  OF  YETEEINAEY  PHYSIOLOGY 

is  found  in  the  wool ;  it  exists  in  two  forms,  (1)  as  a  fatty 
acid  united  to  potash  to  form  a  soap,  and  (2)  a  fatty  acid 
combined  with  cholesterin  instead  of  glycerin  ;  the  latter 
is  known  as  lanolin,  and  is  largely  used  as  a  basis  for 
ointments.  It  is  also  found  in  hair,  horn,  feathers,  etc. 
The  fatty  substance  in  the  wool  is  known  to  shepherds 
aiVd  others  as  '  suint.'  In  merino  sheep  it  may  amount  to 
more  than  one-half  the  weight  of  the  unwashed  fleece,  but 
in  ordinary  weather-exposed  sheep  it  may  be  15  per  cent, 
or  less.  The  large  amount  of  potash  in  unwashed  wool  is 
very  remarkable ;  a  fleece  sometimes  contains  more  potash 
than  the  whole  body  of  the  shorn  sheep  (Warrington), 

Respiratory  Function  of  the  Skin. — Certain  vetebrates  such 
as  the  frog  can  respire  by  the  skin  in  the  entire  absence 
of  lungs ;  in  this  way  they  absorb  oxygen  and  excrete 
carbonic  acid.  Observations  made  on  animals  and  men 
have  demonstrated  that  similar  changes  occur  through  the 
skin,  but  on  a  very  small  scale. 

Varnishing  the  skin  rapidly  causes  death  in  rabbits,  and 
more  slowly  in  horses.  Death  is  due  to  loss  of  body  heat, 
and  not  to  the  retention  of  poisonous  products  as  was  at 
one  time  supposed.  Bouley*  states  that  horses  shiver 
when  varnished,  and  the  surface  of  the  body  and  the 
expired  air  become  colder,  the  visible  membranes  respond 
by  becoming  violet  in  tint,  and  the  animals  die  after 
several  days.  According  to  Ellenberger,  if  only  partly 
varnished  they  do  not  die,  l)ut  exhibit  temporary  loss  of 
temperature,  and  show  signs  of  weakness.  The  effect  of 
varnishing  the  skin  is  to  cause  the  capillaries  to  dilate, 
and  so  produce  a  great  discharge  of  heat. 

For  absorption  from  the  skin,  see  'Absorption,'  p.  258. 

Pathological. 

The  chief  pathological  conditions  of  the  skin  are  those  due  to 
parasitic  invasion ;  they  may  produce  widespread  disease  in  all 
animals. 

*  Colin's  '  Physiologie.' 


CHAPTEE    X 

THE  URINE 

The  urine  is  sometimes  spoken  of  as  a  secretion,  but  this 
is  not  strictly  correct ;  speaking  broadly,  we  may  say  a 
secretion  is  something  which  is  formed  in  a  part  for  the 
purpose  of  being  eventually  utilized  by  the  system.  This 
does  not  apply  to  the  urine,  the  chief  constituents  of  which 
are  not  even  prepared  in  the  kidneys  but  onl}'  separated  by 
them  ;  moreover,  the  urine  having  once  been  formed  is  of 
no  further  use  to  the  body  and  is  excreted.  An  excretion, 
therefore,  is  something  removed  from  the  system  as  being 
no  longer  required,  and  the  retention  of  which  would  be 
harmful.  This  removal  is  effected  by  the  kidneys,  which 
may  in  a  sense  be  regarded  as  the  filters  of  the  body, 
regulating  the  composition  of  the  blood  by  removing  from 
it  waste  and  poisonous  products,  and  maintaining,  as  will 
be  later  explained,  its  proper  degree  of  alkalinity. 

In  consequence  of  the  discoveries  which  have  been  made 
of  internal  secretions,  physiologists  have  forecast  that  the 
kidneys  may  yet  be  shown  to  take  some  important  part  in 
nitrogenous  metabolism.  This  forecast  is  based  on  the  fact 
that  in  the  dog,  with  only  one-quarter  the  normal  amount 
of  kidney  substance  left,  double  the  normal  amount  of 
urea  is  excreted.  We  have  seen  how  both  nourishment 
and  waste  materials  are  poured  into  the  circulation,  and 
have  studied  several  of  the  channels  by  which  the  latter 
are  removed,  viz.,  by  the  lungs,  skin,  and  intestinal  canal ; 
we  have  now  to  examine  the  last  excretory  path,  viz.,  the 
kidneys. 

285 


28(3     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

The  vascular  arrangements  of  the  kidney  are  intimately 
connected  with  the  function  of  the  organ.  The  renal 
artery  is  short,  it  comes  off  close  to  the  posterior  aorta, 
and  the  pressure  within  it  is  practically  the  pressure  in 
that  vessel ;  the  pressure  in  the  renal  vein  on  the  other 
hand  is  low,  nearly  as  low  as  that  in  the  posterior  vena 
cava.  It  will  be  observed  that  the  same  amount  of  blood- 
pressure  as  is  required  to  fill  the  vessels  of  the  lumbar 
region  and  hind  limbs  is  expended  on  driving  the  blood 
through  the  kidneys. 

At  every  increase  in  the  amount  of  blood  in  the  kidney 
the  organ  swells,  at  every  decrease  it  contracts.  These 
movements  on  the  part  of  the  kidney  have  been  carefully 
studied  by  means  of  Eoy's  oncometer.  An  oncometer  is  a 
metallic  capsule  in  which  the  living  kidney  is  enclosed,  and 
so  arranged  that  the  expansion  and  collapse  of  the  organ 
can  readily  be  detected.  A  tracing  given  by  the  use  of 
this  instrument  shows  that  the  volume  of  the  kidney  is 
affected  by  every  beat  of  the  heart,  and  even  the  re- 
spiratory undulations  in  the  blood-pressure. 

Structure  of  the  Kidney. — The  kidney  consists  of  a  central 
part,  the  medulla,  surrounded  by  an  external  part,  the 
cortex  ;  the  boundary  of  the  two  is  easily  visible  in  a  sliced 
kidney.  The  branches  of  the  renal  artery  break  up  at  the 
boundary  of  the  cortical  and  medullary  portions ;  the 
cortex  of  the  kidney  is  the  essential  secreting  region,  and 
it  is  here  that  the  Malpighian  tufts  or  capsules  are  found. 
These  consist  of  small  balls  of  capillaries,  the  glomeruli, 
derived  from  the  renal  artery ;  the  artery  entering  the 
Malpighian  tuft  is  larger  than  the  vein  leaving  it,  the 
result  is  that  a  high  blood-pressure  is  maintained  in  the 
glomerulus.  The  vessel  which  supplies  these  tufts  also 
sends  branches  to  form  a  plexus  around  the  urimferous 
tubules ;  these  branches  do  not  enter  the  Malpighian  body. 
The  whole  glomerulus  is  contained  in  a  capsule  in  which  it 
is  suspended  by  its  afferent  and  efferent  vessel ;  when  the 
vessels  are  dilated  the  tuft  fills  the  capsule,  when  they 
are  collapsed  there  is  a  space  between  them  (Fig.  65). 


THE  URINE  287 

The  minute  vein  or  efferent  vessel  leaving  the  tuft 
breaks  up  into  capillaries  around  the  uriniferous  tubule ; 
thus  the  blood  in  the  plexus  of  capillaries  around  the 
tubule  is  derived  from  two  sources,  viz.,  from  the  tuft,  and 
directly  from  the  renal  artery. 

The  capsule  of  Bowman  which  surrounds  the  tuft  is 
lined  by  cells  resembling  the  epithelioid  plates  seen 
in  capillaries ;  they  are  flat  polygonal  cells  containing  a 
nucleus.  The  capsule  is  practically  the  dilated  beginning  of 
a  uriniferous  tubule,  and  the  latter  is  continued  from  the 


Fig.  65. — Diagram  showing  the  Relation  of  the  Malpiohian 
Body  to  the  Uriniferous  Tubules  and  Bloodvessels 
(KiRKE,  after  Bowman). 

a,  An  interlobular  artery;  a',  branch  of  artery  passing  into  the 
glomerulus ;  c,  capsule  of  the  Malpighian  body  forming  the  com- 
mencement of,  and  continuous  with  t,  the  uriniferous  tube  ;  e'e'e', 
vessels  leaving  the  tuft,  forming  a  plexus  p  around  the  tube,  and 
finally  terminating  in  e,  a  branch  of  the  renal  vein. 

capsule,  taking  a  course  of  extraordinary  complexity  in  order 
to  reach  the  pelvis  of  the  kidney  ;  further,  the  cells  found 
in  the  tubule  are  no  longer  the  flat  polygonal  cells  of  the 
capsule,  but  a  something  special  to  the  tubule  and  even 
to  different  parts  of  it. 

If  we  briefly  follow  the  course  of  a  uriniferous  tubule 
(Fig.  66),  it  is  found  that  on  leaving  the  capsule  it  becomes 
twisted  in  the  cortex  forming  the  convoluted  tube;  it  then 
forms  a  spiral  tube,  and  leaving  the  cortex  runs  straight 
into   the  medulla,  forming  the  descending  limb  of  Henle ; 


•288    A  MANUAL  OF  YETEEINAEY  PHYSIOLOGY 


Fig.  66. — Diagram  of  the  Course  of  the  Urixiferous  Tubules 
(Klein  and  Noble  Smith). 

A,  Cortex  of  kidney  ;  a,  subcapsular  layer  not  containing  glomeruli ; 
a',  inner  structure  of  cortex  also  without  glomeruli;  B,  boundary 
layer  of  medulla;  C,  papillary  part  of  the  medulla  ;  1,  Bowman's 
capsule  of  the  glomerulus  ;  2,  neck  of  capsule  ;  3.  proximal  convo- 
luted tube  ;  4.  spiral  tube  ;  5,  descending  limb  of  Henle  ;  6,  loop  of 
Henle  ;  7,  thick  part  of  ascending  hmb;  S,  spiral  part  of  ascending 
limb  ;  9,  narrow  ascending  limb  in  the  medullai-y  raj- ;  10,  the 
irregular  tubule  :  11.  distal  convoluted  tube;  12,  curved  collecting 
tube  ;  13,  straight  collecting  tube  ;  14.  collecting  tube  of  boundary 
layer;  15,  large  collecting  or  discharging  tubule  of  papillary  layer. 


THE  UEINE  289 

it  now  makes  a  sharp  turn,  the  loop  of  Henle,  and  travels 
back  to  the  cortex,  in  the  same  way  that  it  left,  by  the 
ascending  limb  of  Henle.  The  descending  limb  is  straight 
and  narrow,  the  ascending  limb  is  wavy  in  character  and 
larger.  Having  reached  the  cortex  the  ascending  limb 
becomes  distinctly  wider  and  twisted,  forming  the  zhjzag 
or  irregular  tubule ,-  from  this  a  tubule  is  continued  which 
resembles  in  its  contortions  the  first  convoluted  portion  ;  it 
is  termed  the  second  convoluted  tubule.  This  now  leaves  the 
cortex  and  enters  the  medulla  as  a  straight  tube,  known  as 
the  collecting  tiibe ;  it  runs  towards  the  apex  of  the  pyramid 
and  joins  other  collecting  tubes;  by  so  doing  it  becomes 
larger,  and  on  reaching  the  apex  is  known  as  a  discharging 
tube  or  duct  of  Bellini. 

The  epithelial  cells  lining  the  tubules  are  not  of  the 
same  character  throughout ;  broadly,  they  may  be  divided 
into  a  striated  cell  staining  readily,  and  a  clear  transparent 
cell  staining  with  difficulty.  The  first  epithelium  is  sugges- 
tive of  secreting  cells,  the  latter,  on  the  other  hand, 
possesses  more  the  characteristics  of  the  epithelial  lining 
of  ducts. 

The  amount  of  blood  passing  through  the  kidney  is 
something  very  considerable ;  it  has  been  calculated  that  in 
24  hours  1-46  lbs.  of  blood  will  pass  through  the  kidneys 
of  a  dog  weighing  ii6  lbs. 

Vascular  Mechanism. — The  vascular  arrangements  of  the 
kidney  are  under  the  control  of  a  rich  supply  of  vaso-con- 
strictor  nerves,  while  dilator  nerves  are  also  known  to  exist. 
If  the  general  blood-pressure  be  constant,  dilatation  of  the 
renal  vessels  means  an  increased  secretion  of  urine,  while 
constriction  of  the  vessels  means  a  reduced  secretion.  An 
increase  in  the  general  blood-pressure  produces  an  increase 
in  the  amount  of  blood  in  the  kidney,  and  this  is  rendered 
evident  by  the  swelling  of  the  organ  in  the  oncometer  and 
an  increased  production  of  urine.  If  the  increased  general 
blood-pressure  is  accompanied  by  a  constriction  instead  of 
a  dilatation  of  the  small  arteries  of  the  kidney,  such  for 
instance  as  when  the  vaso-constrictor   nerves  are  stimu- 

19 


'290    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

lated,  then  the  increased  l)lood-pressure  cannot  lead  to 
increased  secretion,  but  on  the  contrary  the  amount  of 
urine  becomes  less  and  the  kidney  shrinks.  A  fall  in 
general  blood-pressure,  such  as  is  caused  by  dividing  the 
spinal  cord,  brings  al>out  a  reduction  in  the  flow  through 
the  kidney,  and  the  blood-pressure  becomes  so  low  that  the 
secretion  of  urine  is  entirely  suspended.  It  is  thus  evident 
that  the  vaso-motor  influence  over  the  kidney  is  of  the 
greatest  importance,  and  largely  regulates  the  amount  of 
urine  manufactured.  If  the  renal  vein  be  obstructed,  the 
pressure  of  lilood  in  the  kidney  rises,  but  no  urine  is 
secreted  ;  evidently  therefore  an  increased  flow  of  blood 
through  the  kidney  is  as  essential  to  secretion  as  is 
increased  blood-pressure. 

Two  theories  of  urinary  secretion  have  hence  been  put 
forward,  one  l)eing  based  on  the  physical  conditions  which 
are  favourable  in  the  kidney  to  filtration,  while  the  other  is 
based  on  the  supposition  that  the  cells  are  secretory.  It  is 
o])vious  that  there  are  two  portions  of  the  kidney  engaged 
in  the  manufacture  of  urine,  viz.,  the  glomerular  and  the 
tubular.  In  the  former  the  conditions  for  filtration  from 
the  bloodvessels  of  the  tuft  into  Bowman's  capsule  exist, 
yet  the  experiment  of  obstructing  the  renal  vein,  referred  to 
above,  has  impressed  on  physiologists  the  influence  of  the 
activity  of  the  endothelial  cells  of  the  glomerulus,  for  if 
filtration  pure  and  simple  could  obtain  water  from  the 
Malpighian  tufts,  more  urine  should  have  been  secreted 
immediatelij,  though  not  continuously,  after  ligaturing  the 
renal  vein  than  before.  As  a  matter  of  fact  we  know  that 
secretion  ceases. 

The  evidence  of  secretory  activity  in  the  tubules  of  the 
kidney  is  based  on  the  following  experiment.  If  sulph- 
indigotate  of  soda  be  injected  into  the  blood  of  the  dog, 
within  a  short  time  the  urine  acquires  an  intensely  blue 
colour,  though  the  blood  may  be  only  slightly  afl^ected.  If 
the  kidney  be  removed  and  examined,  all  parts  but  the 
Malpighian  bodies  are  found  stained  l)lue.  In  order  to 
determine  what  portion  of  the  tubule  excretes  the  dye  it  is 


THE  UEINE  291 

necessary  to  stop  the  secretion  in  the  glomeruli,  otherwise 
the  dye  gets  carried  through  the  whole  length  of  the  tubule. 
In  order  to  stop  glomerular  secretion  the  spinal  cord  is 
divided  in  the  neck,  the  blue  colouring  matter  injected,  and 
the  kidney  examined.  The  blue  is  now  found  in  the  cortex 
only,  and  within  the  striated  epithelial  cells  of  the  first  and 
second  convoluted  tubes,  where  the  indigo  may  be  seen  in 
granules.  From  this  experiment  it  is  clear  that  the  cortical 
tubules  elected  to  turn  out  the  indigo,  while  the  medullary 
tubules  were  unable  to  effect  this,  from  which  it  is  judged 
that  a  specific  secretory  activity  of  these  cells  is  shown  for 
indigo,  and  it  is  assumed  that  a  similar  function  may  be 
exercised  towards  other  bodies,  for  instance,  urea  and  the 
other  constituents  of  the  urine. 

Stating  these  points  briefly  in  connection  with  secretion 
they  amount  to  this,  that  in  the  glomeruli  the  water  of  the 
urine,  and  perhaps  the  salts,  are  passed  out  chiefly  as  the 
result  of  varying  glomerular  blood-pressure,  while  in  the 
tubules  the  organic  matter  is  excreted  as  the  result  of  a 
distinctly  secretory  activity  of  their  cells.  These  substances 
are  carried  along  by  the  fluid  which  trickles  down  the 
tubules  into  the  pelvis  from  the  kidney  and  so  becomes 
urine.  Under  pathological  conditions  the  glomeruli  admit 
of  the  exit  of  both  albumin  and  sugar. 

The  secretion  of  proteid  in  the  tuft  and  its  reabsorjDtion 
in  the  tubule  was  at  one  time  believed  to  be  true,  but  inas- 
much as  no  proteid  is  found  in  the  normal  urine  of  any 
animal,  it  is  safe  to  assume  that  in  an  undamaged  state  the 
epithelial  cells  of  the  glomerulus  allow  none  to  pass. 

There  are  no  secretory  nerves  to  the  kidney ;  the  in- 
fluence of  the  nervous  system  is  confined  to  its  action 
on  the  bloodvessels.  The  action  of  diuretics  has  been 
studied  in  connection  with  the  question  of  urinary  secretion, 
and  most  observers  find  that  though  these  determine  a 
greatly  increased  flow  of  blood  to  the  kidneys,  yet  they 
also  exert  a  directly  stimulating  effect  on  the  secretory  cells. 

The  function  of  the  cells  of  the  tubules  does  not  end  with 
the  removal  from  the  blood  of  the  substances  presented  to 

19—2 


292     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

them  ;  they  are  also  capable  of  originating  material  on  their 
own  account.  Thus  the  union  of  glycine  with  benzoic  acid, 
resulting  in  the  formation  of  hippuric  acid,  takes  place  in 
the  cells  of  the  tubules,  and  observations  have  shown  that 
providing  the  benzoic  acid  be  presented  to  it,  the  kidney  is 
capable  of  providing  the  needful  glycine.  It  can  hardly  be 
doubted  that  what  is  true  of  glycine  and  benzoic  acid  may 
also  be  true  of  other  substances,  and  that  transformations 
may  occur  in  the  cells  leading  to  the  production  of  colour- 
ing matters,  etc.,  our  knowledge  of  which  is  at  present 
obscure. 

The  Composition  of  the  Urine  depends  upon  the  class  of 
animal ;  in  all  herbivora,  with  certain  minor  differences, 
the  urinary  secretion  is  much  the  same :  not  so  with 
omnivora  or  carnivora  which  possess  a  distinctive  urine, 
especially  the  latter.  When  herbivora  live  on  their  own 
tissues,  as  during  starvation,  they  become  carnivora  and 
their  urine  alters  completely  in  character,  corresponding 
now  to  the  urine  of  flesh  feeders ;  the  young  of  herbivora, 
if  still  sucking,  have  a  urine  possessing  much  the  same 
characteristics  as  that  of  carnivora.  But  apart  from  this 
general  statement,  it  is  necessary  to  point  out  that  in 
animals  of  the  same  class  the  composition  of  the  urine  may 
vary  within  very  wide  limits,  depending  upon  several 
causes,  of  which  diet  is,  perhaps,  the  most  important. 

Urine  consists  of : 

Water. 

/Nitrogenous  end-products:  urea,  uric  acid,  hippuric 
acid,  creatine,  creatinine. 
Organic  matter  -^  Aromatic  compounds  :  benzoic  acid,  ethereal  sulphates 
of  phenol,  cresol,  etc. 
I  Colouring  matter  and  mucus, 
g  1,      _  _  /Sulphates,    phosphates,    and    chlorides    of     sodium, 

I     potassium,  calcium,  and  magnesiuiu. 

The  Nitrogenous  Substances  taken  up  into  the  blood,  either 
from  the  disintegration  of  proteids  in  the  digestive  canal 
or  from  the  metabolism  of  the  tissues,  supply  the  total 
nitrogen  of  the  urine.     A  distinction  is  made  between  the 


THE  UEINE  293 

nitrogen  from  without,  viz.,  that  supplied  by  the  food, 
and  the  nitrogen  from  within,  viz.,  that  from  the  tissues, 
and  this  is  more  especially  of  interest  in  connection  with 
urea  and  uric  acid. 

The  total  nitrogen  of  the  urine  consists  of : 

1.  Urea  nitrogen. 

2.  Uric  acid  nitrogen. 

3.  Ammonia  nitrogen. 

4.  Creatinine  nitrogen. 

Speaking  generally,  the  nitrogen  varies  directly  with  the 
amount  of  proteid  taken  as  food. 

Urea. — It  is  by  no  means  decided  how  urea  is  produced. 
It  must  presumably  arise  from  the  disintegration  of  pro- 
teids,  derived  either  from  proteid  food  or  proteid  tissues. 
As  the  result  of  their  destruction  it  is  extremely  probable 
that  ammonia  compounds  are  formed  which  are  discharged 
into  the  blood,  and  are  then  subsequently  converted  into 
urea  in  some  organ,  which  is  probably  the  liver.  Some 
suppose  that  the  proteids  undergo  hydrolytic  cleavage  with 
the  formation  of  amido-bodies,  such  as  leucine,  tyrosine, 
aspartic  acid,  glycocoll,  etc.,  and  that  these  bodies  undergo 
oxidation  in  the  tissues  yielding  ammonia,  carbonic  acid, 
and  water.  The  ammonia  and  carbonic  acid  unite  to 
form  ammonium  carbamate,  which  is  carried  to  the 
liver,  and  by  the  loss  of  a  molecule  of  water  is  readily 
converted  into  urea.  The  oxidation  of  the  amido-bodies 
is  essential  as  a  preliminary  step  towards  urea,  in  order 
to  get  rid  of  some  of  the  carbon  they  contain  ;  in  amido- 
bodies  this  is  in  excess  of  the  nitrogen,  whereas  in  urea 
the  reverse  is  the  case.  It  seems  fairly  clear  that  the 
nitrogenous  waste  leaves  the  muscles  as  ammonia  com- 
pounds, and  in  this  form  the  nitrogen  of  the  proteid  food 
may  be  found  in  the  portal  vein,  the  blood  of  which  contains 
three  to  four  times  as  much  ammonia  compounds  as  does 
arterial  blood.  If  the  blood  of  the  portal  vein  be  experi- 
mentally compelled  to  pass  into  the  posterior  vena  cava 
without  circulating  through  the  liver,  the  ammonia  com- 


294     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


pounds  in  the  arterial  blood  become  equal  in  amount  to 
those  in  the  portal  blood.  The  ammonia  in  the  blood  is 
considered  by  some  to  be  in  the  form  of  carbonate  ;  by 
others,  and  perhaps  more  generally,  as  carbamate,  though 
ammonium  carbamate  is  readily  produced  from  the  car- 
bonate by  the  loss  of  one  molecule  of  water.  The  sub- 
sequent disposal  of  the  ammonia  compounds  is  evidently  by 
means  of  the  liver,  this  gland  standing  between  the  portal 
and  systemic  circulation,  and  converting  the  poisonous 
ammonia  compounds  into  the  less  poisonous  urea. 

In  the  urine  the  urea  exists  in  a  free  and  uncombined 


Fig.  67. — Crystals  of  Nitratp:  of  Urea  (Funke). 

state,  though  it   is   capable   of   forming  salts   with   acids 
(Fig.  67).     It  is  a  substance  very  soluble  in  water. 

The  proportion  of  urea  in  urine  varies  dependently  on 
the  nature  of  the  diet.  As  a  rule  the  larger  the  amount  of 
nitrogen  in  the  food  the  more  urea  excreted,  but  this  is  not 
invariable,  for  some  observers  have  stated  that  on  a  diet 
consisting  principally  of  hay  more  urea  is  excreted  than  on 
one  of  oats  and  hay.  Urea  was  at  one  time  considered  to 
be  a  measure  of  the  amount  of  work  performed  by  the 
animal  body,  but  this  view  has  long  been  known  to  be 
wrong,  though  there  can  be  no  doubt  that  under  the 
influence  of  work  rather  more  urea  may  be  excreted  than 
during  rest. 


THE  UEINE  295 

Judging  from  our  observations  on  the  horse,  great  varia- 
tion in  the  amount  of  urea  ma}^  be  met  with  even  when  the 
conditions  as  regards  diet,  rest  and  work  are  identicaL  It 
is  probable  that  this  applies  also  to  other  animals.  The 
percentage  of  urea  present  in  urine  may  broadly  be  stated 
to  vary  between  3  and  4  per  cent.,  but  it  is  obvious  that 
the  percentage  present  is  influenced  by  the  total  secretion 
for  the  twenty-four  hours.  If  this  is  small  in  amount,  the 
percentage  is  higher  than  when  an  average  production  of 
water  occurs. 

Creatinine  has  been  regarded  as  another  source  of  urea, 
but  the  physiological  history  of  this  substance  is  imperfectly 
known.  In  flesh-feeding  animals  part  of  it,  no  doubt,  is 
derived  from  the  food,  while  another  portion  is  produced 
within  the  body,  probably  originating  from  the  metabolism 
of  muscle.  Though  the  conversion  of  creatinine  into  urea 
may  be  brought  about  as  a  laboratory  process,  there  is  no 
definite  proof  that  the  conversion  occurs  in  the  body  ; 
if  creatine  be  injected  into  the  blood  it  does  not  lead  to  an 
increase  of  urea  but  of  creatinine.  It  is  possible  that  under 
physiological  conditions  creatine  before  it  leaves  the  muscles 
may  undergo  a  further  change,  being  decomposed  into  urea 
and  sarcosine,  the  latter  passing  to  the  liver  and  there 
being  converted  into  ammonium  carbonate  and  subsequently 
into  urea. 

Uric  Acid. — The  origin  of  uric  acid  is  not  clearly  deter- 
mined in  mammals.  In  birds  it  is  known  to  be  formed  in 
the  liver  from  ammonia  compounds,  and  probably  from 
lactic  acid.  In  mammalia  it  is  known  that  in  herbivora 
the  amount  of  uric  acid  is  extremely  small,  or  this  substance 
may  be  even  entirely  absent ;  in  carnivora  and  omnivora  it 
is  present,  though  only  in  a  small  proportion  of  the  total 
nitrogen  excreted.  The  influence  of  diet  in  flesh  feeders  is 
very  marked,  meat  causing  a  rise  in  the  uric  acid  output, 
while  cellular  organs,  such  as  liver  and  sweetbreads, 
produce  a  still  greater  rise ;  this  fact  has  afforded  a  clue 
to  the  probable  origin  of  uric  acid  in  the  body,  viz.,  from 
the  nucleo-albumins  and  nucleins,  both  of  which  largely 


296     A  MANUAL  OF  YETERINAEY  PHYSIOLOGY 

exist  in  the  cellular  organs.  Pathologically  there  is  an 
increase  in  uric  acid  in  the  disease  known  as  leuco- 
cythaemia,  in  which  a  great  increase  in  the  white  blood- 
corpuscles  occurs.  These  corpuscles  contain  a  quantity  of 
nuclein,  and  the  work  of  Emil  Fischer  has  shown  the  close 
chemical  relationship  between  the  nitrogenous  bases  so 
easily  obtained  by  the  decomposition  of  nuclein — the  purin* 
bases— and  uric  acid.  The  purin  bases  are  hypoxanthine, 
xanthine,  and  adenine,  and  from  these  uric  acid  may  arise 
by  the  process  of  oxidation.  For  example,  one  atom  of 
oxygen  allied  to  purin  (C-H^XJ  gives  rise  to  hypoxanthine 
(C^H^X^O),  two  atoms  of  oxygen  to  xanthine  (C^HjN^Oo),  and 
three  atoms  of  oxygen  added  to  purin  lands  us  finally  in 
uric  acid  (QH^N^Og). 

If  hypoxanthine  or  uric  acid  be  given  to  dogs  the  uric  acid 
is  not  increased ;  if  adenine  be  given  the  animal  dies  from 
suppression  of  urine  and  crystals  of  uric  acid  block  the 
renal  tubules.  It  is  evident  that  very  little  is  known  of  the 
subject  of  uric  acid  formation.  Even  the  seat  of  production 
is  not  definitely  ascertained,  though  the  liver  will  probably 
be  found  to  play  no  small  part  in  the  process,  as  it  is 
already  known  to  do  in  the  case  of  birds.  The  spleen  has 
been  pointed  to  as  a  probable  seat,  though  possibly  it  is 
not  relatively  more  so  than  other  lymphoid  tissues. 

As  previously  stated,  the  production  of  uric  acid  is 
affected  by  diet,  being  largest  on  animal  food  and  smallest 
on  vegetable.  The  acid  is  therefore  present  in  the  dog  fed 
on  meat  and  in  the  pig,  but  entirely  absent,  so  far  as  our 
observations  go,  in  the  horse  in  health,  and  probably  in  all 
herbivora  unless  still  suckling  at  the  mother.  It  is  important 
to  note  that  during  sickness,  especially  when  there  is  fever 
and  the  animal  is  living  on  its  own  tissues,  uric  acid  may  be 
readily  found  in  the  urine  of  herbivora.  The  explanation 
is  simple  :  the  animal  for  the  time  being  is  practically 
carnivorous. 

*  Purin  is  the  name  given  by  Fischer  to  the  nucleus  common  to  the 
uric  acid  group  of  substances  from  which,  by  simple  transformations, 
the  several  members  of  the  group  may  easily  be  obtained. 


THE  UEINE 


297 


Uric  acid  does  not  occur  free  in  the  urine,  but  in  com- 
bination with  soda  and  potash.  Its  crystalHne  formation  is 
shown  in  Fig.  68  ;  it  is  a  substance  very  insoluble  in  water, 
but  soluble  in  alkaline  solutions. 

The  ammonia  salts  present  in  urine  are  an  index  to  the 
neutralization  of  acids  in  the  body.  The  acid  substances 
are  produced  as  the  result  of  metabolism  ;  when  they  are 
in  excess  there  is  an  increase  in  the  ammonia  of  the  urine, 
the  formation  of  ammonia  in  the  muscles  being  the  natural 
protection  of  the  body  against  acid  poisoning.  When,  as 
occurs  in  herbivora,  there  is  already  an  excess  of  alkali  in 


Fig.  68. — Crystals  of  Uric  Acid  (Funke). 

the  diet,  a  sufficiency  of  bases  is  present  to  neutralize  the 
acid,  and  ammonia  is  absent  from  the  urine.  With  flesh 
feeders  the  amount  of  ammonia  is  kept  at  a  minimum 
owing  to  its  poisonous  nature ;  on  a  vegetable  diet  it  all 
disappears  from  the  urine,  being  converted  into  urea. 

The  injection  of  dilute  mineral  acid  into  the  veins  of  a 
dog  does  not  alter  the  reaction  of  the  blood,  but  the 
ammonia  is  increased  as  a  natural  protection  and  appears 
in  the  urine,  with  a  resulting  decrease  in  the  urea.  A 
similar  injection  of  dilute  mineral  acid  in  herbivora  reduces 
the  alkalinity  of  the  blood,  after  having  used  up  the  store 
of  vegetable  alkaline  salts.  In  consequence  of  the  reduced 
alkalinity   the   carrying   power   of    the    blood   for   carbon 


298     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

dioxide  is  reduced ;  it  is  retained  in  the  tissues,  and  gives 
rise  to  symptoms  which  may  prove  fatal.  If  ammonium 
carbonate  be  given  by  the  mouth,  it  does  not  appear  as 
such  in  the  urine,  but  as  urea. 

Hippuric  Acid. — This  acid,  characteristic  of  the  urine  of 
the  herbivora,  may  arise  in  two  or  three  different  ways.  It 
is  known  that  hay,  grass,  and  grains,  contain  in  their 
cuticular  covering  a  substance  which  yields  hippuric  acid 
in  the  body  ;  if  these  foods  be  extracted  with  caustic  potash 
the  hippuric-acid-forming  substance  is  removed,  and  if 
animals  are  fed  on  forage  so  treated  no  hippuric  acid  is 
formed  in  the  body ;  even,  it  has  been  said,  if  the  husk  be 
removed  from  grain  the  latter  is  incapable  of  giving  rise  to 
hippuric  acid. 

The  chief  source  of  hippuric  acid  in  the  herbivora  is  from 
the  above  hippuric-acid-yielding  body.  Benzoic  acid  is 
derived  from  various  aromatic  combinations  contained  in 
plants,  and  this  combined  with  glycocoU,  derived  from  the 
decomposition  of  proteid  substances,  yields  hippuric  acid. 
The  synthesis  occurs  in  the  kidney,  and  is  brought  about 
by  the  cells  of  the  gland  in  conjunction  with  the  oxygen  of 
the  red  corpuscles.  Outside  the  body  the  synthesis  may  be 
produced  by  using  ground-up  kidney  tissue  mixed  with 
blood,  and  kept  at  the  body  temperature.  It  is  probable 
that  the  active  agent  in  the  synthesis  is  an  enzyme.  A 
second  source  of  hippuric  acid  is  the  aromatic  (benzoic) 
products  formed  in  the  intestinal  canal  as  the  result  of  the 
putrefaction  of  proteids  ;  lastly,  it  is  believed  that  hippuric 
acid  may  be  formed  from  the  aromatic  residues  of  tissue 
proteids. 

Hippuric  acid  never  exists  in  the  free  state  in  the  urine, 
but  either  as  hippurate  of  lime  or  potash,  probably  the 
former.  Crystals  of  hippuric  acid  are  shown  in  Figs.  69 
and  70.  The  amount  of  hippuric  acid  excreted  varies  with 
the  diet ;  it  is  increased  by  using  meadow-hay  and  oat-straw, 
and  decreased  by  using  clover,  peas,  wheat,  oats,  etc. ;  as 
the  urea  rises  the  hippuric  acid  falls. 

Liebig  many  years  ago    stated   that   benzoic   acid   was 


THE  UEINE 


299 


found  in  the  urine  of  working  horses,  and  hippuric  acid  in 
the  urine  of  those  at  rest.  Our  observations  show  that 
hippuric  acid  is  generally  found  in  the  urine  of  working 
horses,  and  seldom  found  in  the  urine  of  horses  at  rest — in 
fact,  the  reverse  of  Liebig's  view.     Owing  to  its  easy  and 


Fig.  69. — Crystals  of  Purified  Hippuric  Acid  (Funke). 


Fig.  70. — Crystals  of  Impure  Hippuric  Acid. 


rapid  fermentative  decomposition  hippuric  acid  is  rarely  to 
be  found  in  urine  twenty-four  hours  old ;  in  fifty-four 
specimens  we  only  found  it  eight  times.  This  decomposi- 
tion may  be  prevented  by  the  addition  of  a  slight  excess 
of  milk  of  lime,  and  then  boiling  the  freshly  voided  urine. 


300    A  MANUAL  OF  VETEEINAEY  PHYSIOLOGY 

Benzoic  Acid  is  the  antecedent  of  hippuric.  As  just 
mentioned,  it  is  derived  from  the  benzoic -acid -forming 
substances  in  vegetable  food  ;  its  crystalline  formation  is 
show  in  Fig.  71. 

Sulphuric  Acid  in  the  urine  of  carnivora  and  omnivora  is 
almost  wholly  derived  from  the  decomposition  of  proteid 
bodies  undergoing  digestion,  and  its  amount  is  employed  as 
a  measure  of  proteid  disintegration  in  the  system.  The 
sulphur  is  derived  from  the  sulphur  of  the  proteid  body,  and 
the  acid  is  united  with  indol,  phenol,  skatol,  cresol,  all  of 


Fig.  71. — Crystals  of  Benzoic  Acid. 


which  are  products  of  proteid  disintegration  in  the  intes- 
tinal canal.  In  herbivora  indol,  phenol,  and  skatol  may  be 
derived  from  the  benzene  compounds  in  food,  so  that  the 
amount  excreted  is  no  measure  of  proteid  disintegration. 
Phenol,  skatol,  and  cresol  are  poisonous  bodies  ;  part  of 
them  are  got  rid  of  by  the  faeces,  part  are  absorbed  into  the 
blood,  and  after  oxidation  are  conjugated  with  sulphuric 
acid  and  eliminated  by  the  urine.  By  this  conjugation  the 
poisonous  aromatic  compounds  are  rendered  harmless. 

From  the  conjugation  between  indol  and  sulphuric  acid 
indican  is  produced,  which  may  be  made  to  yield  indigo,  a 
substance  common  in  the  urine  of  herbivora.  From  the 
conjugation  between  phenol  and  sulphuric  acid  a  colouring 


THE  UEINE  301 

matter  is  formed  which  is  found  in  stale  urine ;  phenol- 
sulphuric  acid  undergoes  oxidation  in  the  presence  of  the 
air,  and  yields  pyrocatcchin,  to  which  the  brown  colour  in 
the  stale  urine  of  the  horse  is  due. 

Some  of  the  indol  and  skatol  may  be  united  with  gly- 
curonic  acid,  a  substance  co-related  to  dextrose,  and  often 
present  in  the  urine  of  the  dog.  It  exerts  a  reducing  action 
on  salts  of  copper. 

Oxalic  Acid  in  combination  with  lime  is  constantly  found 
in  the  urine  of  herbivora  ;  its  deposit  presents  a  character- 
istic microscopical  appearance  (Fig.  72).  In  dogs  it  has 
been  produced  in  considerable  quantity  by  feeding  on  uric 
acid ;  its  origin  in  the  herbivora  is  doubtless  from  the 
oxalates  contained  in  the  food. 

The  Colouring  Matter  of  the  urine  is  not  yet  completely 
worked  out.  The  chief  substance  is  urochrome ;  this  is 
probably  an  oxidation  product  of  urobilin,  as  on  suitable 
treatment  a  pigment  is  obtained  which  gives  a  spectrum 
identical  with  urobilin.  Urobilin  is  not  found  in  normal 
urine,  but  there  is  present  a  chromogen,  or  another 
substance,  which  yields  urobilin.  The  origin  of  urobilin 
is  from  bile  pigment ;  the  stercobilin  formed  in  the  in- 
testines is  identical  with  it. 

The  Inorganic  Substances  found  in  the  urine  are  calcium, 
magnesium,  sodium,  and  potassium,  existing  in  the  form 
of  chlorides,  sulphates,  phosphates,  and  carbonates.  The 
origin  of  these  salts  is  from  the  food  taken  into  the  body, 
but  mainly  from  metabolic  processes  occurring  in  the 
tissues.  The  nature  and  amount  of  the  salts  vary  with 
the  class  of  animal  and  the  character  of  the  food.  In  the 
urine  of  the  horse  potassium  salts  predominate,  sodium 
and  magnesium  are  present  in  small  amounts,  phosphates 
are  practically  absent,  while  sulphates  and  chlorides  are 
in  considerable  quantity.  It  has  been  found  that  in 
ruminants  the  calcium  salts  are  mostly  excreted  with  the 
fseces,  whereas  in  the  horse  they  principally  pass  through 
the  kidneys.  It  is  certain  that  phosphates,  which  form 
such  a  prominent  feature  in  the  urine  of  carnivora  and 


302     A  MANUAL  OF  VETEPJNAEY  PHYSIOLOGY 

omnivora,  are  in  the  horse  ahnost  wholly  excreted  by  the 
intestines. 

Calcium. — More  lime  exists  in  the  urine  of  the  horse  than 
is  soluble  in  an  alkaline  fluid,  so  that  both  suspended  and 
dissolved  lime  exists ;  the  former  increases  with  the  age  of 


Fig.  72. — Crystals  of  Oxalate  of  Lime  (Funke) 


Fig.  73. — Crystals  of  Carbonate  of  Lime  (Funke). 


the  urine,  owing  to  the  development  of  ammonia,  until 
nearly  the  whole  of  the  lime  is  precipitated.  The  lime 
exists  in  combination  with  oxalic,  carbonic,  hippuric,  and 
sulphuric  acids ;  all  these  combinations  do  not  necessarily 
exist  in  one  specimen  of  urine,  the  salts  formed  depending 


THE  UEINE  303 

on  the  varying  relative  amounts  of  the  acids  formed  in 
metabolism.  The  amount  of  lime  in  the  food  does  not 
influence  the  elimination  through  the  kidneys,  but  more 
lime  is  found  in  the  urine  of  horses  at  work  than  of 
those  at  rest.  Oxalate  and  carbonate  of  lime  crystals  are 
common  microscopic  deposits  in  the  urine  of  the  horse 
(Figs.  72  and  73). 

Under  any  condition  the  urine  of  a  healthy  horse  is 
turbid  from  suspended  lime  ;  this  may  be  got  rid  of  on 
the  addition  of  acid  with  profuse  evolution  of  gas,  while  a 
clear  transparent  urine  results. 

Magnesium  in  the  urine  is  also  suspended  and  dissolved, 
the  amount  which  is  suspended  being  increased  by  the 
ammonia  generated  in  the  urine  on  standing. 

Potassium  exists  largely  in  the  urine  of  herbivora,  derived 
from  the  potash  of  the  food ;'  it  forms  numerous  combina- 
tions, the  one  with  carbonic  acid  being  the  cause  of  the 
fixed  alkalinity  of  the  urine  in  the  horse.  There  is  more 
potash  found  in  the  urine  of  horses  at  rest  than  of  those  at 
work,  which  is  explained  by  the  considerable  amount  of 
potassium  excreted  v/ith  the  sweat.  Sodium  only  exists  in 
the  urine  of  herbivora  in  small  quantities,  which  is  due  to 
the  fact  that  very  little  sodium  is  found  in  vegetable  food. 

Sulphuric  Acid  in  its  organic  combinations  has  been  dealt 
with  previously ;  the  inorganic  sulphur  is  combined  with 
alkalis  as  ordinary  salts. 

Chlorine  is  supplied  by  the  chlorides  of  the  food.  The 
proportion  of  chlorides  in  the  food  of  herbivora  is  not 
very  high  ;  the  amount  excreted  by  horses,  combined  with 
sodium,  was  found  by  us  to  equal  a  daily  excretion  of 
85j  grains  of  common  salt.  Salkowski  places  it  much 
higher,  viz.,  about  ^  oz.  daily. 

Phosphoric  Acid,  though  existing  largely  in  food  such  as 
oats,  passes  off  almost  wholly  by  the  alimentary  canal  ; 
sometimes  only  traces  are  to  be  found  in  the  urine  of 
herbivora,  at  others  the  amount  is  marked,  but  never  con- 
siderable. Work  does  not  influence  its  production.  In  the 
urine  of  carnivora  the  phosphates  are  an  important  con- 


304     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

stituent.  They  exist  in  the  urine  in  two  forms,  viz.,  alka- 
line phosphates,  such  as  phosphate  of  sodium  or  potassium, 
and  earthy  phosphates,  such  as  phosphates  of  calcium  and 
magnesium ;  these  triple  phosphates  are  common  as  a 
microscopical  object  in  the  decomposing  urine  of  the  horse, 
though  trifling  in  actual  amount  (Fig.  74).  The  phosphates 
are  derived  from  the  food  and  tissues.  According  to  Munk, 
if  there  is  an  abundance  of  lime  salts  in  the  diet,  as  in 
vegetable  food,  the  phosphates  are  not  eliminated  to  any 
extent  by  the  kidneys,  for  the  reason  that  they  combine  in 


Fig.  74. — Crystals  of  TpaPLE  Phosphate  (Funke). 


the  intestinal  canal  with  lime  and  magnesia  and  pass  off 
by  that  channel ;  if,  on  the  other  hand,  there  is  but  little 
lime  and  magnesia  in  the  intestines,  the  phosphates  are 
united  to  soda  and  potash,  pass  into  the  blood,  and  are 
eliminated  by  the  urine. 

Ammonia. — Free  ammonia  exists  in  the  urine  of  the 
horse.  It  may  be  owing  to  ammoniacal  fermentation  in 
the  bladder,  but  it  is  quite  certain  that  perfectly  fresh 
urine  may  fgive  marked  evidence  of  the  presence  of  free 
ammonia.  On  standing  a  short  time  outside  the  body, 
especially  in  summer  weather,  the  urea  decomposes  and 
carbonate  of  ammonium  is  largely  formed. 

The  Reaction  of  the  urine  of  herbivora  is  alkaline,  the 


THE  UKINE  305 

alkalinity  being  due  to  carbonate  of  potash.  The  urine  of 
all  vegetable  feeders  is  alkaline,  owing  to  the  excess  of 
alkaline  salts  of  organic  acids  contained  in  the  food,  such 
as  malic,  citric,  tartaric  and  succinic.  During  their  passage 
through  the  body  these  salts  are  converted  into  carbonates, 
and  appear  as  such  in  the  urine,  where  they  produce  con- 
siderable effervescence  on  the  addition  of  an  acid.  The 
nature  of  the  food  influences  the  reaction,  for  if  hay  be 
withheld  from  the  diet,  the  urine  of  the  horse  may  be 
rendered  acid  by  feeding  entirely  on  oats ;  this  is  probably 
due  to  the  formation  of  acid  phosphates  from  the  food.  A 
considerable  quantity  of  the  alkalinity  present  in  the  stale 
urine  of  the  horse  is  due  to  the  exceedingly  rapid  fermenta- 
tive change  which  occurs  in  it  on  standing,  leading  to 
the  breaking  up  of  part  of  the  urea  and  the  formation  of 
ammonium  carbonate. 

In  the  dog  the  urine  is  acid,  due  to  the  acid  phosphate  of 
soda,  and  not  to  any  free  acid ;  no  free  acids  exist  in  the 
urine  of  any  animal.  In  the  pig  the  reaction  is  either  acid 
or  alkaline,  depending  on  the  diet :  an  animal  diet  producing 
an  acid  and  a  vegetable  diet  an  alkaline  urine. 

Urine  of  tlie  Horse. 

Specific  Gravity. — This  varies  considerably  dependently 
on  the  diet  and  the  amount  of  dilution.  The  mean  of  a 
large  number  of  observations  was  1036,  the  highest  regis- 
tered was  1050  and  the  lowest  1014. 

The  Quantity  oi  urine  is  liable  to  very  considerable  varia- 
tion depending  on  the  season  and  the  diet ;  the  more 
nitrogen  the  food  contains  the  larger  the  amount  of  water 
consumed  and  the  greater  the  bulk  of  urine  excreted.  The 
mean  of  a  large  number  of  observations  was  8^  pints 
(4'8  litres)  in  24  hours,  the  diet  being  moderately  nitrogenous, 
but  in  individual  instances  very  much  more  than  this  may 
be  met  with,  viz.,  12,  15,  or  even  20  pints  (11*3  litres). 

Horses  at  work  excrete  less  urine  than  those  at  rest, 
probably  owing  to  the  loss  by  the  skin.     In  winter,  owing 

20 


306     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

to  the  reduced  action  of  the  skin,  more  urine  is  excreted  than 
during  summer. 

The  Odour  of  urine  is  said  to  be  due  to  certain  aromatic 
substances  of  the  phenol  group.  Perfectly  fresh  urine  has 
commonly  a  most  distinct  though  faint  smell  of  ammonia. 
This  may  be  due  to  fermentative  changes  occurring  in  the 
urea  before  the  urine  is  evacuated. 

The  normal  fluid  is  always  turbid,  some  si^ecimens  more 
so  than  others ;  very  rarely  is  it  clear,  and  then  only  for  a 
short  time.  The  turbidity  is  due  to  the  amount  of  sus- 
pended carbonate  of  lime  and  magnesia  it  contains ;  as 
the  urine  cools,  particularly  if  it  undergoes  ammoniacal  fer- 
mentation, the  amount  of  turbidity  becomes  intense. 

The  Consistence  of  the  fluid  depends  upon  sex,  and  per- 
haps on  the  season.  It  is  certain  that  some  mares  excrete 
a  glairy  tenacious  fluid  which  owing  to  the  amount  of 
mucin  it  contains  can  be  drawn  out  in  strings ;  it  is  very 
common  to  find  it  as  thick  as  linseed-oil,  and  very  rare  to 
find  it  fluid  and  watery.  During  cfstrum  the  urine  is  of 
the  consistence  of  oil.  On  a  diet  of  oats  and  no  hay,  we 
have  seen  the  urine  so  mucinous  as  to  pour  like  white  of 

egg. 

The  Colour  of  urine  is  yellow  or  yellowish-red,  rapidly 
turning  to  brown,  the  dark  tint  commencing  on  the  surface 
of  the  fluid  and  gradually  travelling  into  its  depth.  The 
cause  of  the  colour  on  standing  is  due  to  the  oxidation  of 
pyro-catechin  (see  p.  301). 

The  Total  Solids  consist  of  organic  and  inorganic  matter, 
of  which  on  a  mixed  diet  5  ozs.  are  organic  and  3  ozs. 
inorganic  ;  the  quantities  are  liable  to  great  variation, 
sometimes  being  found  greatly  in  excess  of  that  mentioned. 
The  total  solids  are  considerably  afi"ected  by  the  diet ; 
E.  Wolff*  found  that  when  he  reduced  the  hay  and  in- 
creased the  corn  ration  the  solids  in  the  urine  decreased, 
whereas  on  a  diet  consisting  principally  of  hay  and  but 
little  corn  the  solids  increased. 

The  composition  of  the  mineral  solids  is  given  in  the 
*  Ellenberger. 


THE 

UEINE 

following  table  by  Wolff. 

In  every  100  parts  of 

are  found : 

Potassium  - 

- 

-     36*85  per  cent. 

Sodium 

- 

-       3-71       „ 

Calcium 

- 

-     21-92 

Magnesium 

- 

-       4-41       „ 

Phosphoric  acid 

- 

-       — 

Sulphuric      ,, 

- 

-     17-16       „ 

Chlorine     - 

- 

-     15-36       „ 

Silicic  acid 

. 

•32       „ 

307 


In  the  following  table  are  given  the  results  obtained  by 
us  in  the  examination  of  the  twenty-four  hours'  urine  of 
horses  at  rest  and  work  :* 


Best. 

WorJc. 

Quantity 

- 

8-69  pints 

7-88  pints 

Specitic  gravity  - 

- 

1036 

1036 

Total  solids 

8-11  ozs. 

8-19  ozs. 

Organic  solids 

5-15     „ 

5-37     „ 

Inorganic  solids  - 

2-94     „ 

2-82     „ 

Urea 

- 

3-47  oz 

Ammonium  carbonate  as  urea   - 

•46  „ 

Ammonia 

- 

•09     „ 

-19     „ 

Benzoic  acid 

•23     „ 

Hippuric  acid     - 

- 

-55     „ 

Phosphoric  anhydride     - 

- 

•04     „ 

-06     „ 

Sulphuric           ,, 

- 

•37     „ 

-54     „ 

Other  sulphur  compounds 

- 

•26     „ 

•27     „ 

Chlorine  - 

- 

1-12     „ 

•77     „ 

Calcium  oxide     - 

- 

•12     „ 

-06     „ 

Magnesium  oxide 

-10     „ 

-09     „ 

Potassium       ,, 

- 

1-29     „ 

-95     „ 

Sodium            ,, 

- 

•09     „ 

-06     „ 

Salkowskit  examined  the  urine  of  the  horse,  and  gives 

the  following  as  the  co 

mposition  of  one 

specimen : 

Water          -             -     3'5 

pints 

Phenol 

- 

-     37-89  grains 

Organic  solids         -     6-25 

ozs. 

Organic  sulphur 

-  208-69      „ 

Ash              -             -     1-60 

,, 

Inorganic 

)i 

-     85-77      „ 

Urea            -             -     3-25 

)) 

Phosphoric  acid 

-       3-40      „ 

Ammonia    -            -     5-53 

grains 

Lime 

- 

-     88-50      „ 

Hippuric  acid          -       -49 

oz. 

Sodium  c 

iloride 

-87  oz. 

*  '  Chemistry  of  the  Urine  of  th 

s  Horse,'  Proceedings  of  the  Boyal 

Society,  vol.  xlvi.,  1889. 

f  Ellenberger's  '  Physiologie.' 

20—2 

308    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

In  the  following  summary  of  the  urine  of  animals  other 
than  the  horse,  the  main  facts  are  those  given  by  Tereg.* 

The  Urine  of  the  Ox. 

The  urine  of  the  ox  is  much  the  same  as  that  of  the 
horse,  excepting  that  it  is  secreted  m  larger  quantities,  10 
to  40  pints ;  the  difference  mainly  depends  upon  the 
amount  of  nitrogenous  matter  in  the  diet,  for  it  has  been 
shown  that  the  more  nitrogen  a  diet  contains  the  larger 
the  amount  of  water  consumed.  The  fluid  is  clear,  yellowish, 
and  of  an  aromatic  odour ;  it  is  of  a  lower  specific  gravity 
than  that  of  the  horse,  1007  to  1030  (in  milch  cows, 
according  to  Munk,  1006  to  1015),  owing  to  the  larger 
amount  of  water  secreted. 

The  nitrogenous  matter  found  in  the  urine  is  mainly 
represented  by  urea  and  hippuric  acid,  and  the  amount 
varies  according  to  the  diet.  On  a  diet  of  wheat  straw, 
clover  hay,  beans,  starch,  and  oil,  the  amount  of  urea  may 
be  4  per  cent. ;  while  on  one  of  oat  straw  and  beans  it  may 
fall  to  less  than  1  per  cent.  When  the  urea  is  high,  the 
hippuric  acid  is  low,  and  vice  versa.  The  largest  amount 
of  hippuric  acid  is  produced  by  feeding  on  the  straw  of 
cereals,  the  smallest  is  furnished  by  feeding  on  leguminous 
straw,  whilst  a  medium  amount  is  produced  by  feeding  on 
hay. 

The  urine  of  ruminants  contains  less  aromatic  sulphur 
compounds  than  that  of  the  horse,  and  more  of  the  in- 
organic sulphur ;  like  the  horse,  the  phosphates  are  either 
absent  or  only  occur  in  small  amounts. 

The  following  table  by  Tereg  shows  the  composition  of 
the  urine  of  the  ox  on  different  diets ;  the  observations 
extended  over  four  months  : 

lbs.  lbs.  lbs.  lbs. 

Total  (luantity  of  urine               -     26-02  31-17  29-98  18-32 

„               „          dry  matter    -       1-71  1-51  1-40  1-14 

„               „          ash    -             -         -88  1-01  1-03  -66 

*  EUenberger's  '  Physiologie.' 


THE  UEINE  309 

Calves  still  suckling  excrete  an  acid  urine  which  is  rich 
in  phosphates,  uric  acid,  creatinine,  and  a  peculiar  sub- 
stance known  as  allantoin ;  it  is  poor  in  urea,  and, 
according  to  Moeller,  contains  hardly  1  per  cent,  of  solids. 

The  Urine  of  the  Sheep. 

This  has  an  alkaline  reaction,  a  specific  gravity  1006  to 
1015,  and  the  amount  excreted  varies  from  '5  pint  to  1*5 
pints.  Tereg  gives  the  following  percentage  composition  of 
a  sample  : 

Water  .  .  -  .     86-48 

Organic  matter  _  -  -       7*96 

Inorganic  matter       -  .  -       5-56 

The  organic  matter  contained  :  The  inorganic  matter  contained  : 


Urea 

2-21 

Chlorine  - 

-     1^05 

Hippuric  acid 

3-24 

Potassium  chloride 

-     1-84 

Aanmonia  - 

•02 

Potassium 

-     2-08 

Other  organic  substances  - 

2-07 

Lime 

-       -07 

Carbonic  acid 

•42 

Magnesia 

-       -20 

Phosphoric  acid  - 

-      -01 

7*96 

Sulphuric       ,, 

-       -24 

Silica 

-      ^07 

In  sheep  urea  and  hippuric  acid  stand  in  the  proportion 
of  2  to  3,  whereas  in  cattle  on  the  same  diet  the  proportion 
is  2  of  urea  to  I'l  of  hippuric  acid. 

The  food  most  productive  of  hippuric  acid  in  the  horse 
is  old  meadow  hay,  whilst  neiv  meadow  hay  has  this  effect 
on  sheep.  It  will  be  observed  from  the  table  how  rich  the 
urine  of  the  sheep  is  in  hippuric  acid. 

In  sheep  there  is  very  much  more  magnesia  than  lime  in 
the  urine,  consequently  the  reverse  obtains  in  the  faeces  of 
this  animal. 

The  Urine  of  the  Pig. 

This  resembles  that  of  carnivora,  but  its  composition 
depends  on  the  character  of  the  food.  The  specific 
gravity  is  1003  to  102.5.     It  is  either  acid  or  alkaline;  the 


310    A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

amount  excreted  varies  between  2^  to  14  pints,  and  it 
contains  uric  acid,  hippuric  acid,  xanthine,  guanine,  and 
much  urea. 

In  the  following  analysis  of  the  urine  the  diet  consisted 
of  peas,  potatoes,  and  sour  milk : 


Total  urine  - 

- 

-    7  pints 

Sp.  gr. 

- 

-     1018 

Dry  substance 

- 

-     2-768  per  cent. 

Total  nitrogen 

- 

-       -604 

Ammonia 

- 

-       -024 

Ash- 

- 

-     1-234 

The  ash  largely  consists  of  phosphates  and  potassium 
salts,  a  moderate  amount  of  magnesium,  and  very  little 
sodium  or  calcium. 

The  Urine  of  the  Hog. 

It  is  impossible  to  give  the  composition  of  the  urine  of 
the  dog,  as  the  amount  of  constituents  secreted  varies 
considerably  in  dependence  upon  the  nature  of  the  diet. 

The  urine  is  acid  in  reaction  on  a  flesh  diet,  the  acidity 
being  due  to  acid  phosphate  of  soda ;  on  a  vegetable  diet 
it  may  be  alkaline.  The  amount  excreted  is  from  f  to 
If  pints  daily,  but  varies  with  the  size  of  the  animal  and 
the  nature  of  the  diet ;  the  specific  gravity  is  from  1016  to 
1060  depending  on  the  diet ;  the  colour  is  pale  yellow  to 
straw  yellow ;  the  urea  varies  from  4  per  cent,  to  6  per 
cent.  On  an  animal  diet  uric  acid  is  excreted,  but  dis- 
appears on  giving  vegetable  food ;  hippuric  acid  in  small 
quantities  appears  with  fair  regularity ;  indican  and  phos- 
phoric acid  are  well-marked  constituents,  and  a  substance 
known  as  glycuronic  acid  may  be  found  which  exercises  a 
reducing  action  on  salts  of  copper.  The  presence  of 
bilirubin  in  the  urine  of  the  dog  has  been  noted  by 
Salkowski  (see  p.  221). 

As  an  illustration  of  the  variation  of  the  dog's  urine 
dependently  on  the  nature  of  the  diet,  we  may  take  an 
example  from  a  long  series  of  experiments  by  Bischoff  and 
Voit. 


THE  UKINE  311 

On  a  diet  consisting  of  meat  '57  lb.,  starch  "71  lb., 
salts  77"5  grains,  a  specimen  of  urine  gave  the  following 
composition : 

Amount  ...  -44  pint 

Sp.  gr.  -             -             -  1049 

Urea  -             -             -  326'6  grains 

Salts  -             -            -       85'6      ,, 

On  a  diet  consisting  of  meat  2*75  lbs.  and  fat  "55  lb., 
the  following  was  the  composition  : 

Amount  -  -  -  1"23  pints 

Sp.  gr.  ■  -  -  1054 

Urea  -  -  -  1,351  grains 

Salts  -  -  -  189     „ 

Glycuronic  acid  exists  only  in  traces,  but  after  the 
administration  of  camphor  or  chloral  it  is  obtained  in 
well-marked  quantities.  It  is  a  point  of  practical  import- 
ance to  avoid  regarding  urine  which  reduces  salts  of  copper 
as  necessarily  containing  sugar  (see  p.  301). 

The  Discharge  of  Urine. — The  urine  is  constantly  being 
secreted,  and  it  either  trickles  down  or  is  propelled  down 
the  ureters  to  the  bladder  by  rhythmic  muscular  contrac- 
tions. It  is  quite  likely  that  both  movements  are  employed 
depending  upon  the  condition  of  bladder  distension ; 
whereas  '  trickling '  is  suitable  for  an  empty  bladder,  some 
muscular  effort  on  the  part  of  the  ureters  would  be  required 
W'hen  the  bladder  was  full. 

Either  drop  by  drop  or  by  '  spirts '  the  urine  enters  the 
bladder,  which  gradually  advances  in  the  pelvis,  and  rises 
up  so  as  to  touch  the  rectum.  All  reflux  of  urine  into  the 
ureters  is  prevented  by  the  oblique  manner  in  which  the 
coats  of  the  bladder  are  pierced,  so  that  the  greater  the 
internal  strain  the  tighter  are  the  ureters  closed.  If 
circumstances  prevent  the  evacuation  of  the  bladder  con- 
tents, the  organ  gradually  advances  to  the  brim  of  the 
pelvis,  and  then  impinges  on  the  abdominal  cavity  ;  in  a 
state  of  extreme  distension  it  may  project  for  some  distance 


312     A  MANUAL  OF  YETEEINARY  PHYSIOLOGY 

into  the  cavity,  the  weight  of  the  fluid  having  a  tendency 
to  cause  the  organ  to  incKne  towards  the  floor  of  the 
abdomen. 

The  entrance  to  the  urethra  is  controlled  ])y  a  circular 
layer  of  unstriped  muscle,  part  of  the  bladder  muscle,  but 
outside  this  is  a  band  of  voluntary  muscle  which  must  be 
regarded  as  part  of  the  urethra.  Bladder  pressure  pro- 
duces a  desire  to  evacuate,  an  act  which  may  be  a  purely 
reflex  one,  as  in  the  case  of  the  dog,  with  its  spinal  cord 
divided  far  forward,  or,  what  is  more  common,  as  a  voluntary 
act  in  obedience  to  the  summons  issued  by  the  bladder 
wall. 

Physiologists  are  not  agreed  as  to  how  the  act  of 
micturition  is  carried  out,  but  through  the  bladder  wall 
impulses  are  transmitted  to  the  cord,  resulting  in  a  con- 
traction of  the  organ  and  a  relaxation  of  the  sphincters, 
though  there  is  some  difference  of  opinion  as  to  this.  At 
the  moment  the  bladder  wall  begins  to  contract,  it  is  assisted 
by  the  abdominal  muscles  and  a  fixed  diaphragm,  and  the 
flow  is  never  as  powerful  in  the  female  as  in  the  male,  the 
final  expulsion  of  the  last  drops  from  the  urethra  of  the 
latter  being  given  by  the  rhythmical  contraction  of  the 
perineal  muscles  and  accelerator  urines. 

The  bladder  receives  a  motor  nerve  supply  through 
fibres  coming  off  from  the  lumbar  cord,  which  reach  it 
by  the  mesenteric  ganglion,  and  fibres  coming  off  from 
the  sacral  cord  which  reach  the  bladder  through  the 
nerri  erigcntes.  It  is  this  latter  group  which  causes  an 
energetic  contraction  of  the  bladder.  The  sensory  nerves 
run  in  the  fibres  from  the  lumbar  cord. 

During  the  act  both  the  horse  and  mare  stand  with  the 
hind-legs  extended  and  apart,  resting  on  the  toes  of  both 
hind  feet,  thereby  sinking  the  posterior  part  of  the  body  ; 
the  male  animal  also  often  advances  the  fore-legs  in  order 
to  avoid  getting  them  splashed ;  in  this  position  the  penis 
is  protruded,  and  the  tail  raised  and  quivering.  The 
stream  which  flows  from  the  two  sexes  is  very  different  in 
size,  depending  on  the  relative  diameters  of  the  urethral 


THE  URINE  313 

canal.  The  mare  after  urinating  spasmodically  erects  the 
clitoris,  the  use  of  which  it  is  difficult  to  see ;  it  may  be 
due  to  the  passage  of  a  hot  alkaline  fluid  over  a  remarkably 
sensitive  surface.  The  horse  can  under  ordinary  circum- 
stances only  pass  urine  when  standing  still,  though  both  sexes 
can  def aecate  while  trotting ;  but  in  a  condition  of  oestrum 
the  mare  can  empty  her  bladder  while  cantering.  In  the  ox 
the  urine  simply  dribbles  away,  owing  to  the  curves  in  the 
urethral  canal,  and  is  directed  towards  the  ground  by  the 
tuft  of  hair  found  on  the  extremity  of  the  sheath.  The  ox 
can  pass  his  urine  while  walking.  The  cow  arches  her 
back  to  urinate,  but  instead  of  extending  her  hind-limbs  as 
does  the  mare,  she  brings  them  under  the  body,  at  the 
same  time  raising  her  tail. 

The  upright  position  is  essential  to  micturition ;  no  horse 
of  either  sex  can  evacuate  the  bladder  while  lying  down, 
a  point  of  extreme  importance  in  practice.  Further,  it  will 
be  remembered  that  in  an  over-distended  bladder  the  fundus 
hangs  into  the  abdominal  cavity,  and  is  thus  brought  on 
a  lower  level  than  the  urethra,  both  of  which  contribute  to 
the  difficulty  of  emptying  an  over-distended  organ.  As  a 
horse  cannot  micturate  at  work,  it  is  obvious  that  oppor- 
tunity for  this  should  be  regularly  afforded,  or  much 
suffering  results. 

Pathological. 

There  is  scarcely  any  organ  of  the  horse's  body  so  free  from  disease 
as  the  kidneys.  The  material  in  the  pelvis  which  looks  hke  pus  is 
really  the  natural  mucus  of  the  urine,  mixed  with  insoluble  lime  salts. 
We  have  never  found  sugar  in  the  horse's  urine ;  proteid  is  not  un- 
common, but  only  as  the  result  of  inflammatory  affection  of  the  lungs 
and  pleura. 


CHAPTER  XI 
NUTRITION 

Wear  and  tear  is  continually  taking  place  in  the  bodies 
of  all  animals,  and  as  fast  as  destruction  occurs  repair  must 
follow.  We  have  previously  studied  the  various  channels 
in  the  body  which  supply  the  income  and  furnish  an  outlet 
for  the  expenditure,  but  this  is  only  the  beginning  and  the 
end  of  the  process.  To  attempt  to  trace  the  exact  changes 
which  occur,  say  in  the  body  of  a  pig,  in  producing  1  lb. 
of  living  material  from  5  lbs.  of  barley-meal,  is  an  impossi- 
bility. All  we  can  do  is  to  interpret  the  coarser  or  more 
obvious  processes  which  take  place,  that  of  the  conversion  of 
dead  into  living  tissues  being  quite  beyond  our  knowledge. 
Composition  of  the  Body. — The  animal  body  consists  of 
proteids,  fats,  salts,  water,  and  a  very  small  proportion  of 
carbohydrate.  Every  food  must  either  contain  these 
principles,  or  be  capable  of  conversion  into  them  within 
the  animal  body.  The  following  table  from  Lawes  and 
Gilbert  shows  the  relative  proportion  of  these  various 
tissues  in  oxen,  sheep,  and  pigs,  in  '  store '  condition  : 


Water       - 

Ox. 
-     59-0 

Sheep. 
58-9 

Pig. 
57-9 

Proteids    - 

-     lS-3 

16-0 

15-0 

Fat 

-     17-5 

21-3 

24-2 

Ash 

-       5-2 

3-8 

2-9 

The  water  is  always  in  the  largest  and,  excluding  the 
carbohydrate,  the  salts  in  the  smallest  proportion.  The 
amount  of  fat  depends  upon  the  condition ;  in  fat  animals  it 
may,  roughly  speaking,  be  three  times  the  amount  given 

314 


NUTEITION  315 

in  the  above  table.  The  great  bulk  of  the  body  is  repre- 
sented bj  the  muscles,  and  these  hold  half  the  water  and 
half  the  proteid  found  in  the  system.  The  following  table 
shows  the  proportion  of  the  chief  body  constituents  of  an 
adult  horse  weighing  1,100  lbs.,  and  it  may  be  compared 
with  that  of  a  cat : 

Horse.  Cat. 

Muscles  and  tendons         -     45   per  cent.  45  per  cent. 

Bones        -             -             -     12-4     „  14-7     „ 

Skin           -             -             -       6-02    „  12-0     „ 

Blood         -             -             -       5-90   „  6-0 
Abdominal  viscera             -       5"49    „ 
Thoracic          ,,                  -       1*60    ,, 

According  to  Lawes  and  Gilbert  the  following  are  the 
relations  of  parts  in  the  ox,  sheep,  and  pig  for  every  100  lbs. 
of  living  weight : 

Heart,  lungs,  livei',  blood  and  spleen 

Internal  loose  fat 

Stomach  and  contents 

Intestines  ,,         ,,     - 

Other  oflfal  parts 

Muscle,  bone  and  surrounding  fat     - 

Income  and  Expenditure. — In  order  to  arrive  at  a  know- 
ledge of  the  processes  involved  in  nutrition,  tables  of  the 
income  and  expenditure  of  the  body  have  been  drawn  up. 

The  Income  of  the  body  consists  of  carbon,  hydrogen, 
oxygen,  nitrogen,  sulphur,  phosphorus,  salts  and  water; 
these  are  contained  in  the  food,  the  oxygen  being  mainly 
supplied  by  the  air  taken  in  at  the  lungs. 

The  Expenditure  consists  of  the  same  elements,  which 
are  got  rid  of  by  the  lungs,  urine,  and  skin. 

The  nitrogen  is  excreted  almost  wholly  by  the  urine, 
excepting  in  the  horse,  where  there  is  a  loss  by  the  skin. 
It  is  usual  to  regard  the  urine  nitrogen  as  a  measure  of  the 
proteid  changes  in  the  system,  and  this  is  got  rid  of  mainly 
as  urea,  and  in  smaller  proportion  as  uric  and  hippuric 
acids,  and  minor  nitrogen  compounds.  The  hydrogen  is 
excreted  as  water  by  the   lungs,  skin,  and   urine.      The 


Ox. 
7-0 

Sheep. 
7-3 

Pig. 
6-6 

4-6 

6-9 

1-6 

1-6 

7-5 

1-3 

2-7 

3-6 

6-2 

13-0 

15-0 

1-0 

59-3 

59-2 

82-6 

316     A  MANUAL  OF  YETERINAEY  PHYSIOLOGY 

carbon  is  largely  got  rid  of  by  the  lungs  and  urine,  and  in 
the  horse  by  the  skin.  The  salts  are  excreted  by  the 
kidneys  and  skin,  and  in  the  form  of  secretions.  The  sulphur 
is  lost  through  the  kidneys,  epithelium,  hair,  and  horn. 

It  is  hardly  necessary  to  add  that  in  calculating  the 
true  income  of  the  body  the  f?eces  may  be  subtracted 
without  leading  to  any  great  error,  since  they  consist  chiefly 
of  food  which  has  either  escaped  being  digested  or  is  not 
digestible.  At  the  same  time,  they  do  contain  a  certain 
amount  of  material  which  represents  products  of  tissue 
change  which  have  been  excreted  from  the  blood  into  the 
alimentary  canal  (p.  208).  When  the  income  balances  the 
expenditure  the  body  is  in  equilibrium :  if  the  expenditure 
exceeds  the  income  the  body  loses  weight,  and  if  the  income 
is  in  excess  of  the  expenditure  the  animal  gains  weight. 

Metabolism. — By  this  term  is  understood  the  changes 
occurring  in  living  tissues.  It  is  evident  from  what  has 
been  said  that  constant  breaking  down  and  building  up  is 
taking  place  in  the  body ;  every  muscular  contraction, 
every  respiration,  the  beating  of  the  heart,  and  the  move- 
ments of  the  bowels,  all  mean  wear  and  tear,  and  as  rapidly 
as  a  part  is  destroyed  it  must  be  replaced.  The  process  of 
construction  is  known  as  anabolism,  and  of  destruction  as 
katabolism ;  in  a  perfect  state  of  health  these  should  be  in 
equilibrium.  Both  repair  and  destruction  are  dependent 
upon  definite  chemical  changes  occurring  in  the  system, 
of  some  of  which  we  have  a  fair  knowledge,  while  others 
are  wrapped  in  obscurity. 

The  metabolism  of  the  tissues  is  apparently  under  the 
influence  of  the  nervous  system.  We  have  previously 
studied  a  good  example  of  this  in  dealing  with  the  secretory 
nerves  of  the  submaxillary  gland,  and  it  is  probable,  though 
our  information  on  the  point  is  very  defective,  that  under 
the  guiding  influence  of  the  nervous  system  the  nutrition 
of  the  body  is  largely  maintained.  We  constantly  observe 
muscular  wasting  in  some  forms  of  lameness  and  injury  in 
the  horse,  which  is  out  of  all  proportion  to  the  atrophy  a 
part  suffers  by  being  simply  thrown  out  of  use,  and  it  can 


NUTRITION  317 

only  be  explained  by  injury  to  the  trophic  nerves  which 
regulate  the  nutrition  of  the  part.  Even  a  better  example 
is  the  peculiar  changes  which  sometimes  follow  direct 
injury  to  trophic  nerves,  as  in  plantar  neurectomy  of 
the  horse ;  the  sloughing  of  the  entire  foot,  or  gelatinous 
degeneration  of  the  phalanx,  is  due  to  injury  of  the  trophic 
nerves.  Injuries  to  the  fifth  pair  of  nerves  have  been 
followed  by  sloughing  of  the  cornea,  and  pneumonia  has 
followed  division  of  the  vagi,  in  both  cases  being  possibly 
due  to  the  loss  of  trophic  influence,  though  much  may 
be  said  in  support  of  the  view  that  the  effects  observed 
may  be  due  to  failure  of  the  mechanically  protective 
arrangements  of  the  parts  affected,  the  failure  resulting 
from  section  of  the  merely  motor  and  sensory  fibres  which 
the  respective  nerves  contain. 

But  disordered  nutrition  of  a  tissue  may  show  itself 
without  any  obvious  injury  to  trophic  nerves,  as  for 
example  in  the  phenomenon  known  as  inflammation,  or 
the  well-known  sympathy  existing  between  the  digestive 
system  of  the  horse  and  the  laminae  of  the  feet.  Further 
evidence  of  nervous  action  is  afforded  in  nutrition  which  is 
normal  in  character,  such  as  the  change  of  the  coat  with 
the  season  of  the  year.  The  influence  of  light  on  meta- 
bolism is  also  probably  effected  through  the  nervous  system  ; 
it  appears  certain  that  a  connection  between  visual  sensa- 
tions and  the  nutrition  of  the  skin  occurs  in  blind  men 
and  animals,  and  the  popular  belief  that  a  blind  horse 
carries  a  heavy  coat  in  summer  and  a  short  one  in  winter 
may  be  something  more  than  mere  superstition.  In  making 
these  statements  we  must  guard  against  the  error  of  con- 
sidering that  no  growth,  repair,  or  reproduction  can  take 
place  excepting  under  the  influence  of  the  nervous  system ; 
the  trophic  influence  exercised  by  nerves  appears  to  be 
directed  to  maintaining  in  equilibrium  the  processes  of 
building  up  and  breaking  down  which  are  occurring  in 
all  tissues.  Though  the  metabolism  of  the  body  is  largely 
regulated  by  the  nervous  system,  yet  the  process  cannot 
be  carried  out  without  food.    It  is  true  that  metabolism  goes 


318     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

on  during  starvation,  but  even  then  food  is  being  supplied, 
inasmuch  as  the  animal  is  living  on  its  own  tissues. 

The  food  must  contain  the  elements  required  by  the 
tissues,  viz.,  water,  proteid,  fat  (or  carbohydrate)  and 
salts ;  each  of  these  must  be  in  proper  proportion,  neither 
deficient  nor  in  excess  of  the  animal's  requirements  ;  each 
must  be  present,  fat  cannot  be  substituted  for  proteid, 
nothing  can  take  the  place  of  salts,  and  a  water-free  diet 
sustains  life  less  long  than  does  the  entire  absence  of  food 
as  long  as  water  is  consumed.  We  have,  therefore,  to  in- 
quire why  it  is  these  substances  are  absolutely  essential  in 
every  diet,  and  how  they  behave  in  the  system. 

Nitrogenous  Food. — The  history  of  proteid  in  the  body 
may  be  conveniently  taken  up  at  the  point  where  it  was 
left  when  dealing  with  absorption,  viz.,  in  the  bloodvessels. 
It  will  be  remembered  that  as  peptone  the  material  passed 
from  the  bowel  to  the  blood,  yet  no  peptone  can  be  detected 
in  blood,  showing  that  a  regeneration  has  occurred,  the 
peptone  being  converted  back  to  proteid.  We  do  not  know 
whether  the  whole  of  the  proteid  in  its  downward  course 
from  complexity  to  simplicity  becomes  body-proteid,  such 
as  would  be  represented  by  the  serum  of  blood,  or  whether 
a  part  only  undergoes  this  change  while  the  remaining 
portion  is  converted  into  leucine,  tyrosine,  arginine,  etc., 
and  is  not  built  into  proteid,  but  becomes  urea. 

The  interest  attached  to  knowing  how  the  proteid  of  the 
food  behaves  in  the  body,  arises  from  the  remarkable  fact 
that  nearly  the  whole  of  the  nitrogen  in  it  can  be  recovered 
from  the  excretions  ;  very  little,  and  under  some  circum- 
stances none,  is  stored  up.  So  that  the  question  arises  as 
to  whether  the  nitrogen  of  the  excreta  arises  from  pre- 
formed proteid  tissue,  or  from  the  nitrogen  last  consumed, 
and  if  the  latter  whether  it  was  from  recently  formed  body 
proteid,  or  only  material  in  the  leucine  or  tyrosine  condition? 
It  is  in  an  endeavour  to  answer  these  questions  that  the 
bulk  of  the  work  on  metabolism  has  been  carried  out,  and 
the  results  group  themselves  into  two  theories,  Pfluger's 
and  Voit's.     Pfluger  holds  that  the  whole  of  the  absorbed 


NUTEITION  319 

material  must  first  be  converted  into  proteid  before  any 
destruction  of  it  can  occur  ;  in  other  words,  that  there  is 
no  short  cut  to  urea  excepting  through  the  disintegration 
of  the  living  cell.  Voit  contends  that  the  proteid  when 
absorbed  is  divided  into  two  portions  :  one,  the  smaller, 
repairs  wear  and  tear  in  the  body  and  is  spoken  of  as 
tissue  proteid ;  the  other,  the  larger  portion,  circulates  with 
the  blood  and  lymph  and  bathes  the  body  cells,  but  does 
not  form  part  of  them.  This  is  destroyed  by  the  tissues 
with  the  liberation  of  heat  and  the  formation  of  nitrogenous 
end-products,  the  chief  of  which  is  urea  ;  this  portion  Voit 
describes  as  the  circulating  proteid.  Voit's  theory  has  been 
subjected  to  severe  criticism,  and  the  experiments  on  which 
it  is  based  have  been  shown  to  be  not  entirely  free  from 
error,  yet  there  is  much  in  it  which  explains  the  observed 
facts  of  nitrogenous  metabolism. 

Nitrogenous  Equilibrium. — If  an  animal  in  poor  condition, 
or  a  young  growing  animal,  be  fed  on  an  ordinary  diet,  it 
will  be  found  that  the  whole  of  the  nitrogen  is  not  recover- 
able from  the  excreta,  as  described  above,  so  that  evidently 
some  has  been  retained  in  the  body  and  stored  up ;  further, 
under  the  conditions  of  a  liberal  diet  and  active  muscular 
work,  the  muscles  grow  and  for  this  purpose  they  retain 
nitrogen.  But  speaking  generally  all  the  nitrogen  con- 
sumed is  practically  recoverable  from  the  excreta.  If  the 
nitrogen  of  the  food  be  increased  that  of  the  excreta  is 
increased ;  if  it  be  reduced  the  nitrogen  excreted  becomes 
reduced,  and  this  may  be  maintained  through  long  periods 
of  time. 

It  is  certainly  a  very  remarkable  fact  that  the  body 
should  be  able  to  work  under  ordinary  circumstances 
equally  well  on  a  moderate  as  on  a  large  supply  of  proteid. 
The  influence  of  this  may  be  still  further  tested  by  placing 
the  animal  on  a  diet  entirely  proteid.  The  effect  of  such 
a  diet  is  to  cause  at  once  an  increased  elimination  of 
nitrogen  by  the  kidneys,  so  that  more  is  actually  being  cast 
off  from  the  body  than  enters  by  the  mouth.  If  in  oj'der 
to  meet  this  loss  further  proteid  be  given,  a  larger  and  still 


320    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

excessive  excretion  of  nitrogen  continues.  Experiment,  in 
fact,  determines  that  it  is  not  until  three  times  the  usual 
amount  of  proteid  is  given  that  the  nitrogen  entering  by 
the  mouth  equals  that  excreted  by  the  kidneys.  "When  this 
condition  is  reached  the  animal  is  said  to  be  in  nitrogenous 
eqidlihrium.  It  is  obvious  that  this  is  an  artificial  state  and 
cannot  possibly  be  maintained  for  long ;  further,  it  is  im- 
possible to  bring  it  about  unless  the  animal  starts  the  experi- 
ment with  some  stored-up  fat  in  the  body. 

The  diet  necessary  for  the  production  of  nitrogenous 
equilibrium  is  seriously  deficient  in  carbon,  and  the  reason 
why  the  animal  goes  on  consuming  proteid  and  increasing 
its  excretion  of  urea  is  in  order  to  obtain  the  needful 
carbon ;  proteid  contains  54  per  cent,  of  carbon,  while  fat 
contains  76*5  per  cent.  It  is  quite  possible  on  a  large 
nitrogenous  diet  for  the  animal  to  continue  to  lose  weight 
in  consequence  of  the  body  carbon  being  drawn  on  ;  on  the 
other  hand,  should  it  gain  weight  the  material  which  is 
stored  up  is  not  proteid,  for  we  have  shown  that  all  the 
nitrogen  appears  in  the  urine.  The  stored-up  substance  is 
carbohydrate  and  perhaps  fat,  though  this  latter  point  is  not 
yet  decided.  Proteid  in  the  body  splits  into  a  nitrogenous 
and  non-nitrogenous  moiety ;  it  is  from  the  latter  that 
glycogen  and  perhaps  fat  are  obtained,  the  former,  of 
course,  furnishing  the  urea. 

In  nitrogenous  equilibrium  there  is  carbon  starvation,  and 
if  the  explanation  given  above  is  correct,  that  the  increased 
consumption  of  proteids  is  due  to  the  urgent  need  for  their 
carbon,  then  the  addition  of  carbon  to  the  diet  will  cause 
a  reduction  in  the  amount  of  proteid  metabolised.  And 
this  is  found  to  be  the  case.  The  addition  of  either  starch 
or  fat  to  the  diet  at  once  causes  a  reduction  in  the  amount 
of  proteid  necessarily  ingested  in  order  to  maintain  the 
condition  of  nitrogenous  equilibrium ;  this  is  known  as 
the  proteid  sparing  action  of  starch  and  fat,  and  is  one  of 
the  few  well-established  facts  in  metabolism  and  the  basis 
of  rational  dieting. 

It  has  long  been  observed  that  many  diets  were  exces- 


NUTEITION  321 

sively  nitrogenous  and  therefore  costly  and  wasteful ;  in 
exact  experiments  on  men  it  has  been  shown  they  can  be 
kept  in  health  for  months  on  a  diet  far  poorer  in  proteid 
than  what  is  generally  accepted  to  be  necessary.  It  is 
extravagance  to  give  2  pounds  of  proteid  daily  to  a  horse 
if  8  ounces  will  meet  all  necessities.  It  is  at  this  point 
the  physiologist  comes  into  conflict  with  practical  experi- 
ence. Theory  says  the  quantity  of  nitrogen  required  is 
nearly  independent  of  muscular  work ;  practice  says  the 
harder  the  machine  is  worked  the  more  nitrogen  must  be 
given.  Theory  says  proteids  are  not  the  source  of  muscular 
energy,  as  this  is  the  function  of  non-nitrogenous  food  ; 
practice  replies  that  may  be  so,  but  we  know  from  experi- 
ence that,  whether  we  are  getting  a  horse  fit  for  hard  work 
or  cattle  and  sheep  ready  for  the  butcher,  the  diets  given 
must  be  strongly  nitrogenous  and  limited  only  by  the 
appetite.  In  this  matter  our  personal  experience  places  us 
on  the  side  of  practice  and  opposed  to  theories.  Why  the 
hard-worked  horse  needs  more  nitrogen  we  are  not  pre- 
pared to  explain.  The  suggestion  that  the  machine  works 
more  easily  and  smoothly  on  a  liberal  nitrogenous  diet  does 
not  bring  us  any  nearer  to  a  solution  of  the  problem,  but 
the  fact  remains  that  whatever  may  be  the  energy  obtainable 
from  starch  and  fat  this  energy  is  in  some  unknown  way 
directed  by  proteid.  All  nitrogen  over  and  above  that 
required  for  repair  was  considered  a  wasteful  or  '  luxus 
consumption,''  a  condition  to  which  we  by  no  means  sub- 
scribe. That  a  wasteful  consumption  of  proteid  occurs 
where  horses  are  not  fed  in  accordance  with  the  work 
they  are  performing  is  undoubted  ;  the  excess  of  nitro- 
genous material  throws  an  additional  strain  on  the  ex- 
creting channels,  and  it  is  certain  that,  clinically,  we  are 
able  to  recognize  the  effects  of  a  highly  nitrogenous  diet 
in  the  liver  disorders  of  tropical  climates,  and  lymphangitis 
and  azoturia  of  the  temperate  latitudes. 

Doubtful  and  difficult  of  solution  as  many  of  the  im- 
portant points  are  in  nitrogenous  feeding,  they  are  nothing 
in  comparison  with  the  problem  of   how  the  dead  food- 

21 


322     A  MANUAL  OF  YETEEINAEY  PHYSIOLOGY 

proteid  is  converted  into  the  living  body-proteid,  and  how 
the  same  kind  of  proteid  can  be  utilized  in  building  up 
material  so  different  in  structure  as  bone  and  brain,  muscle 
and  fat,  liver  and  skin. 

It  is  here  convenient  to  summarize  what  we  have  learnt 
regarding  nitrogenous  food  : 

1.  The  body  requires  nitrogen  ;  no  diet  is  complete  with- 
out it,  nor  can  life  be  permanently  supported  in  its  absence. 

2.  The  body  having  obtained  its  nitrogen  stores  up  the 
small  amount  required  to  replace  wear  and  tear  and  excretes 
the  whole  of  the  remainder  mainly  in  the  form  of  urea. 

3.  The  assumption  that  the  proteids  are  the  source  of 
muscular  energy  is  incorrect,  this  being  the  function  of 
non-nitrogenous  food,  yet  increased  muscular  efforts  must 
be  met  by  an  increased  nitrogenous  ration,  the  assump- 
tion being  that  in  some  unknown  way  it  directs  the  produc- 
tion of  energy  in  the  muscle  machine,  after  which  it  is 
completely  cast  off. 

4.  Proteid  is  stored  up  in  young  growing  animals  and  in 
those  out  of  condition ;  some  is  also  stored  up  in  working 
animals  so  long  as  their  muscles  are  increasing  in  bulk.  It 
is  considered  that  proteid  cannot,  under  ordinary  circum- 
stances, be  stored  unless  there  is  accompanying  muscular 
effort,  and  even  then  there  is  a  limit  to  the  growth  of 
muscular  tissue,  just  as  there  is  to  the  skeleton  to  which  it 
is  attached. 

All  true  proteids  are  equally  capable  of  becoming  part  of 
the  tissues  when  taken  as  food,  but  when  alhnminoids,  such 
as  gelatin,  are  consumed,  they  produce  the  same  amount  of 
urea  as  an  assimilable  proteid,  but  the  animal  loses  flesh, 
viz.,  none  of  the  material  is  stored  up  in  the  system ;  when 
gelatin  is  mixed  with  proteid  it  exercises  a  sparing  action 
upon  the  latter,  and  less  of  it  is  used  up  in  the  body. 

Non-nitrogenous  Food. — The  whole  of  the  carbo-hydrate 
matters  found  in  food,  viz.,  the  starch,  sugar,  gum  and 
cellulose,  must,  as  we  have  seen,  \)Q  first  rendered  soluble 
before  they  can  enter  the  system.  Further,  they  can 
only  enter  as  some  form  of  sugar,  and  are  then  stored  up 


NUTRITION  323 

.for  future  use  as  fat  in  certain  depots,  and  as  glycogen 
in  the  muscles  and  liver,  while  for  present  use  they  exist 
as  glucose  in  the  circulating  blood. 

The  supply  of  carbo-hydrates  is  added  to  by  the  splitting 
up  of  proteids  into  a  nitrogenous  and  non-nitrogenous 
portion  (p.  320)  ;  whether  the  non-nitrogenous  portion  of 
proteid  can  form  fat  is  uncertain,  but  it  is  undoubted  that 
it  forms  glycogen. 

Carbo-hydrates  are  readily  oxidized  as  the  molecule  pro- 
vides sufficient  oxygen  to  oxidize  all  its  hydrogen,  and  only 
needs  to  obtain  from  the  tissues  oxygen  for  the  oxidation 
of  the  carbon.  In  this  respect  they  are  a  great  contrast  to 
the  fats,  in  which  the  proportion  of  oxygen  to  hydrogen  in 
the  molecule  is  not  sufficient  to  oxidize  all  the  hydrogen  to 
water,  so  that  fats  have  to  obtain  oxygen  both  for  their 
hydrogen  and  carbon.  In  dealing  at  p.  96  with  the  ques- 
tion of  the  respiratory  quotient  it  was  explained  that  this 
fraction  represented  the  relative  amounts  of  carbonic  acid 
produced  and  oxygen  absorbed.  The  theoretical  value  of 
the  respiratory  quotient  on  a  carbo-hydrate  diet  is  1,  but 
with  fats  the  volume  of  oxygen  absorbed  is  greater  than 
the  volume  of  carbonic  acid  produced,  and  the  respiratory 
quotient  becomes  "707. 

1  gramme  (15i  grains)  of  carbo-hj'drate  requires  '832  litre  (50'8  cubic 
inches)  of  oxygen,  and  produces  •832  litre  (50'8  cubic  inches)  of 
CO.a. 

1  gramme  (15i  grains)  of  fat  requires  2-8875  litres  (176  cubic  inches) 
of  oxygen  and  produces  1-434  litres  (87*5  cubic  inches)  of  CO2. 

Great  interest  attaches  to  the  carbo-hydrates  in  the 
feeding  of  herbivora,  as  so  little  fat  exists  naturally  in 
vegetable  food.  We  have  learnt  that  the  carbo-hydrates 
are  one  of  the  sources  of  muscular  energy,  and  with  horses 
they  are  the  chief  source.  This  material  is  '  fired  off ' 
by  the  muscles  during  contraction,  and  so  markedly  are 
carbo-hydrates  the  source  of  muscular  work,  that  the  whole 
store  in  the  body  may  be  used  up  under  the  influence  of 
muscular  work  and  starvation. 

As  the  result  of  the  oxidation  of  carbo-hydrates  heat  is 

21—2 


324    A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

generated,  so  that  these  substances  supply  not  only  energy 
but  heat  to  the  body.  The  seat  of  the  necessary  oxidation 
is  in  the  tissues  and  not  in  the  blood  ;  the  tissues  produce 
enzymes  which  break  up  the  sugar  with  the  formation  of 
carbonic  acid  and  water ;  these  enzymes  are  called  into 
activity  by  the  internal  secretion  of  the  pancreas.  The 
amount  of  heat  generated  by  the  oxidation  of  sugar  can 
easily  be  measured,  1  gramme  (15i  grains)  yielding 
4,100  calories,  or  4'0  large  calories  of  heat.*  Oxidations  are 
constantly  going  on  throughout  the  life  of  the  animal ;  those 
occurring  during  rest  are  providing  for  the  internal  work 
and  heat  of  the  body,  while  during  work,  in  addition  to 
these,  they  furnish  the  muscular  energy. 

The  influence  of  carbo-hydrate  as  a  proteid  sparer  has 
already  been  mentioned ;  10  per  cent,  less  proteid  is  re- 
quired with  the  food  when  carbo-hydrates  are  present  in 
sufficient  quantity.  Experiment  even  shows  that  under  the 
influence  of  a  considerable  quantity  of  carbo-hydrate,  no 
more  proteid  is  required  by  the  system  than  is  equivalent 
to  the  urea  excreted  during  starvation.  In  spite  of  the 
immense  value  of  carbo-hydrates  in  feeding,  a  diet  of 
carbo-hydrate  without  proteid  means  starvation. 

The  Fats. — As  previously  noted  there  is  very  little  fat  in 
the  diet  of  herbivora,  in  fact  the  amount  is  so  small  that  in 
the  fattening  of  animals  fat  is  always  specially  added  to 
the  diet. 

It  might  hence  be  natural  to  conclude  that  the  fat  in 
the  body  is  derived  from  the  fat  in  the  food,  but  this 
does  not  cover  the  whole  ground ;  great  stores  of  fat  may 
exist  in  animals  receiving  a  trifling  amount  of  fat  in  the 
diet :  a  cow,  for  instance,  may  produce  more  fat  in  her  milk 
than  she  receives  in  her  food,  so  that  it  is  evident  a  some- 
thing not  fat  is  furnishing  it.  This  something  is  the 
carbo-hydrate  which  when  in  excess  of  requirements  is 
stored  up  as  fat  in  the  permanent  fat  reserve  depots  of  the 

*  A  large  calorie  is  the  amount  of  heat  necessary  to  raise  1  kilo. 
(2'2  lbs.)  water  1°  C.  (1*8°  F.),  and  is  conveniently  named  a  kilo- 
calorie. 


NUTRITION  325 

body,  and  subsequently  doled  out  to  the  system  as  required. 
Perhaps  also  the  non-nitrogenous  portion  of  the  proteid 
molecule  may  contribute  to  fat  formation,  though  this 
point  is  not  settled. 

The  storing  up  of  fat  is  a  physiological  process,  though 
under  certain  circumstances  it  may  constitute  a  patho- 
logical condition.  By  its  oxidation,  which  is  referred  to 
more  fully  at  p.  332,  fat  furnishes  heat  and  energy,  and  in 
this  respect  is  of  higher  value  than  an  equal  quantity  of 
carbo-hydrate.  One  gramme  (15j  grains)  of  fat  yields 
9*3  large  calories  on  oxidation.  How  it  is  prepared  for 
oxidation  is  unknown  ;  the  fat  as  it  lies  in  masses  in  the 
body  cannot  be  oxidized  until  it  is  brought  back  into  the 
blood  and  carried  to  the  tissues,  and  it  is  suggested  that 
the  fat- splitting  ferment,  lipase,  decomposes  the  fats  into 
fatty  acids  and  glycerin,  in  much  the  same  way  that  the 
same  ferment  splits  the  fat  in  the  intestinal  canal  before 
absorption.  Should  this  be  the  case  the  lipase  regulates 
the  supply  of  fat  to  the  blood. 

There  are  certain  fat  reserve  depots  natural  to  the 
animal,  and  on  which  under  ordinary  circumstances  little 
or  no  drain  occurs ;  such  are  found  beneath  the  peritoneum, 
around  the  kidneys,  in  the  mesh  of  the  omenta,  and  sur- 
rounding the  base  of  the  heart.  It  is  only  under  the 
influence  of  starvation  that  the  fat  in  these  places  is  drawn 
on.  The  chief  means  to  induce  the  laying  on  of  fat  is  a 
liberal  diet  and  freedom  from  exercise  and  work.  The 
farmer  feeding  for  beef  or  mutton  understands  the  value 
of  keeping  the  animals  as  quiet  as  possible,  and  recognizes 
also  that  there  are  certain  breeds  which  have  a  distinct 
predisposition  to  store  up  fat.  He  further  learns  how 
necessary  it  is  to  introduce  animals  gradually  to  a  fattening 
diet  until  toleration  is  established,  and  he  knows  from 
practical  experience  that  he  will  not  succeed  in  fattening 
within  a  reasonable  time  unless  to  the  diet  of  carbo- 
hydrate and  fat  he  also  adds  proteids  liberally.  The 
measure  of  the  diet  is  that  of  the  animal's  appetite  ;  they 
can  never  eat  enough  to  please  the  feeder,  who  cheerfully 


326     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

accepts  the  heavy  initial  outlay,  as  he  knows  the  sub- 
sequent saving  in  time  effected.  The  obesity  aimed  at  with 
*  show'  cattle,  sheep,  and  pigs  is  a  pathological  condition  re- 
pugnant to  common  sense,  and  the  outcome  of  a  barbarous 
fashion. 

The  consensus  of  opinion  is  in  favour  of  castration  as 
facilitating  fattening,  though  this  view  has  not  stood  the 
test  of  scientific  enquiry.  It  is  conceivable  that  if  it  has 
some  such  effect,  it  may  easily  be  explained  on  the  ground 
of  greater  freedom  from  excitement.  It  is  quite  certain  that 
geldings  have  no  greater  disposition  to  accumulate  fat  than 
mares,  and  if  castration  favoured  fattening  there  would 
be  no  need  for  that  constant  striving  after  fatness  instead 
of  'fitness,'  which  is  so  characteristic  of  all  who  have 
charge  of  horses.  There  are,  of  course,  some  animals 
which  have  a  tendency  to  store  up  fat  and  others  which 
never  do  any  credit  to  their  '  keep,'  but  this  is  an  in- 
dividual peculiarity  not  explained  by  castration. 

The  fat  of  horses  is  soft  and  of  sheep  hard ;  that  of  cattle 
occupies  a  middle  position.  Each  animal  has  fat  of  a 
certain  melting-point  to  store  up,  and  whether  this  be 
derived  from  oil,  carbo-hydrate  or  food  fat,  makes  very 
little  if  any  difference.  In  the  fattening  of  the  herbivora  it 
is  considered  that  carbo-hydrates  are  better  fat  producers 
than  food  fat.  The  form  in  which  the  fats  in  food  are 
stored  up  has  been  made  the  subject  of  many  experiments : 
a  dog  fed  on  a  hard  fat  converts  it  into  canine  fat  which  is 
soft ;  cattle  fed  on  fluid  fats,  like  linseed  oil,  convert  them 
into  hard  body  fats  ;  still,  experiments  go  to  show  that 
foreign  fats  used  for  feeding  may,  if  given  in  sufficient 
amount,  be  recognized  in  the  tissues.  It  is  considered 
that  green  food,  hay,  and  carbo-hydrates,  produce  a  hard 
body  fat,  while  grain  feeding,  such  as  oats,  conduce  to  a 
soft  fat.  Fats,  like  carbo-hydrates,  exert  a  sparing  action 
on  proteids,  and  for  this  reason  a  fat  animal  takes  longer 
to  starve  to  death  than  one  which  is  less  fat. 

Inorganic   Food.  —  The   salts   in   the  body  perform  im- 


NUTRITION  327 

portant  functions  in  connection  with  secretion  and  ex- 
cretion ;  as  Foster  expresses  it,  they  direct  the  metabolism 
oi  the  body,  though  how  they  do  so  is  unknown.  To  their 
presence  is  due  the  normal  composition  of  the  body  fluids 
and  tissues,  for  they  regulate  the  water-flow  from  blood  to 
tissues  and  vice  versa.  Proteids  which  are  free  from  salts 
are  quite  altered  in  their  essential  characters,  while  the 
part  taken  by  the  salts  of  the  body  in  blood-clotting, 
rhythmical  contraction  of  the  heart,  irritability  of  muscle 
and  nerve,  milk-curdling,  and  growth  is  of  supreme 
importance.  The  distribution  of  the  salts  throughout  the 
structure  is  remarkably  regular,  sodium  being  found  in  the 
blood  plasma,  potassium  and  iron  in  the  red  cells,  sulphur 
in  hair  and  horn,  potassium  in  sweat,  sulphur  in  proteid, 
and  lime  in  bones,  etc.  Animals  fed  on  a  diet  which  is  as 
far  as  possible  rendered  free  from  salts  soon  die.  When  a 
deficiency  in  salts  occurs,  the  body  apparently  for  some 
time  draws  on  its  own  store,  and  then  certain  nutritive 
changes  follow.  Cattle  in  South  Africa  suffer  from  inflam- 
matory conditions  of  the  skeleton  (osteo-malacia)  in  con- 
sequence of  deficiency  of  phosphate  of  lime,  and  the  disease 
can  be  cured  by  its  administration.  Young  animals  may 
exhibit  nutritive  changes  in  the  bones  owing  to  a  diet 
poor  in  calcium  salts. 

The  chief  salt  used  by  herbivora  is  potassium,  whilst 
sodium  is  used  by  carnivora.  Both  carnivora  and  herbivora 
obtain  in  their  natural  diet  a  sufficiency  of  these  salts, 
though  the  general  impression  is,  that  the  wild  herbivora 
long  for  sodium.  It  is  quite  certain  that  under  the  con- 
ditions of  domestication  horses  can  be  kept  in  perfect  health 
without  receiving  any  sodium  chloride,  other  than  that 
contained  in  the  food,  and  the  amount  of  this  in  vegetable 
substances  is  small.  The  iron  required  by  the  blood  is 
probably  furnished  in  some  organic  combination.  It  is 
evident  that  the  daily  quantity  of  salts  required  must 
depend  upon  the  age  of  the  animal,  young  growing  animals 
requiring  more  than  adults. 


328     A  MANUAL  OF  A^ETEEINARY  PHYSIOLOGY 

Storage  of  Tissue. — Every  diet  must  contain  the  food 
principles  we  have  been  considering,  viz.  : 

Proteid. 

Fat  or  carhohydrate,  or  both. 

Salts. 

It  is  interesting  to  learn  in  what  proportion  these  are 
stored  up  in  animals  being  fattened,  also  the  amount  of 
food  required  for  a  definite  increase  in  weight,  and  the  rate 
at  which  that  increase  occurs.  This  is  shown  in  the 
following  table  from  the  classical  experiments  of  Lawes 
and  Gilbert : 


Proportion  of    Food    Principles  stored   up  for    every   100  lbs. 
Increase  of  Body  Weight. 


Oxen 
Sheep 
Pigs 


Proteid. 

Fat. 

Salts. 
1-6 

9-0 

58 

7-5 

63 

2-0 

7-0 

66 

0-8 

Amount  of  Dry  Sub- 
stance in  Food  re- 
quired to  produce 
100  lbs.  Increase  in 
Weight. 


Weekly  Increase 
in  Body  Weight. 


1,109 
912 
420 


1-0  % 

1-75  % 

6  to  6-5  % 


The  table  shows  that  in  all  cases  the  chief  increase  in 
body  weight  is  due  to  the  deposition  of  fat.  The  ox  lays  on 
the  most  proteid,  the  sheep  stores  up  the  largest  amount  of 
salts,  the  ])\g  puts  on  the  most  fat,  and  fattens,  not  only  on 
the  smallest  amount  of  food,  but  in  the  shortest  time. 

Water. — The  amount  of  water  found  in  the  tissues  of 
animals  is  very  constant,  as  may  be  seen  from  the  table  on 
p.  314,  where  the  body  water  in  different  animals  varies 
only  from  57*9  per  cent,  to  59  per  cent.  The  muscles  of 
creatures  as  far  removed  as  the  pig  and  the  snail,  the  ox 
and  the  lobster,  contain  78  to  79  per  cent.,  and  other 
tissues  are  equally  uniform. 

Under  the  influence  of  rest  and  work  varying  quantities 
of  water  are  lost,  and  in  hot  weather  the  loss  is  still  further 
increased.  It  has  been  calculated  that  a  man  may  lose 
water  at  the  rate  of  5  per  cent,  of  his  body  weight  on  a  hot 


NUTEITION  329 

day,  and  that  muscular  work  in  hot  weather  may  increase 
the  output  of  water  as  much  as  six  times,  but  we  are  not 
av/are  of  any  exact  experiment  on  this  question  on  animals, 
though  we  know  practically  that  the  loss  of  water  is  con- 
siderable. Of  the  total  water  received  in  the  food  or 
consumed,  the  bulk  passes  away  by  the  kidneys  ;  during 
work  a  considerable  amount  is  lost  by  the  skin  and  lungs, 
and  less  in  consequence  passes  by  the  kidneys. 

The  very  constant  proportion  of  water  in  the  tissues 
shows  that  the  consumption  of  excessive  amounts  of  fluid 
does  not  lead  to  storage.  Adjustments  are  readily  effected 
and  the  excess  of  fluid  in  the  blood  is  rapidly  got  rid  of. 
All  animals  withstand  a  deficiency  of  water  badly  ;  the 
horse  is  probably  the  weakest  in  this  direction,  and  shortage 
of  water  is  far  more  immediately  serious  for  any  horse  than 
shortage  of  rations.  A  thirsting  animal  dies  when  it  has 
lost  10  per  cent,  of  its  body  weight  in  water,  though  50  per 
cent,  of  its  proteid  and  the  whole  of  its  fat  will  disappear 
before  death  from  starvation  ensues.  A  man  may  avoid 
putting  on  weight  by  keeping  himself  short  of  fluid,  and 
horses  will  rapidly  lose  condition  by  having  their  water  supply 
limited.  Without  sufficient  water  intestinal  digestion  in 
herbivora  cannot  go  on ;  the  contents  of  the  colon  and 
caecum  of  the  horse  must  be  kept  fluid,  and  much  of  the 
water  consumed  is  devoted  to  the  purposes  of  digestion. 
Further,  the  blood  must  be  kept  fluid  and  concentration 
avoided  ;  in  the  first  instance  the  concentrated  blood  draws 
on  the  tissues  for  fluid,  but  later  on  this  source  dries  up, 
and  unless  dilution  of  the  blood  be  effected,  death  is  only  a 
matter  of  time,  and  with  horses  undergoing  severe  exertion, 
a  very  short  time  before  complete  collapse  occurs. 

Starvation. — When  an  animal  is  starved  it  lives  on  its 
own  tissues ;  in  the  herbivora  the  urine  becomes  acid, 
hippuric  is  replaced  by  uric  acid,  and  the  secretion  becomes 
transparent.  The  elimination  of  nitrogen  by  the  starving 
animal  at  first  falls  rapidly,  then  gradually,  and  shortly 
reaches  a  fluctuating  daily  quantity.  During  starvation 
the  carbonic  acid  excreted  falls  in  amount,  and  the  oxygen 


330    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

absorbed  becomes  reduced,  though  not  in  proportion  to  the 
fall  of  carbonic  acid.  If  water  be  given  life  is  considerably 
prolonged  ;  Colin  records  a  case  where  a  horse  receiving 
water  lived  thirty  days  without  food.  It  is  notorious  that 
herbivora,  though  they  lose  less  proteid  during  starvation 
than  carnivora,  do  not  withstand  starvation  so  well ;  nor 
need  we  go  so  far  as  a  starvation  experiment  to  ascertain 
this  fact.  When  men  and  horses  are  being  hard  worked, 
the  loss  in  condition  amongst  the  horses  sets  in  early,  and 
is  extremely  marked  for  some  time  before  the  men  show  any 
appreciable  muscular  waste. 

Horses  have  been  known  to  live  without  food  or  water 
for  as  long  as  three  and  dogs  for  four  weeks ;  but  it  is 
said  that  if  horses  have  suffered  15  days'  starvation,  the 
administration  of  food  after  this  time  will  not  save  them. 
Colin  records  an  experiment  where  a  horse  weighing 
892  lbs.  died  after  30  days'  starvation,  only  being  allowed 
2|^  pints  of  water  per  diem.  The  animal  was  nourished 
on  its  own  tissues,  the  daily  loss  in  weight  being  5*9  lbs., 
which  must  be  considered  as  exceptionally  small.  Dewar* 
records  two  remarkable  instances  of  the  length  of  time 
sheep  will  withstand  starvation  ;  in  one  instance  eighteen 
sheep  were  buried  in  the  snow  for  six  weeks  and  only  one 
died.  In  the  second  case  seven  sheep  were  buried  for 
eight  w^eeks  and  five  days,  and  all  were  recovered  alive  and 
eventually  did  well. 

In  some  very  accurate  experiments  on  a  starving  cat,  it 
was  shown  that  the  principal  loss  occurred  in  the  fat, 
97  per  cent,  of  which  disappeared  in  13  days.  The  follow- 
ing table  shows  the  percentage  of  dry  solid  matter  lost  by 
the  tissues  : 


Fat 

-     97   per  cent, 

Spleen    - 

-     63-1      „ 

Liver 

-     56-6      „ 

Muscles  - 

-     30-2      „ 

Blood     - 

-     17-6      „ 

The  loss  in  the  glandular  organs  was  very  heavy ;    next 

*   Veterinarian,  May,  1895. 


NUTEITION  331 

followed  the  muscles,  and  then  the  blood.  The  central 
nervous  sjstem  suffered  no  loss  ;  evidently  its  nutrition  was 
kept  up  at  the  expense  of  other  tissues  of  less  importance. 
Old  animals  bear  starvation  much  better  than  young  growing 
ones,  as  their  requirements  are  smaller. 

Cause  of  Body  Waste. — The  work  of  the  body  may  be 
described  as  internal  and  external.  By  internal  loork  we 
refer  to  respiration,  the  action  of  the  heart,  movement  of 
the  bowels,  animal  heat,  etc. ;  by  external  work  is  under- 
stood those  movements  of  the  muscles  which  transport  the 
body.  Every  diet  given  to  an  animal  must  take  these  two 
factors  into  consideration  ;  the  ration  of  subsistence  is  the 
minimum  diet  necessary  for  the  internal  work  of  the  body 
without  incurring  loss  of  weight,  the  animal,  of  course, 
doing  no  work ;  the  ration  of  labour  furnishes  the  actual 
muscular  energy  employed  during  work.  The  changes 
undergone  by  food  in  providing  energy  as  heat  and  motion 
fall  principally,  if  not  exclusively,  on  the  non-nitrogenous 
elements  ;  this  has  been  settled  beyond  all  doubt.  Con- 
sidering that  no  animal  can  live  on  a  nitrogen-free  diet, 
and  that  the  harder  the  work  performed,  the  larger  is  the 
amount  of  nitrogen  required,  one  would  have  thought,  as 
Liebig  did  years  ago,  that  the  source  of  energy  in  food  was 
the  proteid  substance.  This  is  not  so,  therefore  the  urea 
is  no  measure  whatever  of  the  work  performed,  in  fact  is 
hardly  affected  by  work,  though  it  is  largely  affected  by  the 
amount  of  nitrogen  received  in  the  food. 

During  work  the  heart  and  respirations  are  quickened, 
the  horse  sweats,  and  a  larger  volume  of  air  is  warmed  in 
the  kmgs  ;  all  this  means  a  loss  of  heat  to  the  body.  In 
addition  the  muscles  produce  heat  as  the  result  of  contrac- 
tion, in  fact  every  process  seems  to  tell  essentially  on  the 
non-nitrogenous  elements  of  the  body,  which  is  the  explana- 
tion why  carbo-hydrates  are  so  necessary  in  the  diet  of 
hard-worked  horses. 

The  Energy  yielded  by  Food  has  been  ascertained  by 
burning  the  substance  in  a  calorimeter  and  measuring  the 
amount  of  heat  given  off ;  in  this  way  the  potential  energy 


332    A  MANUAL  OF  YETERINAEY  PHYSIOLOGY 

of  proteid,  fat,  and  carbo-hydrate  has  been  ascertamed. 
Every  1  gramme  (15*432  grains)  of  water  in  the  calorimeter 
raised  1°  C.  {VH°  F.)  is  called  a  heat  unit;  by  this  method 
of  investigation  it  has  been  found  that 

1  gramme  of  average  proteid  evolves,  approximately,  when  oxidized, 

5,770  heat  units,*  or  5'7  large  calories. ■•■ 
1  gramme  of  fat  evolves,  when  oxidized,  9,300  heat  units,  or  9  "3  large 

calories. 
1  gramme  of  carbo-hydrate  evolves,  when  oxidized,  4,100  heat  units, 

or  4'1  large  calories. 

Proteids,  unlike  carbo-hydrates  and  fats,  are  not  com- 
pletely oxidized  in  the  body,  inasmuch  as  the  nitrogen  they 
contain  reappears  in  the  excreta  in  the  form  of  urea. 
Now  the  complete  oxidation  of  1  gramme  of  urea  yields 
2,523  calories  (or  2*5  kilo-calories),  which  must  be 
subtracted  from  the  value  given  al)ove  for  the  potential 
energy  of  proteids  in  order  to  ascertain  the  energy-value  of 
proteids  actually  available  by  the  body.  Speaking  approxi- 
mately, 1  gramme  of  proteid  gives  rise  to  1-  gramme  of 
urea,  hence  the  heat  of  combustion  of  proteids  must  be 
diminished  by  i  of  2,523  =  841  calories,  before  we  apply  the 
data  to  the  body.  This  gives  us  a  heat-value  for  average 
proteids  of  4,929  calories  or  4'9  kilo-calories,  as  ])ased  on 
purely  physical  determinations.  As  a  matter  of  fact,  all 
the  nitrogen  given  as  proteid  does  not  reappear  externally 
as  urea,  nor  is  it  all  excreted  through  the  urine ;  some 
passes  off  in  the  faeces.  Making  allowance  for  this,  it 
appears,  from  Piubner's  valuable  experiments  on  living 
animals,  that  the  working  value  for  an  average  proteid  is 
about  4*1  kilo-calories. 

The  Amount  of  Food  Required. — The  minimum  amount  of 
food  required  by  horses  during  idleness  has  been  deter- 
mined experimentally ;  the  amount  required  for  work  can- 
not be  fixed  with  precision  owing  to  individual  variations ; 
what  is  sufficient  for  one  is  insufficient  for  another.     Still, 

*  One  heat  unit  or  small  calorie  is  the  quantity  of  heat  necessary  to 
raise  1  gramme  of  water  1°  C.  in  temperature, 
■f  For  definition  see  footnote,  p.  324. 


NUTRITION  333 

diet  tables  for  working  horses  have  been  constructed  on  the 
basis  of  the  mean  amount  found  by  practical  experience  to 
be  necessary. 

Suhsiste)ice  Diet. — This  is  the  diet  necessary  for  the 
internal  work  of  the  body,  the  weight  of  the  animal  re- 
maining unchanged  ;  it  represents  the  minimum  amount 
of  food  required  by  horses  doing  no  work.  Grandeau  and 
Leclerc  kept  three  horses  for  a  period  of  from  four  to  five 
months  on  a  diet  consisting  of  17"6  lbs.  (8  kilos.)  of  meadow 
hay.  The  animals  led  a  life  of  idleness  with  the  exception 
of  receiving  half  an  hour's  walking  exercise  daily.  The 
17"6  lbs.  of  hay  furnished  as  a  mean  7*02  lbs.  of  dry 
digestible  organic  matter  for  every  1,000  lbs.  of  body 
weight ;  the  7*02  lbs.  of  organic  matter  contained  '538  lb. 
of  digestible  proteid.  The  subsistence  diet  for  three  horses 
for  24  hours  was,  therefore,  as  follows  for  every  1,000  lbs. 
of  body  weight : 

Proteid  ....       -538  lb.  -244  kilo. 

Non-nitrogenous   -  -  -     6'4S2  lbs.  2'946  kilos. 


7-020  lbs.  3-190  kilos. 

This   amount   of   hay   (7*02   lbs.)    contains   the   following 
elements : 

Carbon    -  -  -  3-563  lbs.                           1-619  kilos. 

Hydrogen  -  -  -385  lb.  (6-16  ozs.)         -175  kilo. 

Oxygen  -  -  -  2-986  lbs.                           1-357  kilos. 

Nitrogen  -  -  -086  lb.  (1-376  ozs.)       '039  kilo. 

Assuming  the  correctness  of  Grandeau's  observations,  we 
may  accept  the  above  amounts  of  carbon,  hydrogen,  and 
nitrogen,  as  approximately  representing  a  horse's  require- 
ments for  24  hours  during  idleness,  the  animal  neither 
gaining  nor  losing  weight.  The  ratio  of  nitrogen  to 
carbon  in  the  above  diet  is  1  :  41 ;  the  ratio  of  the  proteids 
to  the  non-nitrogenous  fats  and  carbo-hydrates  is  1  :  12. 

From  a  table  furnished  by  Grandeau  and  Leclerc,  it 
would  appear  that  no  matter  what  the  nature  of  the  diet 
may  be,  horses  require  between  7  lbs.  and  8  lbs.  of  dry 
digestible  organic  matter  daily  for  ever}^  1,000  lbs.  of  body 


334     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


weight,  in  order  to  maintain  the  nutrition  during  idleness. 
The  following  is  the  table  referred  to : 


Diet. 

lu  the 
Ratiou. 

Amount 
digested. 

Amount  for 
1,000  lbs.  of 
Body  Weight. 

Hay  alone 

Maize  and  oat  straw 
Maize,  oats,  hay  and  straw 

>>                   H                   )1                                 11 

Oats  alone  (crushed) 

14-08  lbs. 

11-57   „ 
9-48  „ 
9-49  „ 
8-59   „ 

6-09  lbs. 
8-33    „ 
7-30   „ 
6-74   „ 
6-41    „ 

7-02  lbs. 
8-22    „ 
7-50    „ 
7-45    „ 
7-02    „ 

In  some  German  experiments  made  by  Wolff  on  the  sub- 
sistence ration,  8*3  lbs.  of  digestible  dry  organic  material 
were  found  necessary  to  maintain  the  body  weight,  and 
from  this  the  digestible  fibre,  1*6  lbs.,  was  deducted,  as  in 
the  experience  of  Wolff  the  fibre  digested  by  horses  was  of 
no  value  as  sustenance  either  at  work  or  rest.  In  our  own 
experiments  on  the  essential  diet  for  horses,  we  found  the 
body  weight  could  be  maintained  on  12  lbs.  hay. 

The  essential  diet  presupposes  that  the  food  possesses 
a  sufficient  proportion  of  digestible  proteids.  In  one  of 
Grandeau's  experiments  a  horse  received  33  lbs.  of  wheat- 
straw  per  diem  which  furnished  13  lbs.  of  digestible  matter 
daily  (nearly  twice  the  amount  actually  required),  but  this 
diet  only  supplied  'IS?  lb.  of  digestible  proteids,  or  less 
than  one-third  of  the  minimum,  the  result  being  the  horse 
died  from  starvation.  The  essential  diet  for  an  ox  weighing 
1,000  lbs.  is,  according  to  the  experiments  of  Wolff,  "5  lb. 
to  "6  lb.  of  proteid,  and  7  lbs.  to  8  lbs.  of  non-nitrogenous 
matter  reckoned  as  starch ;  the  ratio  of  nitrogenous  to 
non-nitrogenous  matters  is  as  1  :  14.  According  to  the 
same  authority  sheep  require  a  relatively  larger  essential 
diet,  owing  to  the  growth  of  the  wool  and  its  accompanying 
fat,  viz.,  for  1,000  lbs.  of  live  weight  "9  lb.  of  proteid  and 
10"8  lbs.  of  non-nitrogenous  matter,  the  ratio  being  1  :  12. 

Working  and  Fattening  Diet. — The  diet  for  horses  at  work, 
and  those  for  the  fattening  of  cattle,  sheep,  and  pigs,  is  a 
question  of  hygiene,  and  reference  should  be  made  to  works 
on  this  subject. 


NUTRITION  335 

Patholog-ical. 

Disorders  of  nutrition  occur  with  every  departure  from  the  normal 
condition,  though  much  more  apparent  in  some  disorders  than  others. 

Fever. — The  tissues  are  readily  broken  down  in  supplying  fuel  for 
the  increased  metabolism  which  is  giving  rise  to  the  abnormally  great 
production  and  loss  of  heat ;  both  the  fats  and  proteids  suiler,  andin  some 
disorders  it  is  remarkable  how  rapidly  wasting  occurs  once  it  sets  in. 
In  acute  lung  cases  this  is  very  obvious— in  a  fortnight  the  patient 
may  be  a  wreck.  The  increased  nitrogenous  metabolism  which  this 
indicates  suggests  an  increased  secretion  of  urea,  but  exact  work  in 
this  direction  is  still  much  needed.  During  fever  there  is  an  increased 
excretion  of  COo,  and  absorption  of  oxygen  ;  uric  acid  is  formed  by  the 
herbivora,  and  the  urine  becomes  acid. 

Marked  muscular  waste  may  occur  in  the  absence  of  fever ;  any- 
thing which  causes  a  drain  on  the  system,  such  as  internal  parasites, 
tuberculosis,  internal  growths,  etc.,  may  reduce  the  animal  to  little 
more  than  a  skeleton.  Starving,  under-feeding  or  over-working  of 
animals  are  obvious  causes  of  metabolic  change,  while  defective  teeth 
are  a  frequent  cause  of  the  same. 

Actual  change  in  structure  as  the  result  of  deficiency  of  a  food 
element  is  mentioned  on  p.  327  as  a  cause  of  osteomalacia.  Osteo- 
porosis in  the  horse  has  also  been  considered  as  due  to  a  deficiency  of 
salts  in  the  food,  but  the  weight  of  evidence  is  against  this  view.  An 
excess  of  salts  in  the  bowel  may  be  productive  of  considerable  trouble. 
One  form  of  intestinal  calculus  in  the  horse  is  due  to  the  amount  of 
ammonio-magnesium-phosphate  existing  in  the  bowel  through  feeding 
too  largely  on  bran. 

The  food-supply  may  be  deficient  in  proteids  or  carbo-hydi-ates 
or  both,  or  there  may  be  an  excess.  Disorders  froni  the  latter  cause 
are  very  evident  in  the  horse.  Lymphangitis  and  hsemoglobinuria 
are  diseases  of  the  horse  intimately  associated  with  over-feeding  and 
idleness,  and  have  no  parallel  in  any  other  animal. 

Broken  wind  is  referred  to  at  p.  120  as  having  its  origin  in  errors  in 
dieting  and  management,  such  as  a  bulky  and  innutritions  food  supplj', 
or  heavy  work  on  a  distended  stomach.  Apart  from  these,  there  may 
be  other  disorders  of  nutrition  responsible,  for  even  under  good  manage- 
ment the  production  of  the  disease  is  not  entirely  controlled. 


CHAPTER  XII 

ANIMAL    HEAT 

Oxidations. — In  dealing  with  internal  respiration  on  p.  101 
we  learnt  the  fundamental  fact  that  the  oxidations  of  the 
body  do  not  occur  in  the  blood  but  in  the  tissues.  By 
means  of  these  oxidations  heat  is  produced,  and  the 
substances  which  are  oxidized,  viz.,  proteid,  fat,  and  carbo- 
hydrate, have  already  been  studied  in  the  chapters  on 
digestion  and  nutrition.  The  manner  in  which  oxida- 
tions are  carried  out  in  the  tissues  is  not  clearly  under- 
stood, in  fact,  it  is  by  no  means  decided  how  oxidations 
occur  outside  the  body.  The  view  that  oxygen  directly 
unites  with  the  substances  oxidized  is  no  longer  accepted, 
for  it  is  known  that  oxidations  do  not  occur  in  the  absence 
of  watery  vapour.  In  spite  of  the  fact  that  oxidations 
within  and  without  the  body  are  very  similar,  and  in  their 
results  practically  identical,  the  conditions  under  which 
each  is  effected  are  not  the  same,  the  great  dividing  line 
being  the  relatively  low  temperature  at  which  oxidations  in 
the  body  are  effected.  It  is  probable  that  oxidations  in  the 
tissues  are  effected  under  the  influence  of  enzymes  and  not 
directly  by  the  presence  of  oxygen  in  the  tissues,  for  it  can 
be  shown  that,  provided  sufficient  oxygen  be  supplied,  any 
further  increase  does  not  affect  the  rate  of  oxidation.  We 
have  had  before  us  the  evidence  of  ferments  capable  of 
splitting  fat,  of  oxidizing  sugar,  of  converting  sugar  into 
glycogen,  glycogen  into  sugar,  and  of  acting  on  proteids  ; 
all  of  these  may  be  isolated  from  the  body  tissues,  and  are 
known  as  intracellular  enzymes.     Other  evidence  can  also 

336 


ANIMAL  HEAT  337 

be  adduced  of  the  existence  of  tissue  ferments,  by  the  fact 
that  living  tissue  removed  from  the  body  under  suitable 
conditions  will  be  found  to  digest  itself.  It  is  supposed 
that  the  enzymes  of  the  body  stimulate  the  oxygen  to 
activity ;  such  enzymes  have  been  called  oxidases  and  have 
been  found  both  in  plants  and  in  the  animal  body.  They  have 
not,  however,  been  found  in  connection  with  the  oxidation  of 
proteid,  fat,  or  carbo-hydrate,  though  this  may  yet  be  demon- 
strated. An  oxidase  effects  oxidation  in  the  presence  of 
oxygen,  but  enzymes,  which  only  act  in  the  presence  of 
hydrogen  peroxide,  are  called  pcj-oxidascs.  It  is  considered 
probable  that  the  splitting  up  of  food  stuffs  by  ordinary  hydro- 
lytic  ferments  is  the  first  stage  in  the  process,  and  this  is 
followed  by  the  action  of  oxidases ;  to  the  latter  is  due  the 
formation  of  carbon  dioxide,  water,  etc.,  and  the  production  of 
heat.  The  heat  so  formed  is  derived  from  the  oxidation  of  food 
stuffs,  as  described  in  the  chapter  dealing  with  metabolism, 
the  fats  and  carbo-hydrate  probably  yielding  in  the  body 
the  same  amount  of  heat  as  they  do  in  their  combustion 
outside  the  body,  while  the  nitrogenous  moiety  of  the 
proteid  is  not  fully  oxidized  inasmuch  as  urea  and  other 
waste  products  carry  away  with  them  at  least  one-third  of 
the  available  energy  of  proteid  (p.  332).  How  the  heat  so 
formed  is  distributed,  maintained  and  lost,  must  now  be 
considered. 

The  Body  Temperature. — One  important  division  of  the 
animal  kingdom  is  into  warm-blooded  and  cold-blooded 
animals.  A  p<nhil(Ahermal  or  cold-blooded  animal  is  one 
in  which  the  body  temperature  depends  upon  its  external 
surroundings.  When  these  are  cold  the  bodies  of  such 
animals  are  cold,  being  about  a  degree  or  so  higher 
than  the  medium  in  which  they  are  living.  Such  a 
condition  exists  in  reptiles,  fish,  etc.  A  komoithcrmal  or 
warm-blooded  animal  is  one  in  which  the  body  temperature 
is  independent  within  wide  limits  of  the  temperature  of  the 
medium  in  which  they  are  living :  whether  this  be  high  or 
low  makes  practically  no  difference.  Between  these  two 
come    a    class    partaking    of    the    characters    of    each, 

22 


338     A  MANUAL  OF  YETEEINARY  PHYSIOLOGY 

hihernatiiKj  animals  which  during  the  summer  are 
homoithermal,  and  during  the  long  winter  sleep  are 
poikilothermal. 

The  temperature  of  the  body  is  not  uniform,  the  interior 
is  warmer  than  the  exterior,  and  the  blood  in  the  interior 
veins  is  warmer  than  in  the  corresponding  arteries.  The 
blood  in  the  veins  leading  from  a  gland  in  a  state  of 
activity  has  a  higher  temperature  than  the  blood  which 
enters  the  gland.  In  the  animal  body  the  hottest  blood  is 
found  in  the  hepatic  veins,  while  the  blood  in  the  posterior 
vena  cava  is  hotter  than  that  in  the  anterior.  There  is  also 
a  difference  in  the  temperature  of  the  blood  in  the  right  and 
left  hearts  ;  it  is  generally  considered  that  the  blood  in  the 
right  heart  is  the  warmest,  though  Colin  found  that  in  the 
horse  the  blood  of  the  left  side  was  the  hottest.  The  brain 
has  also  a  high  temperature.  The  practical  aspect  of  the 
question  is  that  the  interior  of  the  body  is  hotter  than  the 
exterior.  A  surface  temperature  does  not  indicate  the 
temperature  of  the  body,  which  for  clinical  purposes  should 
be  taken  in  the  rectum.  With  the  air  at  freezing-point 
there  may  be  as  much  as  5*4°  Fahr.  (3*0°  C.)  difference  in 
temperature  between  the  rectum  and  the  thin  skin  of  the 
breast  in  the  horse,  while  at  the  same  external  temperature 
the  limbs  of  this  animal,  which  are  naturally  cold,  in 
consequence  of  the  underlying  tissues  having  very  little 
vascularity,  may  indicate  44°  Fahr.  (25-4°  C.)  difference 
between  the  pasterns  and  the  rectum. 

The  Normal  Temxierature  of  Animals. — The  wide  differences 
which  exist  in  the  normal  temperature  of  animals  of  the 
same  class  is  remarkable.  The  following  observations 
were  made  principally  by  Siedamgrotzky. 

Horse :  The  temperature  varies  between  100*4°  to 
100-8°  Fahr.  (38-0°  to  38-2°  C).  Age  has  a  slight 
influence : 

From  2  to    5  years  old  the  temperature  is         -     100"6° 

„  5  „  10 '     „             „             „                         -     100-4° 

„  10  „  15       „             „             „                       -     100-8° 

„  20                 „             ,,             „                98-4  to  100-2° 


ANIMAL  HEAT  339 

Cattle  :  The  normal  temperature  is  from  101-8°  to  102-0° 
Fahr.  (387°  to  38-8°  C).  Wooldridge*  places  the  mean 
temperature  at  101-4°  Fahr.  (88-5°  C),  and  gives  the 
variations  at  100-4°  Fahr.  (38°  C.)  to  102-8°  Fahr. 
(39-3°  C).  Compared  with  the  horse  the  daily  variations 
are  small.  Sheej) :  In  these  animals  the  greatest  variation 
in  temperature  occurs,  viz.,  101-3°  to  105*8°  Fahr.  (38-4° 
to  41-0°  C);  probably  the  majority  of  temperatures  lie 
between  103-6°  to  104-4°  Fahr.  (39-7°  to  40-2°  C).  The 
cause  of  the  variation  is  unknown.  Swine :  The  average 
temperature  is  103-3°  Fahr.  (39*0°  C),  varying  from 
100-9°  to  105-4°  Fahr.  (38-2°  to  40-7°  C).  Dog:  The 
dog  is  liable  to  important  variations  depending  on  the 
external  temperature;  according  to  Dieckerhoff  it  varies 
from  99-5°  to  103-0°  Fahr.,  (37  4°  to  39-4°  C);  other 
observers  put  it  at  100-9°,  101-3,°  and  101-7°  Fahr.  (38-2°, 
88-4°,  38-7°  C). 

Variations  in  Body  Temperature. — A  rise  or  fall  in  body 
temperature  does  not  necessarily  imply  an  increase  or 
diminution  in  the  production  of  heat.  A  rise  of  tem- 
perature might  be  caused  by  a  contraction  of  the  vessels  of 
the  skin,  due  to  external  cooling,  sending  a  larger  quantity 
of  blood  into  the  internal  and  therefore  hotter  joarts  of  the 
body ;  or  a  fall  of  temperature  may  be  due  to  the  greater 
cooling  which  occurs  when  the  vessels  are  dilated,  as  by  an 
external  rise  of  temperature.  To  demonstrate  increased  heat- 
production  it  is  necessary  to  show  that  the  metabolism  is 
increased,  that  more  oxygen  is  absorbed,  and  more  carbonic 
acid  produced.  In  all  animals  there  is  a  daily  variation 
in  temperature,  the  lowest  records  being  obtained  in  the 
early  morning,  2  to  4  a.m.,  the  highest  in  the  evening, 
6  to  8  p.m.,  after  which  the  temperature  falls  during  the 
night ;  these  variations  are  due  to  metabolism,  as  will  be 
shown  presently.  Muscular  work  and  the  oxidation  of  food 
are  the  chief  sources  of  heafc ;  during  rest  the  metabolism 
sinks,  the  tide  is  low,  while  during  activity  it  rises.     The 

*  '  The  Temperature  of  Healthy  Dairy  Cattle.'      See  Proceedings 
of  the  Eoyal  Dublin  Society,  vol.  x.,  part  iii.,  1905. 

22—2 


340    A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

temperature  of  the  young  animal  is  higher  than  that  of 
the  adult,  while  the  temperature  of  animals  living  in  the 
open  is  lower  than  those  under  cover ;  in  the  case  of  the 
horse  as  much  as  1°  Fahr.  difference  in  temperature 
has  been  registered  under  this  condition.  Other  causes  of 
variation  in  temperature  will  be  considered  presently.  The 
thermometer  does  not  tell  us  the  amount  of  heat  formed  in 
the  body,  it  only  indicates  the  difference  between  the  heat 
produced  and  the  heat  lost.  These  important  points  must 
now  be  studied. 

Heat  Production. — The  chemical  action  occurring  in 
tissues,  other  than  the  muscles,  as  oxidations  and 
leading  to  the  production  of  heat,  have  previously 
engaged  our  attention  ;  the  rest  of  these  changes  occur 
mainly  in  the  skeletal  muscles,  in  which  four -fifths 
of  the  daily  heat  produced  is  generated,  and  in  active 
glands  such  as  the  liver.  The  heat  furnished  by  glan- 
dular activity  is  amply  demonstrated  in  the  liver,  though 
certainly  not  in  all  secreting  glands.  The  tempera- 
ture of  the  blood  in  the  hepatic  veins  is  higher  than 
in  the  portal,  higher  even  than  in  the  aorta.  It  was 
shown  by  Bernard  that  in  the  dog  while  the  portal  vein 
was  registering  103-5°  Fahr.  (39-6°  C),  the  blood  in  the 
hepatic  veins  was  106*3°  Fahr.  (41-2°  C).  Every  muscular 
contraction  leads  to  the  formation  of  heat  in  the  muscle 
substances.  Experiments  performed  on  the  external 
masseter  muscle  of  the  horse  showed  that  during  contrac- 
tion the  thermometer  registered  5*0°  Fahr.  (2*8°  C.)  higher 
than  in  the  same  muscle  at  rest.  As  the  blood  streams 
out  of  the  muscle  its  temperature  is  higher  than  that  in 
the  corresponding  artery,  and  in  this  way  the  whole  mass 
of  blood  would  have  its  temperature  raised,  were  it  not  for 
mechanisms  by  which  the  heat  is  dissipated.  But  the 
excessive  production  of  heat  is  not  always  met  by  a 
sufficiently  rapid  compensation  by  loss,  and  a  high  tempera- 
ture may  in  consequence  be  produced  as  the  result.  This 
is  a  most  important  point  in  connection  with  working 
horses.     In  the  case  of  man  compensation  is  sufficiently 


ANIMAL  HEAT  341 

rapid,  and  little  or  no  rise  of  body  temperature  occurs  as 
the  result  of  work.  In  the  horse  it  is  otherwise  ;  half  an 
hoar's  trotting  may  raise  the  temperature  from  '7°  to 
2*7°  Fahr.  above  the  normal,  though  the  amount  of  rise 
is  largely  a  question  of  '  condition  ';  temperatures  of  10-4° 
to  105°  Fahr.  after  hard  work,  especially  in  a  hot  sun, 
are  not  uncommon.  With  rest  the  temperature  falls  in  the 
course  of  a  few  hours,  the  mechanism  for  getting  rid  of 
heat  being  able  to  cope  with  it.  With  animals  unfit  for 
work  through  want  of  condition  the  temperature  may  take 
longer  to  fall,  or  even  remain  above  the  normal  sufficiently 
long  to  be  designated  febrile,  and  '  fatigue  fever '  is  not 
unknown  in  man.  Fever  may  be  due  either  to  excessive 
production  of  heat  or  defective  dissipation.  In  the  above 
case  it  is  probable  both  factors  are  at  work. 

The  act  of  feeding,  which  involves  increased  muscular 
activity,  not  only  immediately,  but  subsequently  in  the 
muscles  of  the  whole  alimentary  canal,  raises  the  tempera- 
ture of  the  body.  In  the  dog  the  maximum  is  reached 
from  six  to  nine  hours  after  a  meal,  during  which  time 
from  20  to  25  per  cent,  more  heat  is  produced.  In  the 
horse,  according  to  Siedamgrotzky,  the  temperature  as  the 
result  of  feeding  may  rise  -4°  to  1-4°  Fahr.  ('2°  to  '8°  C), 
but,  according  to  this  observer,  there  is  no  similar  rise  in 
the  ox,  and  Wooldridge  found  not  more  than  "3°  F.  in 
dairy  cattle.  That  heat  is  formed  during  the  masticatory 
processes  we  have  already  seen  from  the  observations  on 
the  masseter  muscles  of  the  horse  ;  but  the  mechanisms  for 
regulating  heat  in  the  body  are  such  that  a  rise  of  anything 
like  1"4°  Fahr.  as  the  result  of  feeding  must  be  regarded  as 
exceptional. 

A  liberal  diet  causes  at  once  an  increased  production 
of  heat.  In  the  tropics,  or  with  a  high  external  temperature, 
even  a  moderate  diet  may  greatly  raise  the  amount  of  heat 
produced. 

Influence  of  the  Nervous  System  on  Heat  Production. — 
The  muscles  of  the  skeleton  are  not  always  actively  con- 
tracting, yet  heat  is  always  being  formed  in  them.     The 


342    A  MANUAL  OF  VETEEINAKY  PHYSIOLOGY 

heat  is  produced  as  the  result  of  muscle  tonus,  viz.,  the 
contracted  condition  of  the  muscles  essential  to  posture. 
There  is  also  in  operation,  even  with  the  most  trifling 
movement,  an  antagonism  to  muscular  contraction.  For 
example,  the  flexors  of  a  limb  cannot  contract  without  the 
extensors  being  thrown  into  a  condition  to  oppose  the 
movement.  This  heat  production  in  muscles  is  under  the 
control  of  the  nervous  system.  If  an  animal  be  poisoned 
with  curare  the  motor  end-plates  in  the  muscles  are 
paralyzed,  less  heat  is  now  being  formed  in  them  and  the 
temperature  sinks  ;  in  fact,  the  animal  becomes  for  the  time 
being  practically  cold-blooded,  the  body-temperature  rising 
and  falling  with  the  surrounding  temperature.  The  same 
condition  may  be  produced  by  dividing  the  spinal  cord 
behind  the  medulla.  In  chloroform  narcosis  heat  produc- 
tion is  greatly  interfered  with,  and  in  prolonged  opera- 
tions this  should  be  borne  in  mind  and  the  loss  of  heat 
provided  against.  Shivering  is  a  physiological  process 
associated  with  the  production  of  heat  to  compensate  for 
a  loss.  The  shivering  which  occurs  with  horses  after  being 
watered  during  winter  is  caused  by  the  consumed  water 
abstracting  heat  from  the  tissues  in  order  that  its  tempera- 
ture may  be  raised  to  that  of  the  body.  The  '  freshness  ' 
of  a  horse  on  a  winter's  morning  is  the  outcome  of  nervous 
impulses  instinctively  started  with  the  object  of  generating 
more  heat. 

Apart  from  contraction  it  is  believed  that  muscles  are 
the  seat  of  a  quiescent  heat  production  under  the  influence 
of  the  nervous  system,  and  that  chemical  changes  resulting 
in  production  of  heat  are  generated  as  the  result  of  nerve 
impulses.  Experimental  injury  to  the  corpus  striatum, 
the  so-called  *  heat  puncture,'  causes  an  increased  produc- 
tion of  heat  which  may  last  for  some  time,  without  apparently 
causing  the  animal  any  inconvenience.  Heat  centres  have 
also  been  located  in  other  portions  of  the  brain,  optic 
thalamus,  septum  lucidum,  etc.,  and  in  the  spinal  cord. 
By  some  it  is  supposed  that  this  extra  heat  production  takes 
place  in  the  liver,  but  the  balance  of  opinion  inclines  to 


ANIMAL  HEAT  343 

locating  it  in  the  muscles.  No  special  set  of  thermogenic 
nerves  has  yet  been  proved  to  exist,  and  it  is  probable 
that  the  chemical  changes  presided  over  by  a  central 
heat  centre  are  reflexly  effected  through  the  motor  nerve 
fibres  of  the  muscles.  The  bearing  of  this  view  on 
the  increased  production  of  heat  in  fevers  and  rapid 
muscular  wasting  in  febrile  conditions  is  obvious,  and 
capable  of  explaining  much  which  has  hitherto  been 
obscure. 

Heat  Loss. — Unless  some  conditions  exist  in  the  body  for 
the  regulation  of  the  temperature,  the  heat  resulting  from 
metabolic  activity  would  continue  to  raise  it  steadily  until  it 
accomplished  the  destruction  of  the  animal,  and  that  this 
is  no  mere  figure  of  speech  is  evident  from  the  fact  that  a 
horse  produces  sufiicient  heat  during  idleness  to  raise  the 
body  to  boiling  point  in  less  than  two  days.  In  order  to 
maintain  the  temperature  at  a  constant  point  heat  pro- 
duction and  heat  loss  must  balance.  This  balance  may 
be  struck  either  as  the  result  of  diminishing  the  production 
of  heat  or  as  the  result  of  increasing  the  loss.  The  tempera- 
ture of  the  body  may  rise  either  as  the  result  of  an  actual 
increase  in  metabolism  or  through  difficulties  in  getting  rid 
of  heat.  The  processes  by  which,  within  narrow  limits, 
accurate  and  prompt  adjustment  is  made  is  known  as  lieat 
regulation. 

If  cold  water  be  poured  on  a  hot  body  the  body  is  cooled ; 
if  the  surface  of  a  heated  body  be  wetted  and  the  water 
allowed  to  evaporate,  the  body  is  cooled.  If  a  cold  body 
be  placed  in  contact  with  one  which  is  hot,  heat  is  lost. 
And  processes  somewhat  similar  to  these  are  occurring  in 
the  animal  body. 

1.  By  Radiation  and  Conduction  heat  is  lost  to  surround- 
ing bodies,  provided,  of  course,  that  they  are  at  a  tempera- 
ture lower  than  the  animal's.  If  the  surrounding  medium, 
air,  wind,  or  such  like,  is  hotter  than  the  animal  body, 
then  heat  is  gained  instead  of  being  lost.  The  natural 
or  artificial  covering  of  animals,  be  it  hair,  wool,  or 
clothing,  checks  the  loss  by  radiation  and  conduction,  as 


344     A  MANUAL  OF  YETEEINAEI  PHYSIOLOGY 

in  a  dry  condition  they  are  bad  conductors  of  heat.  When 
wet,  however,  they  are  good  conductors  and  a  considerable 
amount  of  heat  is  lost  from  sweating  or  rain.  Clothing  acts 
by  imprisoning  a  larger  amount  of  warm  air,  the  air  so  con- 
fined being  a  bad  conductor. 

2.  By  Evaporation  from  the  skin  the  sweat  is  converted 
into  vapour  and  heat  is  lost,  the  rapidity  of  the  process 
depending  on  the  humidity  of  the  air  and  its  rate  of  move- 
ment. The  value  of  this  evaporation  as  a  source  of  heat 
loss  in  the  horse  is  considerable,  probably  higher  than  the 
figure  fixed  for  man,  viz.,  14'5  per  cent,  of  the  total,  but  no 
data  are  available.  Evaporation  is  constantly  occurring ; 
when  the  amount  of  sweat  is  small  it  is  evaporated  as  fast 
as  it  is  produced,  and  this  is  referred  to  in  the  chapter  on 
the  skin  as  insensible  perspiration.  The  sensible  persj)ira- 
tion  is  that  which  is  not  evaporated  as  rapidly  as  it  is 
produced,  and  is  the  source  of  a  much  greater  loss  of  heat. 

3.  Evaporation  from,  the  mouth  and  nostrils,  warming  of 
inspired  air,  and  vapoiirizinri  of  water  from  the  lungs.  The 
former  is  a  very  valuable  means  of  heat  loss  in  those  animals 
which  do  not  sweat  from  the  general  surface  of  the  skin  ;  the 
moist  nose  and  open  mouth  of  the  dog  are  good  examples 
of  the  principle,  and  in  a  much  smaller  degree  the  bedewed 
muzzle  of  the  ox.  The  warming  of  the  inspired  air  and 
the  vapourizing  of  water  from  the  lungs  are  most  important 
sources  of  heat  loss  in  those  animals  which  do  not  sweat. 
The  panting  respirations  of  the  dog,  and  of  cattle  and  sheep 
in  '  show '  condition,  are  simply  a  means  of  cooling  the  body 
by  warming  a  larger  volume  of  air,  and  so  indeed  are  the 
hurried  respirations  of  disease. 

4.  Bij  the  urine  and  feces  a  loss  of  heat  is  incurred  in 
warming  the  food  and  water  to  the  temperature  of  the 
body.  The  amount  of  loss  thus  brought  about  must  be 
relatively  considerable,  especially  in  winter,  at  which  time 
of  the  year,  as  we  have  previously  seen,  the  abstraction 
of  heat  is  so  great  as  to  cause  shivering  ;  experiment 
shows  that  drinking  a  pailful  of  water  at  50°  F.  may  cause 
the  body  temperature  of  the  horse  to  fall  "o"  to  '9°  F.     A 


ANIMAL  HEAT  345 

diet  of  roots,  containing  as  they  do  80  per  cent,  water,  is 
a  heavy  source  of  heat  loss  with  cattle  in  winter,  though 
both  in  the  case  of  the  water  consumed  and  the  succulent 
food  ingested,  no  actual  loss  of  heat  occurs  until  these  are 
excreted  as  urine  and  fteees. 

The  heat  lost  by  conduction,  radiation,  and  evaporation, 
is  greater  in  small  than  in  large  animals,  as  small  animals 
have  a  relatively  greater  surface  exposed  in  proportion  to 
their  body  weight.  A  dog  of  6Q  lbs.  weight  will  lose  79'5  per 
cent,  of  his  body  heat  by  radiation  and  conduction,  and 
20*5  per  cent,  by  the  evaporation  of  water,  whereas  a  dog 
weighing  8  lbs.  will  lose  91  per  cent,  by  radiation,  etc.,  and 
9  per  cent,  by  water  evaporation. 

The  skin  as  a  source  of  loss  of  heat  is  largely  controlled 
by  the  nervous  system.  Through  the  vaso-motor  nerves  the 
vessels  of  the  skin  are  constricted  or  dilated ;  when  the  vessels 
are  constricted  the  skin  becomes  pale  (though  this  may  not 
be  seen  owing  to  hair  and  pigment)  and  the  blood  is  thrown 
upon  the  internal  viscera,  where  it  is  additionally  shielded 
from  loss.  In  consequence  the  skin  becomes  cold  and  the 
loss  of  heat  less,  not  merely  as  the  result  of  the  lessened 
radiation,  but  chiefly  as  the  outcome  of  the  diminished 
evaporation.  When  the  vessels  are  dilated  the  skin  becomes 
flushed  and  hot,  the  veins  stand  out  with  blood,  and  a 
large  amount  of  heat  is  lost.  This  vaso-motor  action  is  an 
automatic  reflex  act,  as  also  is  the  nervous  control  over 
the  sweat  glands,  by  which  more  or  less  water  is  poured 
out  on  the  surface  of  the  body  and  heat  lost  by  its  evapora- 
tion, and  is  normally  set  in  action  by  changes  in  the 
temperature  of  the  surroundings. 

The  loss  of  heat  is  influenced  by  the  thickness  of  the 
natural  covering,  its  colour,  etc.  The  loss  of  heat  from  a 
rabbit  after  shaving  oft'  the  fur  is  one  and  a  half  times 
greater  than  before  shaving.  Sheep  before  shearing  excrete 
less  CO2  and  more  HoO  than  the  same  sheep  after  shearing. 
White  rabbits  lose  75  per  cent,  less  of  the  heat  lost  by  black 
or  grey,  for  white  not  only  absorbs  less  heat  during  the  day 
but  loses  less  heat  at  night.     Grey  horses  are  better  suited 


346     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

to  the  tropics  than  any  other  colour,  and  black  horses  least 
of  all.  The  black  skin  of  the  negro  protects  the  deeper 
tissues  from  the  sun's  rays,  from  which  it  might  be  argued 
that  black  horses  in  theory  should  stand  exposure  to  a 
tropical  sun  better  than  grey,  but  a  grey  horse  has  a  black 
skin  and  the  pigment  prevents  the  rays  from  penetrating. 
.Varnishing  the  skin  causes  a  rapidly  increased  loss  of  heat, 
so  that  the  animal  dies  from  cold  unless  rolled  up  in 
cotton- wool  (see  p.  284). 

Influence  of  Heat  and  Cold. — A  moderate  degree  of  cold 
applied  to  the  external  surface  of  the  body  increases  the 
production  of  heat,  due  to  increased  oxidations.  This 
results  from  reflex  motor  impulses  discharged  from  the  heat- 
regulating  mechanism.  At  the  same  time  the  appetite  is 
increased  to  meet  the  extra  demand,  and  foods  rich  in  fat 
are  instinctively  sought  after  by  man.  The  same  should  be 
observed  in  the  feeding  of  animals,  and  an  increase  allowed 
in  the  food  to  meet  the  extra  oxidations,  fat,  if  possible, 
forming  part  of  it.  The  body  will  stand  a  considerable  degree 
of  cold,  but  a  continuous  fall  in  external  temperature  cannot 
be  withstood ;  a  point  is  reached  where  the  rate  of  heat 
productions  is  below  that  of  heat  loss,  and  the  animal  dies 
from  cold.  Conversely  the  body  is  adjusted  to  withstand  a 
moderately  high  external  temperature ;  the  heat  of  Arabia 
or  India,  which  renders  surrounding  objects  such  as  metal 
too  hot  to  hold,  is  borne  with  impunity  by  the  acclimatized 
horse ;  the  heat-regulating  mechanisms  do  not  allow  the 
external  heat  to  be  stored  up,  but  a  continuous  rise  in  external 
temperature  cannot  be  borne,  and  a  point  arrives  when  the 
heat  kills,  for  the  discharge  of  heat  from  the  body  ceases,  it 
becomes  stored  up,  and  heat-stroke  follows.  A  far  higher 
temperature  can  be  borne  when  the  air  is  dry  than  when 
moist,  as  evaporation  from  the  surface  practically  ceases  in 
a  moist  atmosphere.  When  air  has  its  humidity  increased  by 
1  per  cent,  it  raises  the  loss  of  radiation  and  conduction 
32  per  cent.,  while  an  increase  by  25  per  cent,  in  the  humidity 
of  the  air  is  equal  to  an  increase  of  2°  C.  in  the  external 
air.   At  a  temperature  of  88°  F.  in  an  atmosphere  saturated 


ANIMAL  HEAT  347 

with  vapour  the  regulating  mechanism  of  man  is  exhausted, 
and  a  rise  in  body  temperature  occurs.  Horses  taken  from 
cold  to  hot  latitudes  have  to  learn  to  compensate,  and  until 
they  do  so  a  marked  rise  in  body  temperature  will  occur  as 
the  result  of  standing  in  a  hot  sun,  though  doing  no  work. 
This  passes  away  with  acclimatization. 

The  loss  of  body  heat  among  animals  lying  out  at  night  is 
partly  prevented  by  the  fatty  lining  to  the  peritoneal  cavity, 
which  saves  undue  conduction  of  heat.  Wet,  combined  with 
exposure,  causes  a  more  important  loss  of  heat  than  mere 
cold.  It  has  been  shown  from  exact  observations  on  man  that 
a  limb  clothed  in  wet  flannel  lost  34*4  per  cent,  more  heat 
than  the  same  limb  in  dry  flannel.  Animals  never  look  so 
wretchedly  miserable  as  after  a  night  of  cold  rain ;  under 
the  conditions  of  active  service  a  cold,  wet  night  is  certain 
to  kill  off  the  most  debilitated. 

A  physiological  resistance  to  cold  can  be  obtained  by 
training  ;  the  body  learns  to  regulate  its  loss  and  production 
of  heat,  and  this  brings  us  to  a  consideration  of  the  interest- 
ing practical  point  of  the  necessity  of  clothing  for  animals, 
especially  for  horses,  in  a  state  of  domestication. 

Some  animals,  such  as  the  horse,  ox,  and  sheep,  are  born 
fully  developed  and  clothed  ;  in  a  few  minutes  they  pass 
from  a  temperature  of  certainly  over  100°  F.  within  the 
womb  of  the  parent,  to  perhaps  freezing-point  on  the  bare 
ground.  The  power  of  regulating  their  temperature  is  fully 
established,  and  in  a  very  short  time  this  is  assisted  by 
muscular  movements  of  the  limbs,  which  are  learnt  very 
quickly  ;  the  gambols  of  young  animals  serve  some 
other  purpose  than  that  of  mere  lightness  of  heart.  If 
healthy,  cold  has  no  effect  on  these  young  creatures,  pro- 
vided the  parent  is  able  to  supply  sufficient  nourishment. 
There  are  other  animals,  such  as  newly-born  pups,  kittens, 
rabbits,  and  certain  birds,  such  as  pigeons,  which  are  born 
blind,  helpless,  and  more  or  less  naked  ;  they  cannot  move, 
are  unable  to  regulate  their  temperature,  and  if  taken 
from  the  maternal  warmth  their  body  temperature  steadily 
declines  and  they  die  from  cold.      In  these  the  capacity 


348    A  MANUAL  OF  YETERINARY  PHYSIOLOGY 

for  regulating  body  temperature  does  not  develop  for  some 
little  time  after  birth,  and  until  locomotion  becomes 
possible. 

We  have  seen  then  that  the  young  of  the  horse  comes 
into  the  world  prepared  by  its  heat-regulating  mechanisms 
to  deal  with  the  question  of  external  temperature,  and  as 
time  goes  on  this  is  supplemented  by  an  extra  growth  of 
hair  for  winter  use  and  a  lighter  covering  for  the  summer. 
If  no  interference  with  the  coat  be  practiced  it  is  un- 
doubted that  no  extra  covering  of  any  kind  is  required 
during  the  coldest  weather,  and  even  where  the  covering 
is  of  the  lightest,  as  with  the  thoroughbred  horse,  it  is 
sufficient  for  the  purpose.  The  thoroughbred  brood  mares 
of  this  country,  once  they  go  to  the  stud,  live  in  the  open 
for  the  remainder  of  their  lives  and  never  wear  a  blanket. 
And  practical  experience  tells  us  that  this  may  be  gradually 
imposed  on  all  horses  with  impunity,  even  those  which 
have  been  kept  in  hot  stables.  Coughs,  colds,  and  inflam- 
matory chest  affections,  usually  attributed  to  cold,  are  prac- 
tically unknown  among  horses  living  in  the  open,  even 
during  the  coldest  winter,  and  it  is  easy  to  show  that  these 
diseases  are  largely  the  result  of  the  artificial  conditions  under 
which  working  horses  have  to  live.  Is  it  possible  for  work- 
ing horses  to  be  clipped  and  yet  wear  no  hlankets  f  This 
question  is  not  only  one  of  hygiene,  but  also  of  physi- 
ology. Practical  experience  tells  us  they  may  be  clipped 
two  or  three  times,  even  in  the  coldest  winter,  and  pro- 
vided  tliey  are  well  fed  they  take  no  harm.  Colds  are 
absolutely  unknown,  and  the  explanation  of  these  facts  is 
that  the  horse  possesses  in  a  high  degree  the  power  of 
regulating  his  temperature.  The  nervous  mechanisms  we 
have  been  studying  are  kept  in  active  operation,  diminish- 
ing loss  or  increasing  production  as  the  case  may  be.  A 
somewhat  similar  mechanism  must  exist  among  the 
inhabitants  of  the  Polar  regions,  who  live  during  the 
winter  in  their  huts,  in  a  temperature  which  is  never 
above  freezing-point ;  adults,  and  even  children,  may  expose 
parts  of  their  bodies  to  the  external  air  at  a  temperature  at 


ANIMAL  HEAT  349 

which  mercury  freezes.     Such  exposure  to  the  European 
would  certainly  result  in  frost  bite. 

Clipping. — Siedamgrotzky  observed  the  effect  of  clipping 
on  the  temperature  of  horses.  He  found  that  the  tem- 
perature rose  after  clipping,  and  fell  to  normal  about  the 
fifth  day.  It  was  observed  that  clipped  horses  had  during 
exercise  a  higher  rectal  temperature  by  1'8°  Fahr.  than 
undipped  horses,  and  the  return  to  normal  temperature 
was  more  steady  and  regular  with  them  than  with  undipped. 
The  rise  in  temperature  after  clipping  may  be  due  to 
vaso-motor  action  ;  less  blood  being  in  the  skin,  more  will 
find  its  way  to  the  viscera,  viz.,  to  parts  of  the  body  which 
have  a  naturally  high  temperature,  the  result  being 
that  the  total  mass  of  blood  has  its  temperature  raised. 
Another  way  of  accounting  for  the  rise  in  temperature 
after  clipping  is  by  supposing  that  an  actual  increase  in 
the  production  of  heat  occurs.  This  may  be  due  to  stimu- 
lation of  the  skin  influencing  the  heat-forming  mechanism 
reflexly,  either  as  the  result  of  the  mechanical  stimulus,  or 
of  the  increased  cooling  of  the  skin  due  to  the  removal  of 
the  coat.  Colin  clipped  a  horse  on  one  side  of  the  body 
and  not  on  the  other ;  the  subcutaneous  temperature  in  the 
stable  was  : 

Clipj^ed  Side.         Undipped  Side.         Difference. 
86-9°  95°  8-1° 

The  animal  was  now  taken  out  into  cold  air  at  three 
degrees  below  freezing-point. 


Clip>ped 
Side. 

TJnclip)p)ed 
Side. 

Diferer 

In  30  minutes  the  subcutaneous 

temperature  was 

85-1° 

94-1° 

90° 

2h  hours  later      -             -             - 

79-9 

95-0 

15-1 

1  hour        „         -             -             - 

83-3 

95-5 

12-2 

1     „            „         - 

85-1 

96-1 

11-0 

The  cooling  of  the  clipped  side  is  very  marked,  the  tem- 
perature continuing  to  fall  for  three  hours,  while  the  slight 
fall  in  the  temperature  of  the  undipped  side  was  restored 
to  the  normal  in  three  hours. 

Hibernation. — The  effect  of  a  fall  in  the  temperature  of 


350    A  MANUAL  OF  VETEEINAEY  PHYSIOLOGY 

the  bodies  of  animals  is  to  produce  a  depression  of  meta- 
bolism. This  is  well  seen  in  some  mammals,  such  as  the 
dormouse,  which  sleep  all  the  winter,  during  which  time  they 
live  upon  the  store  of  fat  laid  up  in  the  tissues  during  the 
summer.  Owing  to  their  depressed  metabolism  this  store  is 
found  sufficient  to  keep  them  alive,  though  they  wake  up 
at  the  end  of  the  winter  mere  skeletons.  On  waking  up  the 
body  temperature  rises  by  bounds  to  the  normal,  the 
animal  then  returning  to  the  condition  of  an  ordinary 
warm-blooded  animal,  until  the  recurrence  of  the  next 
period  of  hibernation.  As  to  the  causes  of  this  remarkable 
phenomenon  we  know  but  little.  It  is  not  confined  to  only 
one  class  of  animals,  since  it  occurs  in  mammals,  amphibians, 
reptiles,  etc.  No  purely  anatomical  differences  suffice  to 
explain  why  some  animals  hibernate  and  others  do  not. 
External  cold  is  usually  assumed  offhand  to  be  the  initiating 
factor,  assisted  possibly  by  the  lessened  food  supply  at  the 
approach  of  winter.  But  some  other  more  recondite  cause 
than  either  of  these  must  exist,  since  marmots  may  some- 
times hibernate  in  the  summer,  dormice  will  hibernate  even 
if  kept  warm  in  the  winter ;  cold  will  not  necessarily  cause 
an  animal  to  hibernate  except  at  the  appropriate  season, 
and  severe  cold  may  even  arouse  a  hibernating  animal  from 
its  state  of  torpor. 

The  Amount  of  Heat  produced  by  animals  depends  upon 
the  rate  of  their  metabolism  and  the  surface  area  of  their 
bodies  as  a  factor  which  determines  loss  of  heat,  and  hence 
its  production  if  the  temperature  of  the  body  is  to  be  kept 
easily  constant.  A  large  animal  produces  actually  but  not 
relatively  more  heat  than  a  small  one  ;  a  small  animal,  as 
has  been  previously  stated,  has  a  greater  body  surface 
relative  to  its  weight  than  a  large  animal,  and  in  this  way 
its  loss  is  more  rapid.  As  heat  production  must  balance 
heat  loss,  the  small  animal  must  lose  more  heat,  and  there- 
fore produce  relatively  more  heat,  than  a  large  animal. 

The  heat  produced  is  measured  as  heat-units  or  calories,* 

*  See  footnote,  pp.  324,  332.     The  calorie  referred  to  here  in  the  text 
is  the  large  calorie. 


ANIMAL  HEAT  351 

and  the  amount  produced  per  hour  for  every  2*2  lbs.  of 
body  weight  is  given  by  CoHn  as  follows : 

Horse  -  -  -  -  -     2-1  calories. 

Sheep  -  -  -  -  -     2-6       „ 

Dog      -  -  -  -  -     4-0       „ 

A  horse  loses,  according  to  Colin,  20,684  large  calories  per 
diem,  or  sufficient  heat  to  raise  4,550  gallons  of  water 
1'8°  Fahr.,  or  to  raise  44  gallons  from  freezing  to  boiling 
point,  Wolff,  quoted  by  Tereg,  gives  a  table  showing  the 
heat  lost  per  diem  by  cattle,  horses,  sheep,  and  pigs,  for 
every  1,100  lbs.  of  body  weight : 

Horse  at  moderate  work         -  -  24,500  calories  (large). 

hard  work    -  -  -  37,200 

Ox  resting,  and  on  moderate  diet  -  18,600       ,, 

Sheep,  with  fine  wool  -  -  27,700       ,, 

Pigs,  fattening  -  -  -  35,000       ,, 

According  to  Despretz  a  dog  loses  393  calories  (large)  in 
24  hours,  and  a  man  2,700  in  the  same  time. 

Post-mortem  rises  of  temperature  are  frequently  observed. 
The  explanation  afforded  of  a  post-mortem  rise  in  tempera- 
ture is  that  metabolism  is  still  occurring  in  the  tissues, 
but  since  there  is  no  circulation  to  carry  the  heat  away 
the  temperature  of  the  part  rises. 


CHAPTER  XIII 

THE  MUSCULAR  SYSTEM 

The  muscular  system  is  the  largest  in  the  body,  the 
skeletal  muscles  alone  representing  45  per  cent,  of  the 
body  weight. 

The  regulation  of  the  blood  supply,  the  movements  of 
the  skeleton,  the  contraction  of  the  heart,  and  the  transport 
of  the  ingesta  along  the  intestinal  canal,  are  all  examples 
of  muscular  movement,  and  further  they  are  examples 
of  different  kinds  of  movement ;  the  slowly  moving  intes- 
tinal canal  is  very  different  from  the  active  skeletal  muscles, 
and  these  with  their  long  periods  of  activity  and  rest  are 
greatly  in  contrast  with  the  rhythmical  movements  of  the 
heart. 

Structure  of  Muscle.— There  are  three  varieties  of  muscle 
in  the  body : 

1.  Voluntary,  skeletal,  striped,  or  red  muscle. 

2.  Involuntary,  pale,  or  unstriped  muscle. 

3.  Heart  muscle. 

The  voluntary  muscles  are  generally  in  large  masses 
known  as  flesh,  and  their  function  is  to  move  the  skeleton. 
The  muscle  mass  consists  of  bundles,  the  bundles  are 
composed  of  smaller  bundles,  the  smaller  bundles  are 
made  up  of  fibres.  The  fibre  of  a  muscle  does  not  run  the 
length  of  the  bundle ;  on  the  other  hand  a  primitive  fibre  is 
only  about  1  inch  in  length,  and  of  microscopic  thick- 
ness, viz.,  s^^o-  of  an  inch  as  an  average.  The  fibre  is 
developed  from  a  single  cell,  and  surrounded  by  a  mem- 
brane, the  sarcolemma.  The  contents  of  the  fibre  are 
semi-fluid  and  composed  of  fibrils,  viz.,  minute  thread-like 

352 


THE  MUSCULAK  SYSTEM  353 

masses,  each  of  which  is  found  to  be  alternately  striped 
with  a  dark  and  light  band.  It  is  the  striping  which  gives 
to  muscle  its  characteristic  microscopic  appearance  of 
striation.  Histologists  are  not  agreed  as  to  the  detailed 
structure  of  the  fibrils,  but  Schiifer,  whose  views  are 
accepted  by  most  physiologists,  regards  the  fibrils,  or 
sarcostyles  as  he  terms  them,  as  divided  into  a  series  of 
masses  placed  end  to  end;  each  mass  is  known  as  a  sarco- 
mere, and  possesses  a  dark  centre  and  clear  ends.  The 
dark  and  light  stripes  which  result  from  this  arrangement 
are  composed  of  different  substances,  at  least  they  possess 
different  physical  properties.  During  contraction  the  fluid 
material  in  the  clear  ends  flows  into  the  dark  centre  by 
means  of  certain  pores.  Between  the  fibrils  is  a  coarse 
network  of  material  known  as  the  sarcoplas)n.  It  is 
generally  believed  that  the  fibril  constitutes  the  contractile 
portion  of  the  fibre,  the  sarcoplasm  being  of  a  nutritive 
nature. 

The  nerve  supply  to  muscle  is  both  motor  and  sensory  : 
through  the  soisivi/  nerves  the  brain  is  made  acquainted 
with  the  position  of  the  body  and  the  condition  of  muscular 
tension.  This  involves  the  existence  of  a  special  )ni(scle 
sense  which  plays  such  an  important  part  in  locomotion. 
In  the  muscles  this  sense  is  represented  by  special  bodies 
generally  found  near  tendons,  called  neiiro  -  inustidar 
spindles ;  these  are  from  ^  to  }r  of  an  inch  in  length,  and 
lYT  of  an  inch  in  width  ;  each  spindle  is  of  muscle  sur- 
rounded by  a  sheath,  and  has  a  sensory  nerve  entering  it 
at  one  end.  Nervous  structures,  known  as  the  tendon 
or(jans  of  Gohji,  also  exist  in  the  tendons  at  their  junction 
with  the  muscle  fibres ;  they  consist  of  spindle-like  bodies 
connected  with  one  or  more  fine  medullated  nerve-fibres. 
These  nerves  are  in  communication  with  certain  areas  in 
the  cortex  of  the  brain  which  are  devoted  to  the  '  muscle 
sense'  (see  'Senses,'  Section  4).  The  ordinary  degree  of 
sensibilitj'  in  muscle  is  not  very  great  unless  the  part  be 
cramped  or  inflamed,  though  pain  is  caused  when  they  are 
cut   into.     By  means  of  the    motor   nerves   the    muscle  is 

23 


354     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

supplied  with  the  impulses  which  bring  about  contrac- 
tion, division  of  the  motor  nerves,  or  interference  with 
their  function,  causing  paralysis  of  the  muscle  or  muscles 
supplied  by  them.  Each  motor  nerve  enters  the  primitive 
fibre  about  its  centre,  and  terminates  in  a  special  organ 
known  as  an  e)id  plate.  By  means  of  curari  this  end  plate 
may  be  paralyzed,  in  which  case  stimulation  of  the  nerve 
leads  to  no  muscular  response  in  consequence  of  the  block, 
though  the  muscle  itself  is  still  irritable  and  readily 
responds  to  direct  stimulation. 

Masses  of  material  built  up  on  the  lines  described  above 
are  intended  for  the  transport  of  the  body,  for  which 
purpose  they  are  united  to  the  skeleton  either  by  tendons 
or  by  the  direct  insertion  of  their  own  fibres.  In  the 
muscles  of  the  limbs  the  tendon  attachment  is  the  most 
usual,  wherever  in  fact  the  parts  are  exposed  to  great 
strain.  There  are  certain  muscles  in  the  machine  where 
the  strain  on  them  is  so  considerable  that  tendinous 
material  is  intimately  mixed  up  with  the  muscular  tissue  ; 
this  is  well  seen  in  the  masseters,  the  muscles  of  the  back, 
fore-arm,  and  thigh.  In  the  horse  provision  is  also  made 
for  the  muscles  of  the  limbs  being  rested  without  necessi- 
tating the  animal's  assuming  a  recumbent  position,  viz.,  by 
the  check  ligaments  in  the  leg ;  b}^  means  of  these  an  animal 
can  sleep  standing,  and  may  remain  standing  for  some 
weeks  without  suffering.  During  progression  the  entire 
strain  of  the  body  comes  on  the  feet  and  the  muscles  of 
the  limbs,  and  in  such  paces  as  galloping  the  muscular 
strain  is  enormous  ;  for  example,  during  the  canter  and 
gallop  a  weight  equivalent  to  that  of  the  whole  body  is 
imposed  on  a  foreleg.  But  this  is  a  question  to  be  dealt 
with  in  the  chapter  on  Locomotion. 

Muscle  Antagonism, — Every  muscle  or  group  of  muscles 
possesses  an  antagonist,  and  though  the  antagonist  may  be 
equal  in  size  this  is  not  always  the  case,  as  for  example  the 
great  difference  between  the  bulk  of  the  muscles  which  close 
the  jaw  as  compared  with  the  trifling  size  of  those  which 
open  it.    The  grouping,  in  co-ordination  of  action,  of  volun- 


THE  MUSCULAR  SYSTEM  355 

tary  muscles  is  a  question  to  be  considered  later,  in  the 
chaj)ter  on  the  Senses.  The  interest  which  is  here 
attached  to  antagonistic  muscles  is  connected  with  the  fact 
that  it  is  this  antagonism  which  keeps  the  muscles  of  the 
body  slightly  on  the  stretch,  so  that  if  one  be  cut  across 
it  gapes  in  consequence.  This  elastic  tension  ensures  that 
no  time  is  lost  in  a  muscle  coming  into  action,  as  there  is 
no  slack  to  take  up ;  the  muscle  stands  as  it  were  at  full 
cock. 

InvoliDitart;  or  pale  mnscle  is  not  found  in  masses  as  is  the 
red,  but  in  thin  sheets,  which  in  places  such  as  the  blood- 
vessels are  only  of  microscopic  thickness.  Pale  muscle  is 
employed  throughout  the  whole  length  of  the  digestive 
canal  from  the  stomach  to  the  rectum  ;  it  is  also  found  in 
the  bladder,  uterus,  spleen,  and  bloodvessels.  In  none  of 
these  places  is  the  sharp,  short,  active  contraction  of 
skeletal  muscle  required  ;  slow,  steady,  deliberate  move- 
ments are  essential  in  the  digestive  canal;  slow,  steady, 
expulsive  movements  are  necessary  in  the  bladder  and 
uterus,  and  even  in  the  bloodvessels,  where,  as  we  have 
seen,  the  muscular  tissue  acts  the  part  of  a  tap  :  it  is 
sufficient  if  the  tap  be  turned  on  or  turned  off  slowly  and 
steadily.  "When  we  come  to  study  muscular  contraction 
we  shall  see  how  rapidly  the  wave  passes  along  a  voluntary 
and  how  slowly  along  an  involuntary  muscle. 

In  structure  pale  muscle  consists  of  nucleated  spindle- 
shaped  cells,  dove-tailed,  and  held  together  by  a  cement 
substance ;  it  is  through  the  medium  of  this  cement 
substance  that  the  wave  of  excitation  passes  from  cell  to 
cell,  thus  forming  a  great  contrast  to  red  muscle,  where,  as 
we  shall  see,  the  whole  contracts,  not  by  the  spread  of  the 
stimulus  from  one  fibre  to  another,  but  as  the  result  of  all 
the  fibres  being  stimulated  simultaneously.  There  are 
nerves  and  ganglion-cells  in  abundance  in  pale  muscle ; 
the  nerves,  which  are  chiefly  non-medullated,  form  a  fine 
plexus,  with  the  ganglion-cells  placed  at  the  junctions  of 
the  plexus.  It  is  probably  due  to  these  that  involuntary 
muscle  continues  to  contract  when  all  connections  with  the 

23—2 


356     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

centre  are  destroyed,  though  some  physiologists  see  reason 
for  thinking  that  the  contraction  of  pale  muscle  may  be 
carried  out  just  like  that  of  heart-muscle,  viz.,  as  a  self- 
acting  mechanism,  independent  of  any  nervous  connections. 

The  nerve  supply  of  involuntary  muscle  is  peculiar  and 
presents  a  great  contrast  to  red  muscle ;  "whereas  the  latter 
only  receives  one  variety  of  motor  supply,  pale  muscle 
receives  two,  viz.,  one  set  of  fibres  which  stimulates  con- 
traction, and  another  which  inhibits  it.  Both  sources  are 
derived  from  the  sympathetic  system,  which  again  is  in 
great  contrast  to  the  arrangement  of  the  nerve  supply  to 
red  muscle. 

Heart  muscle  is  in  structure  both  red  and  striated,  never- 
theless it  is  involuntary  ;  the  fibres  are  characterized  by 
being  formed  of  branched,  nucleated,  quadrilateral  cells, 
while  the  sarcolemma  is  absent.  As  we  have  already  seen, 
the  contraction  of  the  heart  is  primarily  dependent  on  the 
properties  of  its  muscle-substance,  though  the  automatism 
is  carefully  directed  by  nervous  mechanisms. 

Muscular  Contraction. — This  apparently  simple  act  is 
extremely  complex,  and  will  require  to  be  dealt  with  in 
some  little  detail. 

Muscles  are  tissues  possessed  of  irritability  and  con- 
tractility, viz.,  they  possess  the  power  of  responding  by  a 
movement  to  the  application  of  a  stimulus.  The  normal 
stimulus  is  effected  through  the  motor  nerves  under  the 
control  of  the  brain  or  spinal  cord,  but  of  the  nature  of 
the  stimulus  we  are  ignorant.  A  coarse  reproduction  of 
it  can  be  effected  by  pinching,  pricking,  chemical,  thermal, 
or  electrical  stimuli,  applied  to  either  the  nerve  or  the 
muscle  itself,  and  to  all  these  the  three  varieties  of  muscle 
are  responsive.  When  a  muscle  contracts,  in  addition  to 
becoming  shorter  and  thicker,  it  also  undergoes  changes  in 
its  extensibility,  elasticity,  and  temperature  ;  there  are  also 
alterations  in  its  electrical  condition  and  chemical  com- 
position, and  these  can  all  be  studied  by  employing  a 
muscle  of  the  frog,  which  retains  its  irritability  for  a  long 
time  after  removal  from  the  body.     Such  a  muscle  suitably 


THE  MUSCULAR  SYSTEM 


357 


prepared  is  knon'ii  as  a  muscle-nerve  'preparation,  and  with 
certain  modifications  what  is  found  to  occur  in  this  as  the 
result  of  contraction,  occurs  also  essentially  in  the  living 
mammalian  muscle. 

The  muscle  most  usually  and  conveniently  employed  for 
investigating  the  phenomena  of  muscular  contraction  is  the 
gastrocnemius  of  a  frog,  dissected  out  in  such  a  way  as  to 
leave  its  upper  tendon  connected  with  a  piece  of  the  femur 
and  its  lower  tendon,  the  tendo  AchilUs,  intact  though  free. 


Fig.  75. — A  Muscle-Nerve  Preparation-  (Foster). 

m,  the  gastrocnemius  muscle  of  a  frog;  n,  the  sciatic  nerve  dissected 
out  back  to  5"/)  c,  the  end  of  the  spinal  canal ;  /,  femur  ;  cl,  clamp  ; 
t  a,  tendo  Achillis  with  S-hook  attached. 


At  the  same  time  care  is  taken  not  to  sever  the  connection 
of  the  muscle  with  its  motor  nerve,  the  sciatic,  which  is 
dissected  out  for  some  considerable  distance  back  towards 
its  point  of  exit  from  the  spinal  canal,  the  central  end 
being,  if  desired,  left  connected  to  a  portion  of  the  spinal 
cord  enclosed  in  a  piece  of  the  lower  end  of  the  spinal 
column.  The  muscle  is  then  suspended  by  fixing  the 
remains  of  the  femur  in  a  clamp ;  a  small  S-hook  is  then 
attached  to  the  tendo  Achillis.    This  muscle-nerve  prepara- 


358     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

tion  and  its  arrangement  as  above  described  is  shown  in 
Fig.  75. 

For  purposes  of  experiment  the  clamp  is  fixed  inside 
a  small  chamber  with  glass  sides  to  prevent  the  drying 
of  the  muscle  and  nerve ;  this  is  effected  by  placing 
a  few  pieces  of  wet  filter-paper  inside  the  chamber.  The 
sciatic  nerve  is  laid  over  a  pair  of  electrodes  connected  by 
wires  to  binding-screws  outside  the  chamber;  by  this  means 
any  desired  electrical  stimulus  may  be  applied  to  the  nerve. 
A  thread  attached  to  the  hook  in  the  tendo  Achillis  passes 
through  a  hole  in  the  floor  of  the  moist  chamber,  and  is 
connected  with  a  horizontal  lever  free  to  move  in  a  vertical 
plane,  a  small  weight  being  hung  under  the  lever  to  give 
the  muscle  the  '  load '  necessary  for  its  efficient  contraction. 
The  free  end  of  the  lever  is  then  brought  to  bear  against 
the  vertical  surface  of  some  recording  apparatus,  usually  a 
cylindrical  drum,  covered  with  smoked  paper,  made  to 
rotate  by  clockwork  about  a  vertical  axis. 

The  most  conveniently  controllable  stimulus  is  that 
obtained  as  single  induction  currents  or  the  interrupted 
current  of  an  induction  coil.  These  have  the  advantage  of 
being  extremely  efiicient  as  stimuli  and  of  giving  rise  in 
the  nerve  to  impulses  which  we  may  regard  as  the  nearest 
artificial  approach  to  the  impulses  which  in  the  body  are 
discharged  along  the  nerves  by  the  cells  of  the  central 
nervous  system.  The  complete  arrangement  of  all  the 
apparat  us  is  clearly  shown  in  Fig.  76. 

An  inspection  of  Fig.  76  shows  at  once  that  if  the  drum 
of  the  recording  instrument  alone  rotates,  the  end  of  the 
lever  connected  to  the  muscle  must  trace  out  a  horizontal 
line  on  the  smoked  surface.  If  the  drum  is  stationary  and 
the  muscle  is  made  to  contract,  the  lever  will  trace  a 
vertical  line.  If  now  the  muscle  is  made  to  contract  while 
the  drum  is  rotating  these  two  lines  are  compounded  into  a 
curve  whose  ordinates  at  each  point  of  the  curve  and  whose 
general  shape  give  us  exact  information  as  to  the  details 
of  the  contraction  from  start  to  finish.  Such  a  curve  is 
called  a  '  muscle-curve  '  and  is  typically  shown  in  Fig.  77. 


^^ci-rnr; 


360     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

At  the  moment  of  contraction  the  muscle  becomes  shorter 
and  thicker,  but  there  is  no  change  of  volume. 

A  muscle  curve  consists  of  three  parts :  (1)  A  period 
following  stimulation  during  which  no  contraction  occurs  ; 
it  is  known  as  the  latrut  i:)eriod,  and  during  this  time  the 
muscle  is  preparing  itself  for  work ;  (2)  a  period  of  con- 
traction  or  shortening  ;  (3)  a  period  of  relaxation.  Some 
physiologists  consider  that  a  portion  at  least  of  the  latent 
period  may  be  due  to  instrumental  defects. 

The  length  of  time  involved  in  the  various  phases  of  a 


Fig.  77. — The  Curve  from  a  Single  Contraction  of  the 
Gastrocnemius  Muscle  of  the  Frog  (Waller). 

From  1  to  2  is  the  latent  period  ;  from  2  to  3  the  period  of  shortening; 
from  3  to  4  the  period  of  relaxation.  The  sinuous  line  below  the 
curve  indicates  periods  of  joo  °^  *  second. 

contraction  may  be  measured  by  bringing  a  time-recorder, 
vibrating  fractions  of  a  second,  to  bear  on  the  smoked 
surface  of  the  revolving  drum  ;  the  sinuous  line  in  Fig.  77 
below  the  muscle  curve  is  produced  in  this  way.  Though 
the  nerve  was  stimulated  at  the  point  marked  1  on  the 
tracing,  it  was  not  until  2  was  reached  that  the  muscle 
responded,  the  time  value  for  the  latent  period  being  ^^q 
of  a  second.  The  entire  contraction  from  the  instant  of 
stimulation  to  the  end  of  relaxation  occupies  about  yV  of 
a  second.  The  time  occupied  in  a  contraction  is  influenced 
by  the  temperature  of  the  muscle,  and  whether  it  be  fresh 
or  fatigued.  The  degree  to  which  a  muscle  contracts  is 
influenced  by  the  weight  it  has  to  lift,  the  strength  of  the 
stimulus  employed,  and  its  state  as  to  fatigue  ;  the  latter 
prolongs  the  latent  period  and  renders  the  contractions 
slower  and  smaller. 


THE  MUSCULAE  SYSTEM  3G1 

Muscle  Wciic. — When  the  impulse  enters  a  muscle  at  the 
middle  of  each  fibre,  the  part  nearest  the  end  plate  contracts 
first,  and  the  impulse  spreads  each  way  to  the  end  of  the 
fibre,  but  this  process  is  so  rapid,  the  fibre  only  being  about 
1  inch  in  length,  that  for  all  practical  purposes  the  whole 
muscle  contracts  at  one  and  the  same  time.  If,  however, 
the  nerve-endings  in  a  muscle  be  paralyzed  by  curare, 
the  part  becomes  at  once  practicall}^  nerveless,  and  if  now 
one  end  of  the  muscle  be  stimulated,  a  wave  of  contraction 
passes  along  it  to  the  other  at  a  rate  of  about  10  or  12 
feet  a  second  in  the  curarized  muscle  of  the  cold-blooded 
frog.  In  the  muscles  of  warm-blooded  man,  where  the 
metabolic  processes  are  more  active  than  in  the  frog,  the 
rate  of  propagation  is  greater,  and  may  be  taken  as  30  to 
40  feet  per  second.  The  time  which  the  wave  takes  to  pass 
any  one  point  of  the  muscle  is  extremely  short,  ^  of  a 
second,  and  if  (in  frog's  muscle)  we  take  its  velocity  as  at 
least  10  feet  per  second,  a  simple  calculation  shows  that 
the  leiir/th  of  the  wave  is  about  1  foot.  This  is  a  fact  of 
great  interest  in  connection  with  the  efficiency  of  a  muscle 
as  a  machine.  It  ensures  that  each  single  fibre,  whose 
length,  as  we  have  said,  is  only  1  inch,  of  which  the 
skeletal  muscles  are  composed,  and  hence  the  whole 
muscle  itself,  is  in  a  state  of  complete  contraction,  from 
end  to  end  at  the  same  moment  during  each  contraction. 

Suunnatioii. — If  instead  of  passing  a  single  stimulus  into 
a  muscle-nerve  preparation,  two  are  sent  in,  so  arranged 
that  the  second  follows  the  first  before  the  muscle  has  time 
to  relax  from  the  first  contraction,  the  contraction  due  to 
the  second  stimulus  is  superposed  on  the  first,  and  the  effect 
oblained  is  a  stronger  total  contraction.  If  a  third  stimulus 
be  sent  in  before  relaxation  occurs  from  the  second,  the  lever 
of  the  muscle  preparation  will  describe  a  curve  still  higher 
than  the  preceding,  and  so  on,  until  a  maximum  is  reached 
when  it  can  go  no  higher.  Such  a  piling  up  of  contraction  on 
contraction  is  known  as  summation  of  contractions  (Fig.  78). 
It  will  be  remembered  that  one  characteristic  of  heart 
muscle  is  the  absence  of  summation;  the  fibres  of  the  heart 


362    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

yield  their  best  possible  contraction  to  a  single  stimulus,  be 
it  weak  or  strong,  single  or  multiple. 

Tetanus. — If  an  induction  current  be  applied  to  a  muscle 
or  its  nerve,  a  rapid  succession  of  stimuli  is  thus  intro- 
duced, and  there  is  no  time  for  complete  relaxation  to  occur 
between  each  successive  stimulus.  This  may  be  seen  in 
the  lower  curve  of  Fig.  79,  in  which  ten  stimuli  per  second 
were  passed  into  the  muscle,  and  partial  relaxation  only 
will  be  observed  to  have  occurred  between  each  of  them.  In 
the  middle  curve  twenty  stimuli  per  second  were  used,  and 
the  amount  of  relaxation  is  represented  by  a  slightly  wavy 


Fig.  78. — Superposition  of  Contractions  (Stewart). 

1  is  the  curve  of  contraction  due  to  the  first  stimulation  ;  2  is  the  curve 
of  the  second  contraction,  superadded  to  1  by  applying  the  second 
stimulus  at  the  moment  when  the  first  contraction  had  nearly 
reached  its  maximum. 

line ;  in  the  upper  curve  thirty  stimuli  per  second  were 
employed,  and  the  curve  shows  no  relaxation ;  the  muscle 
is  in  the  condition  of  tetanus,  and  tetanus,  therefore,  consists 
of  the  summation  of  a  series  of  short  contractions  with  an 
insufficient  interval  for  intervening  relaxation.* 

All  the  ordinary  voluntary  muscular  contractions  of  the 
body  have  usually  been  regarded  as  tetanic  in  nature,  viz., 
as  a  series  resulting  from  a  succession  of  impulses  passed 
into  the  muscle  so  rapidly  that  there  is  no  interval  for 
relaxation.     Other  investigations  have  supported  the  view 

*  The  tetanus  of  the  physiologist  must  not  be  confused  with  the 
tetanus  of  the  pathologist ;  the  latter  is  a  bacterial  affection  producing 
a  poison  which  causes  spasm  of  many  of  the  voluntary  muscles  of  the 
body,  especially  the  limbs  and  jaw. 


THE  MUSCULAR  SYSTEM 


3G3 


that  in  all  probability  a  voluntary  contraction  is  a  pro- 
longed single  contraction,  caused  by  one  long  constant 
stimulus.  On  the  other  hand,  it  has  been  urged,  on  the 
basis  of  experiments  made  by  stimulating  the  motor  areas 
of  the  cerebral  cortex  with  stimuli  of  varying  frequency, 
that  a  motor  cell  cannot  discharge  a  single  impulse,  the 
normal  rate  in  man  under  the  influence  of  the  will  being 
ten  per  second.  The  question  is,  therefore,  evidently  not 
finally  settled. 

Elasticity  and  Extensibility.— Elasticity  is  the  property  a 
body  possesses  of  returning  to  its  shape  after  stretching,  and 


Fig.  79. — Summation  Curves  of  Muscle  Contraction  (Waller). 

The  lower  curve  is  one  obtained  by  stimulating  the  muscle  ten  times 
every  second ;  the  intervals  of  relaxation  are  clearly  seen,  though 
there  is  a  slight  summation  shown  by  the  slanting  rise  of  the 
tracing.  In  the  middle  curve  the  shocks  were  twenty  per  second, 
and  the  relaxation  is  only  of  a  very  partial  kind.  The  upper  curve 
is  that  of  tetanus. 

this  is  very  perfectly  shown  in  living  muscle ;  extensibility 
is  the  power  a  material  possesses  of  stretching,  and  the 
amount  of  extensibility  is  tested  by  loading  it  with  weights. 
If  a  steel  spring  or  a  piece  of  elastic  be  thus  tested,  it  is 
found  that  the  stretching  is  proportional  to  the  weight 
employed,  but  if  muscle  be  weighted  it  is  found  that  the 
greatest  degree  of  extension  occurs  at  the  beginning,  and 
as  the  load  is  increased  the  extensibility  becomes  less.  A 
contracted  muscle  is  more  extensible  than  one  which  is 
uncontracted,  a  protective  mechanism  intended  to  prevent 


3CA    A  MANUAL  OF  VETEEINAEY  PHYSIOLOGY 

rai)ture  of  the  fibres  during  powerful  muscular  effort,  and 
this  is  capable  of  ready  proof  clinically.  A  ruptured  muscle 
is  incomparably  less  frequent  than  a  ruptured  tendon. 

Li  the  body  the  muscles,  as  we  pointed  out  previously,  are 
always  in  a  condition  of  elastic  tension,  viz.,  they  are  not 
slack,  flabby  masses,  but  slightly  on  the  stretch,  as  may 
be  demonstrated  by  the  gaping  which  occurs  when  they  are 
divided. 

The  use  of  this  elastic  tension  is  to  stimulate  the 
changes  which  lead  to  a  contraction,  also  to  ensure  a  rapid 
contraction  without  the  necessity  of  taking  in  any  'slack,' 
and  further  it  is  essential  to  the  proper  action  of  the 
antagonistic  muscles,  which  are  thus  enabled  to  work 
against  an  elastic  resistance,  and  so  cause  a  smoothness 
of  motion  otherwise  unobtainable.  The  antagonistic  action 
of  muscles  may  be  well  seen  in  a  rupture  of  the  flexor 
metatarsi  of  the  horse ;  the  unbalanced  action  of  the 
gastrocnemius  jerks  the  leg  behind  the  body,  and  throws 
the  skin  over  the  cap  of  the  hock  into  folds,  while  the 
Achilles  tendon  is  kinked  and  bent  through  slackness.  The 
elastic  tension  of  muscle  is  not  only  a  valuable  stimulus  to 
contraction  in  all  varieties  of  muscle,  but  is  also  of  the 
greatest  value  in  diminishing  shock  and  strain ;  nowhere  is 
this  better  seen  than  in  the  heart  and  bloodvessels. 

Muscular  tone  is  the  name  given  to  that  condition  of 
continuous  slight  contraction  present  in  all  the  skeletal 
muscles,  and  leads  to  the  elastic  tension  to  which  we  have 
already  referred.  It  is  due  to  the  continuous  discharge 
of  impulses,  originated  reflexly,  by  the  central  nervous 
system  ;  if  the  nerves  concerned  in  the  production  and 
discharge  of  these  impulses  be  divided  the  tone  is  lost,  and 
nutritive  disturbances  follow.  Tone  is  also  influenced  by 
the  quality  of  the  blood-supply  to  the  muscle  and  the 
efi^icient  drainage  of  the  part. 

Work  of  Muscle. — If  a  muscle  preparation  be  loaded  with 
different  weights,  and  the  height  to  which  these  are  lifted 
observed,  it  is  found  that  up  to  a  certain  maximum  the 
load  absolutely  increases  the  amount  of  work  done  by  the 


THE  MUSCULAK  SYSTEM  365 

muscles.  This  is  considered  to  be  due  to  the  Unision 
exercised  on  the  fibres,  as  just  explained.  By  gradually 
increasing  the  weight  the  muscle  preparation  becomes  over- 
loaded, and  the  muscle  may  now  even  elongate.  These  facts 
have  been  shown  to  be  as  true  for  mammahan  as  for  frog's 
muscle,  excepting  that  human  muscle  contains  twice  as 
much  energy  for  the  same  volume. 

If  the  weight  of  the  load  and  the  height  to  which  it  is 
lifted  be  known,  the  work  done  by  a  muscle  is  readily 
calculable.  Work  equals  the  load  lifted  multiplied  by  the 
height  through  which  it  is  raised,  and  may  be  expressed  as 
pounds  or  tons  lifted  1  foot  or  grammes  or  kilogrammes 
lifted  1  metre.* 

In  connection  with  the  work  done  by  muscle  it  is  in- 
teresting to  institute  a  comparison  between  the  work  yielded 
by  the  animal  body  and  that  by  a  well-constructed  machine. 
The  best  triple-expansion  engine  may  yield  as  work  some 
10  to  15  per  cent,  of  the  available  energy  in  the  fuel,  the 
balance  is  lost  as  heat ;  in  other  words,  the  '  efficiency  ' — 
that  is,  the  fraction  of  the  heat  it  receives  which  it  converts 
into  work — of  a  good  engine  is  yV  to  y.  In  the  animal  body 
various  statements  have  been  made  as  to  the  proportion  of 
work  done  to  the  available  energy.  Chauveau  working  with 
the  lip-muscle  of  the  horse  placed  the  work  at  12  per  cent. 
to  15  per  cent,  of  the  energy  liberated,  the  difference  being 
accounted  for  as  heat.  If  this  were  the  case  the  muscle 
machine  would  seem  to  be  very  little  more  economical  than 
the  steam-engine.  Now,  Fick  showed,  some  thirty  years 
ago,  that  the  efficiency  of  an  excised  muscle  of  the  cold- 
blooded frog  may  be  as  much  as  \  or  even  i,  and  we  may 
not  unreasonably  expect  that  mammalian  muscle,  in  the 
body  with  its  circulation  intact,  would  be  still  more  efficient. 
And  this  is  borne  out  by  Zuntz's  experiments  on  the  dog. 
He  calculated  that  one-third  of  the  energy  liberated  appeared 
as  work,  while  by  experiments  on  men  it  was  found  that 
the  proportion  was  25  per  cent,  as  external  work  for  the 

*  A  '  horse-power,'  the  unit  used  in  engineering,  equals  33,000  foot- 
pounds of  work^er  minute. 


3G6    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

muscles  of  the  arms  (turning  a  wheel),  and  35  to  40  per 
cent,  for  the  legs  (m  mountameering),  from  which  it  would 
appear  that  the  muscles  of  locomotion  are  superior  as  work 
producers.  If  this  be  so  it  gives  the  animal  body  an 
'  efficiency '  of  from  .?r  to  nearly  h,  which  far  surpasses  that 
of  the  best  heat-engine. 

Interesting  as  the  comparison  may  be,  a  word  of  caution 
is  necessary.  It  is  true  that  an  engine  and  a  muscle  each 
take  in  energy  and  utilize  a  part  of  it  to  do  external  work, 
but  they  work  in  different  ways  and  along  different  lines  to 
produce  the  same  results.  Thus  the  steam-engine  receives 
its  energy  as  heat,  originated  by  the  combustion  of  fuel  in 
the  boiler-furnace,  converts  a  varying  fraction  of  this  into 
work  and  discharges  the  remainder,  degraded  in  tempera- 
ture but  otherwise  unaltered.  A  muscle,  on  the  other 
hand,  receives  its  energy  in  the  form  of  the  food  it  takes 
from  the  blood.  This  it  metabolizes  by  chemical  processes 
which  are  ultimately  oxidational,  converting  the  potential 
energy  of  the  food  jiartly  into  work  and,  unlike  the  engine, 
partly  into  heat  (see  p.  340),  and  giving  off  degraded 
products  of  its  metabolism,  of  which  one  is  the  same  as 
that  from  the  furnace  of  an  engine,  viz.,  carbon  dioxide. 
These  few  remarks  must  suffice  to  emphasize  the  fact  that 
a  muscle  is  a  cJiemical-engine  and  not  a  heat-engine.  As 
Fick  was  careful  to  point  out,  if  one  tries  to  explain  the 
working  of  a  muscle  on  the  thermodynamic  principles 
which  govern  the  working  of  a  heat-engine,  one  is  landed 
in  the  absurd  result  that  a  muscle  only  converts  into  work 
y^o  part  of  the  potential  energy  it  receives,  the  remaining 
YoV  necessarily  being  converted  into  heat,  and  we  have 
seen  that  the  efficiency  of  a  muscle  may  be  I.  In  the  case 
of  insects,  with  their  astounding  locomotive  activities,  if 
the  efficiency  of  their  muscles  could  be  determined  it  would 
probably  be  found  to  exceed  that  of  mammalian  muscle. 

Muscle  Currents. — Great  controversy  has  taken  place  as 
to  whether  currents  of  electricity  exist  naturally  in  un- 
injured muscle.  It  was  found,  for  instance,  that  a  piece 
of  muscle  isolated  from  the  body,  and  placed  in  connec- 


THE  MUSCULAE  SYSTEM 


367 


tioii  ^Yith  a  galvanometer,  may  be  made  to  demonstrate 
the  presence  of  electric  currents  which  behave  in  a  per- 
fectly regular  manner,  viz.,  under  certain  conditions  they 
are  always  weaker,  and  under  others  stronger,  in  passing 
from  one  definite  point  on  the  muscle  to  another.  These 
are  the  so-called  natural  muscle  curreuts,  or  currents  of 
rest;  they  are  found  to  pass  in  a  certain  direction,  viz., 
from  the  natural  surface  of  the  muscle  to  the  cut  extremity 
(Fig.   80).      It  is  now  distinctly  known  that  the  current 


./■ 


Fig.  80. — Diagram  illustrating  the   Electric  Currents  of  Rest 
OF  Muscle  and  Nerve  (Foster). 

Being  purely  diagrammatic,  it  may  serve  either  for  a  piece  of  muscle 
or  nerve,  excepting  that  the  currents  at  the  transverse  section 
cannot  be  shown  in  a  nerve.  The  arrows  show  the  direction  of 
the  current  through  the  galvanometer. 

a,  b,  the  equator.  The  strongest  currents  are  those  shown  by  the 
dark  lines,  as  from  a  at  the  equator  to  x  or  to  y  at  the  cut  ends. 
The  current  from  a  to  c  is  weaker  than  from  a  to  y,  though  both, 
as  shown  by  the  arrows,  take  the  same  direction.  A  current  is 
shown  from  e,  which  is  near  the  equator,  to  /,  which  is  farther 
from  the  equator.  The  current  (in  muscle)  from  a  point  in  the 
circumference  to  a  point  nearer  the  centre  of  the  transverse  section 
is  shown  at  y,  li.  From  a  to  h,  or  from  x  to  y,  there  is  no  current, 
as  indicated  bv  the  dotted  lines. 


obtained,  as  just  described,  is  one  caused  by  the  iujunj 
inflicted  on  the  muscle  in  its  course  of  preparation  for 
the  experiment,  the  injured  (end)  point  of  the  muscle  being 
always   negative   to   the   less   injured  (equatorial)    points. 


3!)8     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Muscle  at  rest  and  absolutely  uninjured  gives  no  current 
whatever. 

If  while  the  galvanometer  is  registering  the  direction  of 
this  injury  current  the  muscle  preparation  be  stimulated, 
a  ])ack\vard  swing  of  the  needle  of  the  instrument  towards 
zero  indicates  that  the  injury  current  is  diminished  ;  this 
diminution  is  termed  the  ne(jatice  variation. 

If  an  uninjured  muscle,  which  is  giving  no  currents,  be 
stimulated  into  contracting  activity,  it  exhibits  electrical 
phenomena,  the  current  of  action,  which  account  for  the 


Fig.  81. — Sartorius  Muscle  arranged  to  Demonstrate  the  Diphasio 
Variation  of  Action  Current  in  Muscle  (or  Nerve). 

s,  Sartorius  ;  a,  stimulating  electrodes  ;  b,  c,  non-polarisable  electrodes 
as  '  leads '  to  G,  the  galvanometer.  The  electrode  c  is  intentionally 
not  placed  on  the  injured  end  of  the  muscle  as  it  would  be  for 
demonstrating  mere  '  negative  variation,'  since  the  strong 
negativity  of  the  injured  end  would  mask  the  desired  phenomenon. 
A  similar  arrangement  suffices  to  demonstrate  the  same 
phenomenon  in  a  piece  of  nerve. 

negative  variation  of  the  injury  current.  The  causation  of 
the  action  current  is  really  of  such  a  nature  as  to  give  rise 
to  what  is  known  as  a  diphasic  variation  in  the  current  of  a 
muscle,  as  shown  by  the  needle  of  the  recording  galvano- 
meter swinging  first  one  way  and  then  in  the  opposite  direc- 
tion. This  double  variation  is  due  to  the  fact  that  the  point 
on  the  muscle  to  which  the  stimulus  is  applied  becomes 
negative  to  all  points  of  the  muscle  at  which  the  wave  of 
contraction,  resulting  from  the  stimulation,  has  not  yet 
arrived.  This  negativity  arises  during  the  *  latent  period ' 
(p.  360),  and  passes  along  the  muscle  as  a  wave  which 


THE  MUSCULAR  SYSTEM  3G9 

precedes  the  wave  of  contraction.  Thus  if,  as  in  Fig.  81, 
a  muscle  be  stimulated  at  a,  while  the  points  h  and  c  are 
connected  through  a  very  sensitive  galvanometer,  at  the 
moment  of  stimulation  a  becomes  negative  to  the  rest  of 
the  muscle.  As  this  negativity  sweeps  along  the  muscle 
it  passes  first  over  the  point  h,  which  thus  becomes 
negative,  and  the  needle  of  the  galvanometer  swings  in 
one  direction.  Immediately  afterwards  it  passes  over  the 
point  c,  and  the  needle  swings  in  the  opposite  direction. 
Hence  the  diphasic  variation. 

These  phenomena,  while  of  the  greatest  interest  in  the 
case  of  muscle,  become  still  more  important  in  the  case  of 
a  nerve,  since  they  provide  the  only  accurate  means  of 
following  the  passage  of  an  impulse  along  a  piece  of 
isolated  nerve,  which  does  not,  as  does  a  muscle,  change  its 
shape  or  exhibit  other  obvious  changes  when  stimulated. 

The  electrical  phenomena  in  muscle  are  not  an  isolated 
example  of  electric  currents  in  the  body.  Closely  similar 
phenomena  are  demonstrable  in  nerves,  and  electrical 
changes,  accompanying  their  functional  activit}',  occur  in 
secreting  glands,  in  the  eye,  and  to  the  highest  degree  in 
the  electric  organs  of  certain  fish. 

The  Changes  in  Active  and  Resting  Muscles. — The  changes 
occurring  in  muscles  are  remarkably  active.  The  processes 
which  result  in  muscular  contractions  use  up  at  every 
moment  the  combustible  material  of  the  structure,  and  the 
jDroducts  arising  from  their  metabolism  have  to  be  got  rid 
of  at  once  and  repair  brought  about.  Changes  are  also 
constantly  occurring  even  during  the  period  of  muscle  rest. 
Muscle  activity  is  characterized  by  muscle  waste,  muscle 
rest  is  characterized  by  a  preponderance  of  the  process  of 
repair ;  we  must  therefore  learn  the  nature  of  the  waste 
and  repair  occurring  in  muscles. 

The  oxygen  carried  to  resting  muscles  bj'  the  blood  is 
absorbed  in  considerable  quantities,  and  a  volume  of 
carbon  dioxide,  in  slightly  less  quantity  than  corresponds 
to  the  oxygen  absorbed,  is  returned  to  the  venous  blood. 
Whether    a    muscle    be    at    rest    or    active,    it   is   always 

24 


870    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

absorbing  and  storing  up  oxygen,  and  giving  off  carbon 
dioxide.  The  absolute  amount  of  these  varies ;  during 
work  the  oxygen  used  up  and  carbon  dioxide  produced  are 
both  increased,  and  the  increase  provides  some  measure  of 
the  work  performed.  Even  during  rest  a  muscle  is  doing 
"work,  for  we  have  learnt  that  it  is  always  in  a  condition  of 
tonus,  viz.,  of  slight  contraction.  Since  a  muscular  con- 
traction is  essentially  the  outcome  of  an  oxidational 
process,  the  storage  of  oxygen  by  the  resting  muscle  may 
be  said,  in  PflUger's  words,  '  to  wind  it  up '  in  preparation 
for  its  contracting  activity.  This  accounts  for  the  fact  that 
an  excised  muscle  of  the  cold-blooded  frog  can  be  made  to 
give  some  hundred  contractions  when  suitably  treated.  In 
mammalian  muscle,  on  the  other  hand,  with  its  much 
greater  metabolic  activity,  this  quiescent  storage  of  oxygen 
does  not  suffice  to  maintain  its  irritalnlity  for  more  than 
the  briefest  interval  after  its  blood- supply  is  cut  off. 

In  an  active  muscle  the  bloodvessels  are  more  dilated 
than  in  the  muscle  at  rest,  and  this  dilatation  provides  for 
the  increased  quantity  of  blood  now  required  by  the  part. 
By  means  of  the  blood  the  irritability  of  the  muscle,  or  its 
power  of  contraction,  is  maintained ;  whatever  leads  to  a 
smaller  quantity  of  blood  being  sent  to  an  active  muscle, 
produces  partial  or  complete  paralysis  of  the  group  or 
groups  of  muscles  affected.  This  is  well  seen  in  the  horse 
when  suffering  from  thrombosis  of  the  iliac  arteries ;  the 
blood- sujjply  is  sufficient  during  the  time  the  animal  is  at 
rest,  or  even  at  a  walk,  but  if  called  upon  to  trot  muscular 
cramps  occur  followed  by  paralysis. 

Our  study  of  metabolism  has  prepared  us  for  the  state- 
ment that  the  chemical  changes  occurring  during  contrac- 
tion do  not  normally  affect  the  nitrogenous  elements  of 
the  muscle.  There  is  probably  no  increased  output  of 
nitrogenous  substances  such  as  creatine.  The  excretion 
of  any  increased  amount  of  urea  is  variable,  irregular,  or 
even  non-existent,  and  is  in  no  case  even  remotely  pro- 
portional to  the  work  done.  This  is  true  as  long  as  the 
body  is  supplied  with  a  sufficiency  of  the  non-nitrogenous 
carbo-hydrates    and    fats.      If    they    are    deficient,   then 


THE  MUSCULAR  SYSTEM  871 

increased  muscular  activity  does  lead  to  an  increased 
formation  of  urea,  since  the  muscle  now  has  to  metabolize 
its  proteids  to  provide  the  energy  necessary  for  the  work 
performed  and  the  heat  simultaneously  produced.  The 
main  products  of  muscular  waste  are  therefore  to  be 
looked  for  in  the  destruction  of  stored-up  carbo-hydrate 
material.  Muscles  in  a  state  of  activity  contain  less 
glycogen  and  sugar  than  those  in  a  state  of  rest,  due 
to  the  amount  utilized  during  muscular  activity ;  but 
glycogen  is  not  necessarily  the  source  of  the  energy,  since 
muscles  free  from  it  work  normally.  During  muscular 
activity  heat  is  produced ;  the  blood  returning  from  a 
muscle  has  a  higher  temperature  than  that  going  to  it. 
Colin  found  the  temperature  of  the  masseter  muscle  of  the 
horse  to  rise  5°  Fahr.  through  feeding.  The  whole  bod}' 
temperature  in  the  horse  is  raised  during  work,  and  does 
not  fall  for  some  time  after.  In  dogs  a  rise  of  temperature 
of  several  degrees  may  be  obtained  by  stimulating  the  spinal 
cord,  and  thus  producing  muscular  contractions. 

A  contracting  muscle  liberates  energy  in  the  form  of  both 
work  and  heat.  We  have  seen  reasons  for  regarding  the 
oxidation  of  the  non-nitrogenous  carbo-hydrates  as  the 
normal  source  of  this  energy,  and  we  have  referred  to  the 
quiescent  storage  of  oxygen  during  rest  as  accounting  for 
the  prolonged  possible  activity  of  an  isolated  frog's  muscle. 
In  connection  with  this  it  is  not  uninteresting  to  calculate, 
as  Fick  has  done,  the  amount  of  carbo-hydrate  necessarily 
oxidized  to  provide  all  the  energy  as  work  +  heat  furnished  by 
a  single  maximally  vigorous  contraction  of  a  frog's  muscle. 
Knowing  the  heat  of  combustion  of,  say,  glycogen,  and 
converting  the  work  of  the  muscle  into  heat  by  Joule's 
equivalent,  we  find  that  1  gramme  of  frog's  muscle  can 
provide  all  the  energy  it  sets  free  in  a  single  maximal  con- 
traction by  the  oxidation  of  "0006  milligramme  of  carbo- 
hydrate. In  the  case  of  fat  the  necessary  amount  would 
be  still  less,  viz.,  '00025  milligramme.  This  may  serve 
to  diminish  our  surprise  at  the  working  activity  of  which 
an  excised  muscle  is  capable. 

24—2 


372    A  MANUAL  OF  VETEKINAKY  PHYSIOLOGY 

Causation  of  a  Muscular  Contraction. — This  is  a  problem  as 
yet  unsolved  ;  our  previous  studies  in  every  way  point  to  the 
oxidation  of  carbo-hydrate  substance  as  being  the  source 
of  energy,  and  we  have  seen  that  it  is  impossible  for  a 
muscle  to  contract  without  using  up  oxygen  and  producing 
carbon  dioxide.  But  we  are  now  brought  face  to  face  with  a 
paradoxical  condition  ;  if  muscle  be  exposed  to  the  vacuum 
of  a  gas-pump  no  free  oxygen  can  be  obtained  from  it, 
while  if  the  ordinary  nerve-muscle  preparation  be  taken 
and  placed  in  a  jar  of  hydrogen  it  continues  to  contract, 
and,  even  still  more  remarkable,  it  continues  to  produce  CO2, 
though  no  oxygen  exists  in  the  atmosphere  surrounding  it ! 
As  CO2  cannot  be  formed  without  oxygen,  it  is  evident  the 
oxygen  must  come  out  of  the  muscle,  and  to  meet  this 
difficulty  it  is  supposed  that  the  muscle  molecules  store  up 
oxygen  during  rest  in  a  hypothetical  compound  of  hydrogen 
and  carbon  known  as  inogen ;  during  muscular  contraction 
this  compound  breaks  down,  and  the  waste  products  are 
liberated.  The  nature  of  the  process  by  which  the  impulse 
conveyed  to  the  muscle  along  its  motor  nerve  becomes, 
through  the  agency  of  the  end-plates,  the  stimulus  which 
leads  to  this  explosive  compound  being  fired  off  as  a 
muscular  contraction  is  quite  unknown.  The  nitrogen- 
holding  substance  in  muscle  is  only  used  when  the  food- 
supply  is  insufficient  or  the  work  excessive  ;  it  is  therefore 
the  Oo  intake,  and  COo  output,  which  have  to  be  examined 
in  dealing  with  the  question  of  the  influence  of  muscular 
work  on  metabolism. 

Fatigue. — Turning  once  more  to  the  simple  nerve-muscle 
preparation,  it  is  found  that  if  the  muscle  be  kept  at  work 
the  first  few  contractions,  as  shown  by  a  series  of  tracings 
(see  Fig.  82),  may  be  progressively  more  vigorous.  But  if 
the  stimulations  are  continued  the  muscle  rapidly  becomes 
fatigued,  the  latent  period  is  lengthened,  the  height  of 
each  successive  contraction  becomes  less  and  the  duration 
of  each  contraction  is  prolonged,  chiefly  by  a  lengthening 
of  the  period  of  relaxation  :  the  muscle,  in  other  words,  is 
in  a  state  of  fatigue  (Fig.  82).     We  shall  presently  study 


THE  MUSCULAR  SYSTEM  373 

fatigue  from  some  other  aspects,  and  will  now  only  point 
out  that  the  cause  is  here  due  to  the  using  up  of  the  con- 
tractile substance  of  the  fibres  and  the  accumulation  in 
the  muscle  of  the  chemical  products  of  contraction.  In 
fact,  if  a  fatigued  muscle  be  washed  out  with  normal  saline 
solution    and   a  little  weak   alkali  circulated   through  its 


Fig.  82. — Fatigue  Curve  of  Skeletal  Muscle  ;  Gastrocnemius 
OF  Frog  (Stewart). 

Time  tracing  yotj  of  a  second.     The  curve  is  read  from  right  to  left. 


bloodvessels,  it  becomes  restored,  and  regains  its  power  of 
contraction.  A  muscle  at  work  in  the  body  is  protected 
from  ready  fatigue  by  the  ever-circulating  blood,  which 
supplies  it  with  food  and  carries  off  the  waste  products  of 
its  activity. 

A  muscle  so  fatigued  by  repeated  stimulation  may 
be  restored  by  washing  it  out  with  physiological  salt  solu- 
tion containing  a   little  alkali ;    in  course  of  time   mere 


374     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

washing  out  of  the  muscle  is  not  sufficient  to  ensure  its 
recovery,  but  if  serum  or  blood  be  transfused  it  is  enabled 
again  to  start  work.  The  material  in  muscles  which 
gives  rise  to  fatigue  is  probably  sarco-lactic  acid,  and  by 
passing  a  solution  of  this  acid  into  muscles  the  typical 
phenomena  of  muscle-fatigue  may  be  artificially  induced. 
The  production  of  potassium  salts  may  also  be  a  cause  of 
fatigue,  in  spite  of  the  fact  that  they  are  usually  found 
in  muscle,  yet  potassium  salts  in  their  action  on  this 
tissue  rapidly  destroy  its  irritability. 

We  have  seen  (p.  353)  that  muscles  are  connected  by 
elaborated  nerve-endings  with  sensory  nerves,  to  whose 
existence  the  so-called  '  muscular  sense '  is  due.  It  is 
therefore  conceivably  possible  that  the  sensation  of 
general  fatigue  which  arises  from  excessive  muscular 
exertion  is  due  to  a  cerebral  appreciation  of  the  changes 
brought  about  in  the  muscles  as  the  result  of  their 
contracting  activity.  On  the  other  hand,  muscular 
activity  implies  the  action  of  central  nerve-cells  in 
which  the  impulses  which  give  rise  to  the  contractions 
of  the  muscles  are  originated,  of  the  passage  of  these 
impulses  along  the  motor  nerves,  and  of  their  com- 
munication to  the  contractile  fibres  by  the  agency  of  the 
end-plates.  Hence  the  phenomena  of  fatigue,  if  we  regard 
it  as  a  '  weariness'  of  the  body  as  a  machine,  may  be  really 
due  to  a  fatigue  of  the  central  cells,  of  the  motor  nerves,  or 
of  the  end-plates.  We  may  at  once  dismiss  the  motor 
nerves  from  our  consideration  inasmuch  as  nerves  do  not 
appear  to  be  capable  of  fatigue  (p.  385).  Now  the  blood 
of  a  fatigued  animal  contains  fatigue  products,  and  if  it 
be  transferred  into  the  circulation  of  a  normal  animal,  all 
the  symptoms  of  fatigue  are  produced.  If  the  spinal  cord 
be  divided  and  the  distal  end  stimulated,  the  hurried 
respiration  of  fatigue  may  be  produced  as  the  result  of 
muscular  contractions  (see  p.  118).  Possibly,  therefore, 
the  phenomena  are  due  to  the  injurious  action  of  the 
products  of  muscular  activity  on  the  central  motor  nerve- 
cells,  and  some  experiments  on  men  go  to  show  that  the 


THE  MUSCULAR  SYSTEM  375 

central  nervous  system  is  readily  affected  by  fatigue  products. 
On  the  other  hand,  recent  work  by  Wedenski  opens  up  the 
possibility  that  fatigue,  if  we  look  upon  it  as  an  inability  to 
drive  the  muscular  machinery  up  to  its  normal  capacity, 
may  be  due  to  the  deleterious  influence  of  the  products  of 
muscular  contraction  on  the  end-plates.  This  view  receives 
support  from  the  fact  that  a  fatigued  muscle  will  contract 
by  direct  stimulation  when  it  refuses  to  respond  to  a 
stimulus  brought  to  it  through  its  nerve. 

Elaborate  experiments  on  man  performing  muscular 
work  show  how  readily  the  intake  of  0^  may  be  increased  ; 
such  causes  as  difficulties  in  the  road,  rising  ground, 
increase  in  pace,  change  in  the  load  carried,  unpractised 
movements,  even  a  sore  foot,  may  increase  the  consumption 
by  18  per  cent.  Fatigue  produces  wasteful  metabolism, 
and  may  increase  the  CO2  excreted  even  as  much  as 
21  per  cent.  The  abnormal  use  of  certain  muscles,  such 
as  a  man  with  sore  feet  would  employ  in  order  to  save 
himself  pain,  produces  extravagant  combustion  and  fatigue. 
What  applies  to  man  in  these  matters  applies  equally  to 
the  horse ;  ungreased  axles,  badly-fitting  harness  and 
saddlery,  badly-made  roads,  sore  backs  and  lameness,  all 
represent  undue  muscular  wear  and  tear. 

Condition. — That  remarkable  state  of  the  body  described 
as  '  condition,'  into  which  horses  can  be  brought  by  care  in 
feeding,  general  management,  and  carefully  regulated  work, 
must  be  regarded  as  the  highest  pitch  of  perfection  into 
which  muscles  can  be  brought.  In  its  highest  degree  it  is 
not  a  permanent  state  ;  no  horse  can  remain  in  it  for  any 
length  of  time,  and  many  can  never  be  got  into  condition  for 
severe  work.  It  is  easy  in  the  training  of  horses  to  over- 
step the  mark  and  produce  '  staleness,'  a  result  which  is 
usually  recovered  from  by  a  short  judicious  rest,  to  which 
the  system  immediately  responds. 

During  training  all  superfluous  fat  and  water  are  re- 
moved from  the  body,  the  muscle  substance  is  built  up,  and 
the  respiratory  capacity  increased  ;  but  it  is  very  necessary 
to  remember  that  condition,  though  judged  of  largely  by 


376     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  state  of  the  muscles,  has  a  very  important  claim  on 
the  respiratory  and  circulatory  systems.  To  sustain  severe 
and  prolonged  muscular  exertion  an  adequate  supply  of 
oxygenated  hlood  must  be  sent  to  the  muscles ;  this 
necessitates  a  rapid  flow  of  blood  and  adequate  ventilation 
in  the  lungs,  with  strong  regular  pumping  power  in  the 
heart ;  all  these  factors  must  work  in  harmony.  As  a 
matter  of  fact  the  ability  to  endure  the  strain  of  a  violent 
muscular  effort  is  far  more  dependent  on  the  training  of 
the  respiratory  and  cardiac  mechanisms  than  on  that  of 
the  muscles.  Long  walking  exercise  is  given  as  a  muscle 
developer,  and  judicious  gallops  to  give  an  animal  its  'wind,' 
yet  as  a  matter  of  fact  the  '  wind  '  is  largely  a  question  of 
heart.  As  the  circulatory  pump  works  at  high  pressure 
the  bloodvessels  must  be  fit  to  stand  the  strain,  and  to 
return  to  the  heart  at  both  auricles  the  amount  of  blood 
leaving  by  both  ventricles.  A  deficiency  in  this  mechanism 
leads  to  Moss  of  breath';  clogging  in  the  lungs  means 
deficient  oxygenation  in  the  tissues,  and  without  an  adequate 
supply  of  oxygen  the  muscles  are  powerless  to  contract. 
We  are  clearly  shown,  from  what  may  be  witnessed  in  the 
hunting-field,  or  wherever  horses  are  exposed  to  long- 
continued  strain,  that  the  chief  value  in  training  is  located 
in  the  functional  improvement  of  the  muscular  tissue  of 
the  heart  and  in  the  circulatory  system  in  the  lungs  ;  both 
of  these  have  to  be  educated  to  withstand  the  extra  strain 
imposed  and  to  work  economically.  The  voluntary  muscles 
have  also  to  be  educated  to  work  in  the  best  and  most 
economical  manner ;  they  must  be  used  to  advantage, 
smoothly  and  in  combination ;  their  response  must  in- 
crease in  rapidity  and  power,  while  their  relaxation  must 
not  be  too  prolonged  and  so  cause  loss  of  time.  Unpractised 
movements  are  a  serious  source  of  waste ;  by  practice  the 
same  amount  of  work  can  be  performed  with  a  reduced 
expenditure  of  energy,  and  this  is  true  for  both  men  and 
horses. 

There   is   additionally   another   factor   of    supreme   im- 
l^ortance  in  training.     The  respiratory  movements,  as  we 


THE  MUSCULAR  SYSTEM  377 

■have  learnt,  are  dependent  upon  the  rhythmic  activity  of 
the  respiratory  centre  (p.  108).  Hence  this  centre  must  be 
taught  to  withstand  the  extra  strain  imposed  upon  it  during 
violent  exertion.  What  the  centre  has  to  '  learn  '  in  respect 
of  this  is  more  or  less  a  matter  of  conjecture.  Bearing  in 
mind  the  powerfully  stimulating  influence  on  the  respiratorj^ 
centre  of  the  waste  products  of  the  metabolism  involved  in 
muscular  contraction,  it  is  conceivably  possible  that '  wind  ' 
is  the  result  of  an  increased  immunity  of  the  centre  to  the 
action  of  these  products.  However  this  may  be,  one  thing 
is  certain — namely,  that  respiratory  distress  is  more  potent 
than  most  other  factors  in  determining  '  staying  power,' 
the  one  thing  to  which  all  long-distance  athletes  strive  to 
attain.  In  this  connection  we  may  point  out  that  it  is  said 
that  the  deleterious  products  of  metabolism  produced 
during  fatigue  may  be  neutralized  and  immunity  estab- 
lished by  giving  small  doses  of  extracts  of  a  fatigued 
muscle  ;  the  question  has  therefore  arisen  as  to  whether 
'  training '  is  a  process  of  immunizing  against  fatigue 
jDroducts  •? 

Chemical  Composition  of  Muscle. — A  dead  muscle  does  not 
possess  the  same  chemical  composition  as  one  which  is 
living,  and  we  cannot  analyse  living  muscle  without  killing 
it  by  the  methods  necessarily  employed.  Thus  any  tabular 
statement  of  the  quantitative  composition  of  muscle  gives 
really  the  composition  of  dead  muscle.  "We  are,  however, 
assisted  to  some  knowledge  of  the  nature  of  living  muscle- 
substance  by  the  following  facts. 

If  contractile,  and  therefore  living,  frog's  muscle  is  care- 
fully frozen  and  then  very  slowlv  thawed,  it  does  not  lose 
its  irritability  :  it  is  still  alive.  When  frozen  it  may  be 
minced  with  a  cold  knife  and  ground  up  in  a  cold  mortar 
with  four  times  its  weight  of  snow  containing  1  per  cent,  of 
sodium  chloride.  By  this  process  a  viscid  liquid  is  obtained 
which  may  be  filtered,  though  with  difficulty,  at  0^  C.  The 
fluid  filtrate  is  opalescent,  neutral,  or  faintly  alkaline  in 
reaction,  and  is  known  as  'muscle-plasma.'  When  its 
temperature  is  allowed  to  rise  it  coagulates  in  the  same 


378     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

way  as  does  blood-plasma,  yielding  a  dot  which,  unlike 
fibrin,  is  granular  and  flocculent,  and  forming  a  liquid 
serum.  During  the  clotting  the  liquid  becomes  acid,  as  the 
result  of  a  formation  of  sarco-lactic  acid,  and  the  clot  con- 
sists of  myosi)i.  Assuming,  as  we  may  reasonably  do,  that 
the  muscle-plasma  represents  mora  or  less  closely  the 
muscle-substance  in  the  living  fibre,  we  may  take  these 
phenomena  of  the  clotting  of  the  muscle-plasma  as  indicating 
the  most  characteristic  chemical  differences  between  living 
and  dead  muscle  (though  there  are  others),  and  thus  we 
gain  considerable  insight  into  the  composition  of  living 
muscle  as  based  upon  an  analysis  of  the  dead  tissue. 

With  this  preliminary  caution  w^e   may  now  state   the 
composition  of  muscle  to  be  approximately  as  follows : 


Water 
Proteids 

-  75      per  cent. 

-  20 

Fat     - 

■     3 

Carbo-hydrates 
Nitrogenous  waste  products 
Salts  -             -             - 

•4  to  1  per  cent, 

-  -2 

-  1  to  1-5       ., 

Our  knowledge  of  the  nature  of  the  proteids  of  muscle  is 
a  matter  of  no  slight  uncertainty,  which  is  not  made  less 
by  the  existing  confusion  in  the  terminology  employed  by 
various  investigators.  Into  this  we  cannot  here  enter.  It 
must  suffice  to  say  that  the  chief  and  characteristic  proteid 
of  dead  muscle  is  the  myosin  formed  in  the  clotting  of 
muscle-plasma ;  it  belongs  typically  to  that  class  of  proteids 
known  as  globulins.  Bearing  in  mind  the  phenomena  of 
the  clotting  of  muscle-plasma  and  using  the  nomenclature 
employed  for  blood-plasma,  we  may  say  that  living  muscle 
contains  myosinogen,  which  on  the  death  of  the  muscle  is 
converted  into  myosin,  just  as  in  blood-plasma  fibrinogen 
gives  rise  to  fibrin  (p.  18).  It  has  not  as  yet  been  shown 
that  calcium  salts  play  a  part  in  the  coagulative  formation 
of  myosin,  as  they  necessarily  do  in  case  of  fibrin  and  of  the 
casein-clot  in  milk.  The  proteids  of  living  muscle  are  not 
entirely  myosinogen,  nor  are  those  of  dead  muscle  entirely 
myosin.     Other  members  of  the  globulin  class  are  present 


THE  MUSCULAE  SYSTEM  379 

in  both,  as  also  an  ordinary  albumin  closely  resembling 
serum-albumin. 

The  carbo-hydrate  material  is  composed  chiefly  of 
glycogen,  which  diminishes  in  amount,  by  conversion  into 
sugar,  on  the  death  or  after  the  contracting  activity  of 
muscles  ;  these  substances  have  already  been  fully  dealt 
with  in  a  previous  chapter.  The  nitrogenous  waste  products 
or  '  extractives '  are  creatine,  hypoxanthine,  xanthine, 
carnine,  taurine  (in  horse-flesh),  uric  acid  in  minute  traces 
(though  more  abundant  in  reptilian  muscle),  and  traces  of 
urea,  though  this  is  a  question  still  not  decisively  settled ; 
of  these  creatine  is  by  far  the  most  important.  It  is  a 
substance  we  have  already  studied  in  connection  with  the 
production  of  urea  (p.  295).  The  ash  in  muscle  consists 
principally  of  the  salts  of  potassium  and  phosphoric  acid. 
The  gases  are  carbon  dioxide,  together  with  a  small  amount 
of  nitrogen  but  no  free  oxygen. 

Rigor  Mortis. — After  death  a  muscle  passes  into  the 
condition  of  rigor  or  stiffening,  by  which  it  changes  both 
in  its  physical  and  chemical  aspect.  The  muscle  becomes 
firm  and  solid,  loses  its  elasticity,  and  no  longer  responds 
to  electrical  stimuli ;  further,  it  loses  its  alkaline  reaction, 
and  in  course  of  time  becomes  acid  owing  to  the  formation 
of  sarco-lactic  acid.  Through  the  death  of  the  muscle  its 
proteids  coagulate,  and  this  process  is  generally  believed  to 
be  identical  with  the  clotting  of  muscle  plasma  previously 
described.  Eigor  mortis  and  the  production  of  sarco- 
lactic  acid  are  closely  connected,  so  that  if  the  formation 
of  the  acid  be  prevented  by  suitable  means,  rigor  does  not 
occur.  The  view  now  adopted  as  to  the  cause  of  death 
stiffening  is  that  it  is  due  to  a  coagulation  of  the  proteids 
by  the  products  of  metabolism  in  the  muscle,  and  this  ex- 
planation accounts  for  the  rapid  setting  in  of  rigor  in  animals 
hunted  to  death.  Eigor  mortis  is  delayed  in  a  rabbit  in 
which  the  labyrinth  of  the  internal  ear  has  been  destroyed. 
This  is  probably  connected  with  the  obscure  problem  of 
muscle  tonus,  with  which  the  labyrinth  is  connected.  The 
muscles  in   which   delayed  rigor  mortis  occurs  are  those 


380    A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

corresponding  to  the  same  side  of  the  body  as  the  injured 
labyrinth.  During  rigor  mortis  C0._,  is  produced  and 
heat  evolved  ;  some  after-death  temperatures  are  re- 
markably high.  After  a  certain  length  of  time  rigor  mortis 
passes  off  and  decomposition  commences.  It  is  doubtful 
whether  rigor  mortis  occurs  in  involuntary  muscle ;  the 
appearance  presented  in  this  variety  of  muscle  may  be  due 
to  cold,  for  it  has  been  shown  that  two  or  three  days  after 
death  smooth  muscle  may  be  w^armed  up  so  as  to  be  capable 
of  contraction. 

Phenomena  of  Contraction  in  Smooth  Muscle. — Though 
there  is  a  marked  difference  in  appearance  between  red 
and  white  muscle,  the  actual  phenomena  of  contraction 
do  not  differ  excepting  in  the  matter  of  rate.  The 
latent  period,  contraction,  and  relaxation,  are  present,  as 
in  red  muscle,  but  occur  more  slowly  ;  they  are,  in  fact, 
sluggish  and  deliberate.  Owing  to  the  existence  of 
numerous  nerve  fibres  and  ganglia,  white  muscle  may  be 
completely  isolated  from  all  nervous  connections,  and  still 
continue  to  exhibit  the  phenomena  of  muscular  contraction. 

In  response  to  a  continuous  or  induced  current  pale 
muscle  behaves  much  as  does  red,  excepting,  of  course, 
that  the  response  is  slower.  Summation  also  is  present, 
though  not  identical  with  that  observed  in  red  muscle,  for 
no  contraction  follows  the  first  three  or  four  stimuli ;  it  is 
the  stimuli  which  here  accumulate  before  contraction 
follows,  and  after  this  has  occurred  the  muscle  subse- 
quently responds  to  further  stimuli  by  an  increased  height 
of  contraction  as  does  a  red  muscle. 

Mechanical  stimulation  of  pale  muscle  excites  a  sluggish 
but  marked  response ;  pinching  the  intestines  produces 
peristalsis,  and  even  drawing  the  finger  lightly  over  the 
stomach  wall  may  produce  *  weals  '  of  contraction.  The 
muscle  is  markedly  responsive  to  tension,  resembling  in 
this  a  skeletal  muscle  which  up  to  a  certain  *  load  '  does 
more  work  the  greater  the  weight  it  has  to  lift.  Thus  a 
frog's  gastrocnemius,  loaded  successively  with  10,  30,  and 
90  grammes,  will  do  work  proportional  at  each  contraction 


THE  MUSCULAE  SYSTEM  381 

to  the  numbers  106,  312,  and  760'5.  In  the  case  of  the 
digestive  canal  this  is  probably  the  chief  source  of  stimula- 
tion, and  in  this  connection  we  may  call  to  mind  the 
influence  of  cellulose  in  the  diet  of  the  herbivora  (p.  '207), 
the  bulkiness  of  their  food  providing  the  mechanical  stimu- 
lation and  the  distension  necessary  to  produce  the  tension 
to  which  pale  muscle  is  so  responsive.  The  same  remark 
may  also  apply  to  the  physiological  action  of  the  stomach 
and  intestinal  gases ;  nor  must  the  bladder  be  omitted 
from  this  consideration,  from  the  point  of  view  that  distension 
by  fluid  provides  a  tension  of  the  walls  which  acts  as  a 
stimulus  to  contraction. 


CHAPTER  XTY 
THE  NERVOUS  SYSTEM* 

Nerves, — Various  classifications  have  been  adopted  for 
nerves.  Anatomically  they  are  known  as  cranial,  cerebro- 
spinal, and  sympathetic,  but  for  physiological  purposes 
thej'  are  classified  according  to  their  function.  From  a 
structural  point  of  view  there  are  (1)  medullated,  (2)  non- 
medullated  nerves.  Classified  according  to  their  function 
there  are  (1)  afferent,  frequently  called  sensory,  and 
(2)  efferent,  commonly  called  motor.  The  division  into 
motor  and  sensor}'  is  so  obviously  incomplete,  confining  the 
function  of  nerves  simply  to  the  conveyance  of  motor 
impulses,  or  of  those  which  give  rise  to  sensation,  that 
the  terms  afterent  and  efferent  are  better. 

Afferent  nerves  are  those  conveying  an  impulse  from 
the  periphery  of  the  body  to  a  nervous  centre,  that  is  to 
say  conveying  centripetal  impulses.  The  centre  may  be 
situated  in  the  brain  or  spinal  cord,  and  the  impulses 
thus  conveyed  may  be  those  of  (a)  special  sense  such 
as  sight,  hearing,  taste,  smell,  etc.  ;  {h)  impulses  pro- 
ducing sensation  pleasurable  or  painful,  from  skin,  muscle, 
and  viscera  ;  (c)  impulses  producing  the  impression  of  heat 
and  cold,  or  (d)  impulses  leading  to  a  reflex  act  without 
affecting  consciousness  at  all. 

Efferent  nerves  are  those  conveying  impulses  from  a 
centre   to   the    periphery,   that   is   conveying    centrifugal 

*  M3'  best  thanks  are  due  to  Professor  Sherrington,  F.R.S.,  for 
reading  this  chapter,  and  kindly  supplying  that  portion  of  it  dealing 
with  "  stepping  "  and  the  "scratch  reflex." 

382 


THE  NEEVOUS  SYSTEM  383 

impulses ;  these  impulses  in  the  main  are  of  a  motor 
nature  evoking  from  the  muscles,  bloodvessels,  and  viscera 
movements  and  contractions  ;  they  may  also  be  of  an 
inhibitor}'  or  controlling  character,  such  for  instance  as 
the  impulses  which  slow  the  heart,  dilate  the  bloodvessels, 
or  restrain  the  peristaltic  contraction  of  the  bowels.  But 
besides  these,  centrifugal  impulses  may  be  of  such  a 
nature  as  to  cause  glands  to  secrete,  or  to  regulate  the 
metabolism  of  a  part,  or  control,  stop,  or  augment  other 
of  its  actions. 

Structure  of  Nerves. — Meduilated  nerves,  often  spoken  of 
as  white  nerve-fibres  owing  to  their  colour,  are  distinguished 
microscopically  by  the  fact  that  their  component  fibres 
possess  a  white  fatty  sheath  enveloping  the  essential  nerve- 
substance  or  axis-cylinder,  which  lies  like  a  core  within  it. 
The  axis-cylinder  is  the  true  nerve-substance,  and  is  in 
connection  with  either  the  brain  or  spinal  cord,  depending 
upon  the  position  of  the  nerves.  In  these  organs  cells  with 
processes  are  situated  ;  one  of  the  processes  becomes  the 
axis-cylinder  of  a  nerve,  so  that  some  of  the  cells  of  the  brain, 
and  especially  of  the  spinal  cord,  may  be  looked  upon  as 
possessing  processes  of  immense  length.  The  white  fatty 
sheath,  known  as  the  maduUary  sheath,  which  covers  the 
axis-cylinder  does  not  extend  continuously  along  the  nerve, 
but  is  broken  at  intervals  termed  nodes ;  the  portion  of 
nerve-fibre  included  between  two  nodes  has  somewhere  in 
it  a  nucleus.  It  is  at  the  nodes,  where  the  fatty  sheath  is 
absent,  that  the  material  which  supplies  the  nerve  with 
nutrition  gains  access.  Covering  the  medullary  sheath  is 
a  delicate  membrane  which  envelops  the  fatty  matter, 
and  is  known  as  the  neurilemma.  Such  is  the  structure  of 
a  single  nerve-fibre ;  bundles  of  such  fibres,  enclosed  in 
an  appropriate  sheath  of  connective  tissue,  constitute  a 
nerve. 

The  non-medullated  nerves,  often  spoken  of  as  sym- 
pathetic or  grey  fibres,  possess  no  white  fatty  cover  around 
their  axis- cylinder ;  they  are  freely  nucleated  at  intervals, 
and  made  up  in  bundles  as  are  the  meduilated  nerves. 


384    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  essential  feature  in  the  nerve  is  the  axis-cylinder ; 
it  is  the  true  impulse-conducting  substance,  while  fatty 
sheaths  can  only  be  looked  upon  as  a  means  of  insulation 
or  support.  From  what  has  been  stated  about  the  nature 
of  the  axis-cylinder,  it  can  be  readily  understood  that 
every  nerve  runs  direct  from  its  origin  to  its  termination, 
there  is  no  union  of  nerve-fibres,  each  and  every  one  is 
distinct,  though  numerous  divisions  may  exist  at  their 
termination.  In  certain  cases  medullated  nerves  enter 
nervous  bodies  known  as  gaiuiUa  and  leave  them  as  non- 
medullated  fibres.  All  medullated  nerves  before  breaking 
up  in  the  tissues  they  are  intended  to  supply  lose  their 
fatty  sheath  and  eventually  their  neurilemma,  nothing  but 
the  bare  axis-cylinder  being  left. 

Ganglia  on  Nerves. — Placed  on  certain  nerves,  somewhere 
in  their  course,  are  masses  of  nervous  tissue  called  ganglia  ; 
these  ganglia  are  composed  of  an  outer  covering  of  con- 
nective tissue  enclosing  nerve  cells,  between  which  nerve- 
fibres  pass ;  the  cells  are  of  a  particular  shape  depending 
upon  whether  the  ganglion  examined  be  from  the  cerebro- 
spinal or  sympathetic  system.  In  the  former  the  nerve  cells 
are  round  and  possess  a  projection  or  pole,  which  not 
unfrequently  coils  before  issuing  from  the  cell,  and  after 
running  a  short  distance  divides  T-shaped  into  two  branches 
which  travel  in  opposite  directions ;  such  a  body  is  known 
as  a  unipolar  cell.  The  ganglia  belonging  to  the  sympa- 
thetic differ  in  the  shape  of  their  cells,  for  these  instead  of 
having  one  pole  like  the  eerebro-spinal,  have  two,  three,  or 
more  poles,  known  as  bi-polar  or  multipolar  cells.  We 
may  here  say,  though  the  subject  will  be  touched  on  again, 
that  the  cells  of  the  brain,  spinal  cord,  and  sympathetic 
system,  are  mainly  multipolar,  whilst  those  of  the  spinal 
and  cranial  ganglia  are  unipolar.  Bi-polar  cells  may  be 
found  in  the  spinal  ganglia  of  fishes.  One  process  of  a 
nerve-cell  is  the  axis  cylinder  of  a  nerve,  the  other  processes 
branch,  dividing  and  subdividing  like  a  root,  and  become 
primitive  fibrils. 

Nerves  are  remarkable  for  their  want  of  elasticity ;  they 


THE  NEKVOUS  SYSTEM  385 

do  not  retract  on  being  divided ;  further  they  are  capable 
of  very  considerable  stretching  without  breaking.  In  the 
human  subject  the  nerves  of  the  limbs  require  a  weight  of 
from  40  lbs.  to  120  lbs.  to  break  them.  There  are  nerves 
supplying  nerves,  the  nervi  nervorum,  and  the  rationale  of 
nerve- stretching  in  painful  diseases  is  probably  accounted 
for  by  the  damage  done  to  these  minute  nerves  during  the 
process. 

Nerve  trunks  receive  a  poor  blood  supply,  though  ganglia 
and  grey  matter  are  richly  vascular ;  it  is  possible  that 
the  numbness  produced  in  a  sensory  nerve  by  pressure  is 
due  to  its  blood  supply  being  temporarily  cut  off,  the  nerve 
thus  losing  its  irritability.  The  lymphatics  are  numerous, 
and  exist  within  the  lamellae  of  the  perineurium  or  covering 
of  the  nerve  bundle. 

Excitability. — We  have  no  means  of  distinguishing  micro- 
scopically between  an  afferent  and  an  efferent  nerve ;  there 
is  nothing  in  the  structure  of  a  motor,  sensory,  or  secretory 
nerve,  which  enables  its  function  to  be  determined.  Further, 
we  know  that  though  in  the  body  impulses  pass  only  in  the 
one  direction  through  a  nerve,  yet  removed  from  the  body 
and  tested  electrically  it  is  as  easy  to  pass  impulses  in  one 
direction  as  another.  Nerves  however  are  excitable,  the 
living  nerve  can  be  made  to  react  by  means  of  chemical, 
mechanical,  or  electrical  stimuli,  and  when  so  excited 
appears  to  transmit  the  same  impulses  as  when  irritated 
physiologically,  viz.,  as  when  the  normal  body  impulses  are 
being  tra-nsmitted ;  thus  the  stimulation  of  a  sensory  nerve 
gives  rise  to  pain,  of  a  motor  nerve  to  muscular  contraction, 
and  of  a  secretory  nerve  to  secretion.  The  conductivity  of 
nerves  is  diminished  by  cold,  compression,  or  injury,  but 
it  is  noteworthy  that  even  after  long-continued  excitation 
nerves  are  found  practically  unfatigued.  Impulses  are  no 
longer  transmitted  when  nerves  are  ligatured  or  divided. 

Electric  Phenomena  of  Nerves. — Some  very  definite  facts 
are  known  in  connection  with  the  electric  currents  in  nerves, 
and  the  effect  on  the  excitability  of  the  nerve  of  transmitting 
currents  through  it ;  thesefacts  have  been  ascertained  with 

25 


386     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

the  nerves  of  the  frog,  and  so  far  as  can  he  proved  apply 
ecjually  to  those  of  the  higher  animals. 

If  a  nerve  be  removed  from  the  body  and  suitably  applied 
to  an  instrument  which  is  capable  of  measuring  delicate 
electric  currents,  the  galvanometer,  the  needle  of  the  in- 
strument will  be  found  to  be  deflected,  showing  the  passage 
of  a  current ;  it  is  spoken  of  as  the  current  of  rest,  but  it 
does  not  exist  in  uninjured  nerves.  It  is  practically 
identical  in  direction  with  the  natural  muscle  current 
described  on  p.  367.  If  while  the  current  of  rest  is 
passing  shocks  be  sent  into  the  nerve  from  an  induction 
coil,  the  needle  of  the  galvanometer  is  found  to  indicate  a 
momentary  current  in  the  opposite  direction  to  the  current 
of  rest.  This  momentary  opposite  current  is  spoken  of  as 
necfative  variation  or  the  current  of  action,  and  the  phe- 
nomenon is  essentially  the  same  as  in  the  case  of  a  muscle. 

The  negativity  developed  at  the  point  to  which  the 
stimulus  is  applied  travels  as  a  wave  in  both  directions 
along  the  nerve  at  the  rate,  for  frog's  nerve,  of  about 
28  metres  or  some  90  feet  per  second.  In  the  case  of  a 
muscle  the  change  of  shape  when  stimulated  provides  the 
necessary  indication  of  its  receipt  of  an  impulse.  Not  sa 
with  a  nerve.  Here  we  have  no  change  of  shape,  no 
development  of  heat,  to  mark  its  functional  activity.  The 
one  indication  of  the  passage  of  an  impulse  along  an  isolated 
jnece  of  nerve  is  the  electrical  change  taking  2)lace  in  it, 
and  hence  the  study  of  the  '  current  of  action  '  and  the 
onward  rush  of  the  attendant  '  negativity '  is  here  of 
supreme  interest. 

Turning  next  to  the  nerve-muscle  preparations  suspended 
in  the  moist-chamber  (see  p.  359),  and  using  the  contrac- 
tion of  the  muscle  as  an  index  of  the  activity  of  its  nerve, 
we  may  study  other  important  phenomena  of  nerve-excita- 
tion. If  a  moderately  strong  constant  current  be  passed 
into  the  nerve  by  connecting  it  with  the  poles  of  a  battery, 
at  the  moment  the  connection  is  made  the  muscle  gives  a 
twitch  or  contraction,  and  then  remains  perfectl3^  quiet 
though  the  current  is  still  streaming  through  its  nerve ;  if 


THE  NEEVOUS  SYSTEM  387 

the  connection  be  broken  as  by  opening  a  key  in  the  battery 
circuit,  the  muscle  gives  another  contraction.  These  are 
termed  '  making '  and  '  breaking  contraction,'  viz.,  a  con- 
traction produced  on  closing  and  opening  the  electric  circuit. 
If  instead  of  a  moderately  strong  constant  current  a  weak 
or  very  strong  one  be  used,  the  results  on  making  and 
breaking  may  not  be  the  same. 

During  the  period  of  apparent  quiescence  following  the 
closing  of  the  circuit,  though  the  muscle  is  giving  no 
indication  of  the  current,  yet  changes  are  occurring  in 
the  nerve.  If  it  be  tested  by  stimulating  it  with  an  induced 
current,  it  is  found  that  its  irritability,  as  measured  by 
the  greater  or  less  contraction  of  the  attached  muscle,  is 
diminished  in  the  neighbourhood  of  the  positive  pole  (anode) 
of  the  continuous  current,  and  increased  in  the  neighbour- 
hood of  the  negative  pole  (kathode).  This  changed  con- 
dition is  known  as  electrotonus,  the  diminished  irritability 
being  known  as  anelectrotonus,  the  increased  excitability 
as  kathelectrotonus.  Between  the  increased  and  reduced 
irritability  is  a  zone  of  unaffected  irritability  known  as  the 
neutral  point.  During  the  condition  of  electrotonus  there 
is  no  interruption  to  the  natural  nerve  current,  which  is 
simplj^  increased  in  strength  if  the  constant  current  takes 
the  same  direction  in  the  nerve  as  the  current  of  rest ;  but 
if  the  constant  current  be  passed  in  the  opposite  direction 
to  the  nerve  current,  the  latter  is  diminished. 

A  reference  to  Fig.  83  will  suffice  to  make  the  matter 
clear.  At  a  certain  part  of  the  nerve  a  continuous  current 
of  electricity  generated  at  e  is  passed  through  it,  the 
application  and  withdrawal  of  which  gives  rise  to  the 
making  and  breaking  contraction  previously  mentioned  ; 
during  the  passage  of  the  current  the  muscle  is  perfectly 
quiet,  in  spite  of  important  changes  occurring  in  the  nerve. 
Shocks  are  now  sent  into  the  nerve  from  an  induction  coil 
at  a  place  between  the  muscle  and  the  points  of  application 
of  the  continuous  current ;  as  the  result  of  the  stimulation 
the  muscle  either  responds  more  than  it  should  do  for  the 
strength  of  the  stimulus  employed,  viz.,  there  is  increased 

25—2 


388     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 


excitability  of  the  nerve  (kathelectrotonns),  or  the  muscle 
does  not  respond  as  strongly  as  it  should,  viz.,  there  is 
decreased  excitability  of  the  nerve  (anelectrotonus).  The 
increase  or  decrease  of  excitability  in  the  nerve  depends 
upon  whether  the  continuous   current   is  passed  down  it, 


Fig.  88. — Diagram  of  Electrotonus. 

N,  The  nerve  running  to  the  muscle  m  ;  e,  battery  or  cell  for  the  produc- 
tion of  a  constant  current,  the  positive  pole  or  anode  (a)  in  A  being 
placed  furthest  from  the  muscle,  the  current  consequently  flowing 
down  the  nerve,  and  in  B  being  placed  nearest  to  the  muscle,  the 
current  flowing  up  the  nerve.  At  s  the  nerve  is  stimulated  by  an 
mduced  current,  and  its  irritability  determined  by  the  contraction 
of  the  muscle  m  ;  the  irritability  is  increased  in  A,  kathelectronus, 
and  decreased  in  B,  anelectrotonus. 

as  in  A,  or  up  it,  as  in  B  ;  with  a  descending  current 
the  excitability  is  increased,  with  an  ascending  one  it  is 
decreased. 

The  explanation  of  electrotonus  in  nerves  is  that  it  is  a 
vital  phenomenon,  viz.,  the  irritability  of  the  nerve  is  in- 
creased when  its  molecules  pass  from  their  ordinary  con- 


THE  XEEYOUS  SYSTEM  389 

dition  to  one  of  greater  mobility  (kathelectrotonus),  or  it 
is  diminished  when  its  molecules  pass  from  their  ordinary 
condition  to  one  of  less  mobility  (anelectrotonus).  Her- 
mann considers  that  it  is  a  purely  physico-chemical  pheno- 
menon, due  to  the  electric  current  generating  acids  at  the 
positive  pole,  and  alkalis  at  the  negative  ;  the  effect  of  the 
acid  is  to  lower  the  excitability  of  the  nerve,  and  of  the 
alkali  to  increase  it.  One  practical  application  of  this  law  is 
that  the  excitability  of  a  part,  as  in  pain,  cramp,  etc.,  may  be 
removed  b}'  passing  a  current  up  the  nerve,  viz.,  by  placing 
the  positive  pole  nearest  the  muscle,  and  producing  anelec- 
trotonus ;  or  by  reversing  the  process  and  throwing  the 
current  down  the  nerve,  so  that  the  negative  pole  is  nearest 
the  muscle,  the  irritability  of  the  part  ma}'  be  increased. 

The  Nature  of  Nervous  Impulses  is  quite  unknown,  ex- 
cepting that  they  travel  in  the  form  of  a  wave  of  electric 
disturbance,  which  is  shorter,  and  travels  more  rapidly  than 
that  which  traverses  a  muscle.  Impulses  are  not  trans- 
mitted from  one  fibre  to  another  in  a  nerve  bundle. 

Conductivity  of  Nerves. — Compared  with  electricity  a 
nervous  impulse  travels  very  slowly,  and  it  is  necessary  to 
bear  this  in  mind  as  comparisons  between  electric  currents 
and  nerve  impulses  have  been  made.  The  velocity  of  ner- 
vous impulses  in  motor  nerves  has  been  stated  to  be  between 
111  to  140  feet  per  second,  whilst  through  sensory  nerves  it 
is  said  to  be  faster,  160  to  320  feet  per  second ;  in  visceral 
nerves  the  velocity  is  less.  Chauveau  ascertained  in  the 
pharyngeal  branches  of  the  vagus  that  the  velocity 
amounted  to  26  feet  per  second. 

Degeneration  of  Nerves. — "When  nerves  are  cut  they 
degenerate,  the  degeneration  always  taking  place  in  the 
portion  cut  off  from  its  nutrient  centre.  The  nerve  fibre, 
as  has  been  stated  above,  is  but  a  branch  of  a  nerve  cell ; 
if  a  portion  of  a  cell  be  separated  from  the  part  containing 
the  nucleus  of  the  cell  it  soon  dies.  Thus,  when  a  large 
amceba,  or  a  Radiolarian,  is  torn  up  into  several  pieces, 
the  portions  containing  no  nucleus  degenerate  and  die  ; 
but  that  portion  containing  the  nucleus  repairs  itself  and 


390     A  MANUAL  OF  VETEEINAEY  PHYSIOLOGY 

reforms  a  perfect  cell.  The  nerve  fibre  dies  down  after 
being  cut,  just  in  so  far  as  it  is  a  piece  of  cell  cut  off  from 
its  nucleus.  The  sensory  nerve  divided  in  neurectomy,  as 
practised  on  the  horse,  degenerates  towards  the  foot  and  not 
up  the  limb,  for  it  is  the  piece  below  the  wound  which  is  cut 
off  from  its  nutrient  centre  and  not  the  portion  above  ;  had 
this  been  a  motor  nerve  the  degeneration  would  still  have 
taken  place  below  the  wound  and  for  the  same  reason.  All 
spinal  nerves  have  their  seat  of  nutrition  either  in  the 
spinal  cord  or  in  the  ganglia  just  outside  it  (see  p.  384) ; 
the  nearer  to  the  spinal  cord  the  point  at  which  the  section 
is  made  the  greater  the  length  of  nerve  which  degenerates, 
the  further  away  from  the  cord  the  point  at  which  section 
is  practised  the  shorter  the  length  which  degenerates. 
When  the  nerve  degenerates  the  fatty  medullary  sheath 
breaks  up,  forming  globules  around  the  axis-cylinder ;  the 
latter  also  degenerates  and  ultimately  breaks  up.  The 
remarkable  fact  about  these  changes  is  the  rapidity  with 
which  they  occur,  six  days  is  sufficient  to  show  their  com- 
mencement ;  small  nerve  fibres  degenerate  more  quickly 
than  large. 

By  suturing  divided  nerves  union  occurs,  and  though  the 
act  of  division  causes  degeneration,  yet  when  union  takes 
place  regeneration  of  fibres  occurs  ;  a  fresh  axis-cylinder 
grows  through  the  length  of  the  degenerated  nerve,  and 
after  some  weeks  and  often  months  motion  or  sensation 
is  restored,  the  former  always  much  later  than  the  latter. 
Even  suture  of  divided  nerves  is  not  always  necessary  for 
union  ;  we  know  clinically  that  the  plantar  nerves  of  the 
horse  will  often  unite  in  a  few  months  in  spite  of  a  piece 
being  excised,  the  portion  of  nerve  above  sending  down 
an  axis-cylinder  which  soon  finds  out  its  divided  portion 
below. 

Not  only  is  the  nutrition  of  the  nerve  itself  affected 
by  nerve  division,  but  also  the  nutrition  of  those  parts 
supplied  by  it.  Ulceration  more  or  less  severe  has  been 
known  to  follow  injury  of  certain  nerves  ;  sloughing  of  the 
cornea  occurs  in  animals  when  the  ophthalmic  division  of 


THE  NERVOUS  SYSTEM  391 

the  fifth  is  divided ;  and  many  are  practically  acquainted 
with  the  sloughing  of  the  entire  foot  which  sometimes, 
though  fortunately  rarely,  follows  the  operation  of  neurec- 
tomy. It  is  undoubted  that  nerves  influence  the  nutrition 
of  a  part ;  nowhere  is  this  better  demonstrated  than  in  cases 
of  intense  muscular  atrophy  due  to  nerve  injury. 

Nerve  Terminations. — There  are  some  structures  such  as 
glands  where  the  nature  of  the  nerve  termination  is  not 
satisfactorily  made  out,  there  are  other  places  such  as  muscle 
where  definite  and  distinct  motor  nerve-endings  have  been 
found ;  and  on  many  sensory  and  sympathetic  nerves 
special  terminations  known  as  Pacinian  corpuscles  and 
Krause's  end-bulbs  exist.  Nerve  terminations  are  found  in 
the  muzzle  of  animals,  in  tendons,  in  muscles,  in  the 
generative  organs,  conjunctiva,  mouth,  tongue,  epiglottis, 
etc.  ;  some  are  known  as  Krause's  end-bulbs,  those  in 
tendon  are  described  as  the  organ  of  Golgi,  in  muscle  they 
are  known  as  end-plates,  whilst  in  the  skin  of  the  muzzle 
the  nerves  terminate  in  small  swellings  or  enlargements 
known  as  tactile  cells,  which  are  placed  between  the 
epithelial  cells  of  the  epidermis ;  cells  of  this  kind  also 
exist  in  the  foot  of  the  horse.  The  nerves  of  special  sense 
have  each  a  distinct  termination  peculiar  to  themselves, 
such  as  the  hair  cells  of  the  internal  ear,  the  rods  and 
cones  of  the  retina,  taste  bulbs  of  the  tongue,  etc. 

Spinal  Cord. 

The  spinal  cord  extends  from  the  atlas  to  about  the 
second  or  third  sacral  vertebra,  and  is  completely  enclosed 
in  a  dense  membrane,  the  dura  mater.  The  canal  in  which 
it  is  lodged  is  very  much  larger  than  the  cord,  especially  at 
those  parts  where  the  greatest  amount  of  movement  occurs, 
as  in  the  neck.  The  cord  is  not  the  same  shape  nor 
the  same  size  throughout ;  oval  in  the  cervical  region,  it 
becomes  circular  in  the  dorsal,  and  again  oval  in  the 
lumbar  portion.  It  is  largest  where  any  considerable  bulk 
of  nerves  is  being  given  off,  and  thus  there  is  an  enlarge- 


392     A  MANUAL  OF  YETEEINAKY  PHYSIOLOGY 

ment  corresponding  to  the  fore,  and  another  to  the  hmd 
limbs.  On  exposing  the  spinal  canal,  a  large  number  of 
nerves  are  found  to  be  passing  through  the  dura  mater 
either  outwards  or  inwards,  and  these  gain  an  exit  from  or 
entrance  to  the  spinal  canal  by  means  of  the  foramen 
formed  at  the  junction  of  the  vertebrae. 

Spinal  Nerves. — On  opening  the  dura  mater,  it  is  observed 
that  the  nerves  divide  in  such  a  way  that  one  part  of  each 
of  them  runs  to  the  superior,  and  the  other  part  to  the 
inferior  surface  of  the  cord  ;  these  are  spoken  of  as  the 
superior  and  inferior  roots  of  the  spinal  nerves.  Li  the 
horse  each  superior  and  inferior  root  enters  the  cord  not  as 
a  single  cord  but  as  several.  On  the  superior  root,  but 
outside  the  dura  mater,  is  found  a  ganglion  ;  each  rootlet 
of  a  superior  spinal  root  has  a  ganglion  on  it :  no  such  body 
exists  on  the  inferior  root.  Both  inferior  and  superior 
roots  unite  below  the  ganglion  to  form  a  mixed  spinal 
nerve  (see  Fig.  84).  The  function  of  these  two  roots  is 
entirely  different ;  the  superior  root,  possessing  the  ganglion, 
conveys  centripetal  (sensory)  impulses  only;  the  inferior 
root  conveys  centrifugal  (motor)  impulses  to  muscles  and 
glands.  The  superior  roots  are  passing  into  the  cord,  the 
inferior  roots  are  passing  out  of  it. 

Passing  out  with  the  inferior  root  of  the  spinal  nerve, 
but  indistinguishable  from  it,  is  a  branch  of  nerve  known 
as  the  white  ramus  communicans,  which  leaves  the  main 
trunk  after  the  mixed  nerve  has  been  formed,  and  runs  to  a 
distinct  system  known  as  the  sympathetic.  One  part  of  the 
latter,  the  gangliated  cord,  runs  under  the  arches  of  the  ribs 
and  back  as  far  as  the  loins ;  to  this  cord  the  white  ramus 
runs,  and  establishes  a  communication  between  the  cerebro- 
"spinal  and  sympathetic  system ;  in  this  branch  are,  among 
others,  the  nerves  which  constrict  the  bloodvessels.  A 
careful  study  of  Fig.  85  is  necessary  for  the  clear  elucida- 
tion of  the  arrangement  of  the  spinal  nerves. 

Arrangement  of  the  Cord. — If  a  cord  be  suitably  prepared, 
a  transverse  section  shows  that  it  consists  of  two  similar 
halves,  united  by  a  comparatively  small  central  mass  of 


THE  NERVOUS  SYSTEM 


39B 


tissue  through  whose  centre  a  minute  (longitudinal)  canal 
runs.  The  halves  are  separated  by  fissures  on  the  superior 
and  inferior  surfaces  of  the  cord  ;  the  inferior  fissure  is 
wide  and  does  not  reach  down  to  the  centre  of  the  cord, 
while  the  superior  fissure  is  narrow  and  deeper  (Fig.  84). 
Each  half  is  further  seen  to  consist  of  a  superior,  lateral, 


Fig.  84. — Transverse     Section    of     the     Spi.val     Cord     in  the 

Cervical    Region     x  8d.     The    Lines    in    the    Lateral  and 

Superior    Columns    running   from    the    Outer   Margin  are 
Lamin/E  of  the  Pia  Mater  (M'Kendrick). 

a,  Processus  reticularis;  6,  superior  liorn  ;  c,  grey  commissure;  d, 
superior  septum  ;  e,  GoU's  column  ;  /,  superior  column  ;  g,  point  of 
entrj-  of  superior  root ;  h,  substantia  gelatinosa  ;  i,  lateral  column  ; 
j,  large  multipolar  nerve  cells  ;  l\  inferior  horn  ;  /,  white  com- 
missure;  m,  inferior  longitudinal  tissure ;  n,  inferior  column; 
o,  central  canal ;  ^j,  point  of  exit  of  inferior  roots. 


and  inferior  column,  separated  from  each  other  by  a  shallow 
longitudinal  groove. 

A  section  of  the  cord  shows  it  to  be  made  up  of  both  white 
and  grey  matter ;  the  latter  placed  internally,  and  forming 
the  medulla,  is  arranged  something  like  two  commas  placed 
back  to  back,  the  tail  of  the  comma  being  uppermost.  The 
tail  of   the  comm.a  corresponds    to  the  incoming  sensory 


Fig.  85. — Scheme    of 


THE    Nerves  of  a  Segment  of   the    Spina 
Cord  (Foster). 


Gr.,  grey  ;  ^Y,  white 
matter  of  spinal 
cord  ;  A,  inferior ; 
P,  superior  root  ; 
G,  ganglion  on  the 
superior  root ;  N, 
ruixed  nerve,  con- 
sisting of  sensory 
and  motor 
branches  with 
fibres  passing  into 
the  sympathetic 
system  at  V  ;  W, 
spinal  nerve,  con- 
sisting of  sensory 
and  motor 
branches,  termin- 
ating in  M,  skele- 
tal muscles,  S, 
sensory  cell  or  sur- 
face, and  X,  in 
other  ways.  V, 
white  ramus  com- 
municans  uniting 
the  cerebro-spinal 
with  the  sympa- 
thetic S3' stem; 
it  runs  out  from 
the  cord  with  the 
inferior  spinal 
nerve,  and  is  given 
off  from  the  mixed 
nerve  at  Y,  from 
whence  it  passes 
to  2,  a  ganglion  on 
the  sympathetic 
chain,  and  thence 
on  to  V^  to  supply 
the  more  distant 
ganglion  a,  then 
to  V^i  to  the  peri- 
pheral ganglion 
5',  and  ends  in 
ni,  a  visceral 
muscle,  s,  a  vis- 
ceral sensory  cell 
or  surface,  and  x, 
other  possible  vis- 
ceral endings. 

From  2  is  given  off  a  branch,  r.v.,  known  as  the  grey  ramus  com- 
municans,  which  partly  passes  backwards  towards  the  spinal  cord, 
and  partly  runs,  as  v.m.,  in  connection  with  the  spinal  nerve  to 
supply  vaso-constrictor  fibres  to  the  muscles  of  bloodvessels,  ni^,  in 
certain  parts,  for  example,  in  the  limbs. 

Sy,  the  sympathetic  chain  (gangliated  cord  of  the  sympatlietic)  uniting 
the  ganglia  of  the  series  2.  The  terminations  of  the  other  nerves 
arising  from  2  a,  <t',  are  not  shown. 


THE  NERVOUS  SYSTEM  395 

fibres,  the  head  of  the  comma  to  the  outgoing  motor  ones  ; 
the  two  commas  anatomically  known  as  cornua  are  con- 
nected by  a  band  of  grey  matter  called  the  grey  commis- 
sure, in  the  centre  of  which  is  a  canal.  The  white 
substance  of  the  cord  is  not  of  the  same  thickness  through- 
out ;  stated  generally  the  cord  increases  in  white  matter 
from  the  tail  to  the  head  ;  the  grey  matter  is  largest  in 
the  cervical  and  lumbo-sacral  enlargements,  and  this  in- 
crease and  decrease  in  size  corresponds  with  the  increase 
and  decrease  in  the  number  of  nerves  entering  and  leaving 
the  cord  in  these  regions.  The  white  substance  of  the  cord 
is  found  microscopically  to  consist  of  longitudinally  arranged 
medullated  nerve  fibres,  very  much  like  those  previously 
described,  excepting  that  the  fatty  white  substance  has  not 
the  same  covering  found  in  other  medullated  nerves,  but 
is  enclosed  in  a  sheath  of  neuro-keratin  which  is  peculiar 
to  the  spinal  cord.  Between  the  groups  of  fibres  peculiar 
branched  corpuscles  exist  known  as  glia  cells ;  these  belong 
to  a  connective  tissue  peculiar  to  the  cord,  called  the 
neuroglia. 

The  grey  matter  of  the  cord  consists  of  cells,  many  being 
large  and  multipolar ;  amongst  these  are  very  fine  fibres, 
either  the  delicate  processes  of  the  cells,  or  of  medullated 
or  non-medullated  fibres  derived  from  the  white  substance 
of  the  cord,  and  terminating  in  the  grey  matter :  the  whole 
is  held  together  by  neuroglia.  The  nerve  cells  of  the  grey 
matter  of  the  inferior  cornua  arrange  themselves  into 
certain  groups  in  various  parts  of  the  cord,  those  situated 
at  the  lower  part  of  the  cornua  run  the  entire  length  of  the 
cord,  but  others  are  limited  to  certain  regions,  being  at 
some  parts  strongly  in  evidence,  at  others  almost  or  entirely 
absent.  The  cells  are  very  large,  and  possessing  numerous 
branched  processes  or  poles,  are  known  as  multipolar  cells. 
In  the  superior  cornua  the  cells  are  not  in  groups,  and 
moreover  they  are  much  smaller. 

A  peculiar  column  of  cells  known  as  Clarke's  column  is 
limited  to  three  portions  of  the  spinal  cord  ;  the  cells  are 
found  lying  above  the  inferior  cornua  towards  the  middle 


39G     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

line  of  the  cord,  and  are  related  to  the  endings  of  sensory 
nerves  entering  the  cord.  The  cells  of  this  column  have 
few  processes,  and  their  long  axis  lies  in  the  long  axis  of 
the  cord. 

Distribution  of  Nerve  Fibres  in  the  Cord. — The  superior 
spinal  nerve  root  joins  the  superior  cornu,  the  inferior 
proceeds  from   the  inferior  cornu.     We    must    now    learn 


Fig  86. — Lateral  Column  of  a  New-born  Eabbit  to  show  the 
Collateral  Fibres,  and  the  Manner  in  which  the  Longi- 
tudinal Fibres  bend  round  and  end  free  in  the  Grey 
Matter  (Landois  and  Stirling,  after  Eam6n  y  Cajal), 

c,  Collateral  fibres  ;  <??,  bending  round  of  the  longitudinal  fibres  ;  Z,  to  end 
in  the  grey  matter  ;  n,  axis-cylinder  process  of  nerve-cell  bending 
in  amongst  the  longitudinal  fibres  of  the  white  column. 

how  the  fibres  composing  these  roots  are  disposed  with 
reference  to  the  cord  itself.  The  fibres  of  the  superior 
spinal  nerve  root  grow  from  the  spinal  ganglion,  enter 
the  superior  column  of  the  white  matter  of  the  cord,  and 
some  travel  forwards,  others  backwards  in  its  substance. 
After  running  a  short  distance  the  fibres  bend  at  right 
angles,  enter  the  grey  substance  of  the  cord,  and  end  not 


THE  NEEVOUS  SYSTEM  397 

in  nerve  cells  but  simply  in  fine  branches  (Fig.  86). 
During  their  course,  before  the  final  bending  occurs,  they 
all  give  off  collateral  fibres  at  right  angles  to  the  main 
trunk,  which  enter  the  grey  matter,  and  connect  the 
parent    fibre  with  various    segmental   levels  of    the  cord. 


Fig.  87. — Transverse  Section  of  the  Spinal  Cord  in  the  Thoracic 
Region  of  an  Embryo  Fowl,  at  Ninth  Day  of  Incubation 
(Landois  and  Stirling,  after  Ramon  y  Cajal). 

A,  inferior  root ;  P,  superior  root  of  spinal  nerve  ;  C,  axis-cylinder  of 
a  motor  nerve-cell,  issuing  from  the  large  cells  of  the  inferior 
cornu  ;  D,  intra-meduUary  part  of  the  superior  root ;  e,  origin  of  a 
collateral  branch,  which  ramifies  as  /  g,  the  terminal  ramifica- 
tions of  the  collateral  fibres  ;  d,  final  bifurcation;  E,  ganglion  on 
superior  root ;  h,  bipolar  ganglionic  cells ;  i,  a  unipolar  nerve- 
cell  similar  to  those  in  mammals. 

In  this  way  all  the  sensory  fibres  reach  the  grey  matter 
of  the  superior  and  in  some  cases  the  inferior  cornua, 
terminating  in  numerous  fibrils  close  to  but  not  in  con- 
tinuity with  the  nerve  cells  (Fig.  87).  The  superior  column 
of  white  matter  is  made  up  entirely  of  the  fibres  of  the 
superior  roots  arising  in  the  cells  of  the  spinal  ganglion. 


398     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  fibres  of  the  inferior  spinal  nerve  roots  may  be 
traced  into,  or  rather,  out  of,  the  grey  matter  of  the 
inferior  cornua.  There  each  has  its  origin,  for  it  really 
arises  in  one  pole  of  the  large  multipolar  cells  situated 
there,  the  remaining  poles  of  the  cells  giving  off  small 
branched  fibres  (Figs.  87  and  88).  The  axis-cylinder 
process  of  the  inferior  cornual  cell  is,  excepting  in  very 
rare  instances,  without  collaterals  (Fig.  88). 

The  inferior  and  lateral  white  columns  are  composed  of 


Fig.  88. — A  Nervk-Cell  in  the  Inferior  Cornu    of  the  Lumbar 
Region    of    an    Ox   Embryo    (Landois    and    Stirling,    after 

GOLGl). 

n,  Axis-cylinder  process  passing  at  n'  into  a  longitudinal  fibre  of  the 
inferior  column  ;  n"  much  branched  lateral  process  of  n. 

fibres  which  originate  from  nerve-cells  in  the  grey  matter 
of  the  cord  itself ;  the  fibres  in  these  columns  give  off 
collateral  fibres  which  again  enter  the  grey  matter,  and 
finally  the  fibre  itself,  as  we  saw  in  the  superior  column, 
bends  round  and  terminates  in  the  grey  matter  of  the  cord. 
It  is  observed  that  none  of  these  fibrils  are  in  continuity 
with  nerve  cells,  nor  do  the  fibrils  anastomose  among  them- 
selves. The  inferior  and  lateral  columns  of  one  side  of  the 
cord  receive  fibres  from  the  opposite  side,  and  these,  with 
the  aid  of  the  collateral  fibres  which  are  returning  in  the 


THE  NEEYOUS  SYSTEM  399 

opposite  direction,  help   to    constitute   the   anterior   com- 
missure. 

Corresponding  to  this  arrangement  of  the  fibres  it  is 
found  that  three  different  kinds  of  nerve  cells  exist  in  the 
grey  matter  :  (1)  the  large  multipolar  cell  of  the  inferior 
cornua  in  which  the  motor  fibres  begin,  (2)  cells  supplying 
the  fibres  which  pass  into  the  inferior  and  lateral  columns 
of  the  cord,  and  (8)  cells  found  only  in  the  superior  cornua, 
giving  oft'  a  process  which  is  confined  entirely  to  the  grey 
matter  and  breaking  up  in  it.  In  connection  with  all  these 
cells  it  is  important  to  bear  in  mind  that  the  branched 
processes  do  not  anastomose,  although  they  are  often  con- 
tinued for  a  considerable  distance. 

The  chief  features  which  recent  inquiry  has  brought  to 
light  are  the  collateral  branches  of  fibres  ;  these  fibrils 
do  not  anastomose,  but  terminate  by  surrounding  and 
perhaps  coming  into  contact  with  nerve  cells.  This  latter 
fact  would  go  to  show  that  man}^  of  the  impulses  conveyed 
to  the  cord  can  only  act  on  the  nerve  cells  by  contact 
without  continuity. 

Function  of  Spinal  Nerves. — If  the  superior  spinal  roots  be 
divided  all  parts  supplied  by  them  below  the  division  lose 
sensation,  but  if  the  portion  of  nerve  in  connection  with 
the  spinal  cord  be  irritated  pain  is  produced.  If  the 
inferior  roots  be  divided  all  parts  supplied  by  the  nerves 
below  the  seat  of  division  sufter  motor  paralysis  ;  if  the  cut 
end  of  the  nerve  still  in  connection  with  the  tissues  be 
irritated  the  muscles  contract  vigorously,  while  if  the  piece 
of  nerve  in  connection  with  the  cord  be  irritated  nothing 
hai^pens.  In  this  way  it  is  demonstrated  that  the  sensory 
impulses  pass  into,  whilst  the  motor  impulses  pass  out  of 
the  cord.  Sometimes  pain  is  felt  when  the  motor  roots  are 
divided,  due  to  one  or  two  branches  of  the  sensory  nerves 
finding  their  way  in  to  the  cord  by  this  channel ;  the  phe- 
nomenon is  known  as  n-curreiit  sensibility. 

The  function  of  the  inferior  or  motor  roots  is  to  supply 
all  the  voluntary  muscles  with  the  power  of  movement,  the 
bladder,  uterus,  intestines,  and   other  hollow  viscera  and 


400     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  bloodvessels,  with  the  power  to  contract  and  dilate, 
secretory  fibres  to  the  sweat-glands  of  the  skin,  and 
perhaps  '  nutritive '  nerves  to  the  tissues.  Many  of  the 
nerve  fibres  of  the  efferent  or  inferior  roots  are  distributed 
via  the  sympathetic  system  ;  from  which  nerve  cells  in  the 
cord  they  arise  is  not  at  present  known,  but  after  leaving 
the  cord  in  the  inferior  roots  they  join  the  vertebral 
ganglia  of  the  sympathetic  under  the  name  of  the  *  rami 
communicantes'  of  those  ganglia,  thus  establishing  a 
communication  between  these  two  important  systems 
(see  p.  89-1). 

The  spinal  sensory  fibres  endow  the  whole  of  the  body 
with  sensation,  with  the  exception  of  certain  parts  of  the 
face. 

Wallerian  Degeneration. — As  the  result  of  dividing  the 
spinal  nerves  degeneration  of  these  occurs ;  if  the  superior 
root  be  divided  above  the  ;iaii<iUon  degeneration  occurs 
upwards  into  the  spinal  cord,  and  there  affects  certain 
bundles  of  fibres,  which  are  really  the  fibres  of  the 
superior  roots  continued  into  the  spinal  cord,  and  running 
especially  forward  towards  the  head  (Fig.  89 — 1).  If  the 
root  be  divided  heloic  the  ganglion  the  degeneration  takes 
place  in  a  downward  direction,  involving  the  whole  length 
of  the  nerve  below  the  ganglion  (Fig.  89—2) ;  if  the  trunk 
be  divided  both  above  and  below  the  ganglion  degeneration 
above  and  below  occurs,  but  the  ganglion  remains  un- 
affected. If  the  entire  nerve  be  divided  below  the  ganglion, 
both  the  efferent  and  afferent  fibres  of  which  it  is  com- 
posed degenerate  below  the  section  (Fig.  89 — B). 

All  this  is  explained  by  saying  that  the  ganglion  is  the 
seat  of  nutrition  of  the  sensory  nerve-fibres,  and  which- 
ever part  of  the  superior  root  is  cut  off  from  its  nutritive 
influence  degenerates ;  or  in  other  words,  the  ganglion 
contains  the  nerve  cells,  of  which  the  sensory  nerve  fibres 
of  the  nerve  are  the  axis-cylinder  processes. 

If  the  inferior  nerve  roots  be  divided  degeneration  of 
these  also  occurs,  but  the  degeneration  extends  down  the 
trunk  of  the  nerve,  and  does  not  run  up  into  the  spinal 


THE  NERVOUS  SYSTEM 


401 


cord  (Fig,  89 — 4),  the  explanation  being  that  the  seat  of 
nutrition  of  the  motor  nerves  Hes  in  the  spinal  cord,  so 
that  degeneration  occurs  below  the  cut  part  and  not 
above  it.  Their  nutrient  centre  is  the  multipolar  ganglion 
cell  in  the  inferior  cornu  of  the  grey  matter  of  the  cord. 


No.  1. — Degeneration  of  afferent 
tibres  caused  by  a  section  of  supe- 
rior root  above  the  ganglion. 


No.  2. — Degeneration  of  afferent 
fibres  below  a  section  of  superior 
root  below  the  ganglion. 


No.  3. — Degeneration  of  efferent 
and  afferent  fibres  below  a  section 
of  the  entire  nerve. 


No.  4. — Degeneration  of  efferent 
fibres  below  a  section  of  inferior 
root. 


Fig.  89.— Diagrams  to  illustrate  Wallerian   Degeneration 
OF  Nerve  Roots  (Waller). 


These  degenerative  changes  were  first  described  by  Waller, 
and  the  means  they  supply  of  tracing  out  the  tracts  of 
fibres  in  the  central  nervous  system  is  hence  known  as  the 
Wallerian  method. 

Tracts  in  the  Cord. — The  white  matter  of  the  cord  can 
be  mapped  out  into  columns  or   tracts,  which  are  quite 

26 


402     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

distinct  from  the  columns  into  which  the  cord  is 
anatomically  divided.  Some  of  these  tracts  convey  impulses 
from  the  cord  to  the  brain,  and  are  known  as  ascending 
tracts ;  others  convey  impulses  from  the  brain  to  the  cord, 
and  are  known  as  descending  tracts.-  These  ascending 
and  descending  tracts  have  not  been  made  out  by  ordinary 


Fig.  90. — Scheme  showing  the  Degeneration  Tracts,  and  the 
Paths  which  do  not  undergo  Degeneration  in  the  Cord 
(Landois  and  Stirling). 

AMF,  Inferior  median  fissure ;  dpt  and  cpt,  direct  and  crossed  pyra- 
midal tracts.  AR,  inferior  root ;  ph,  superior  root  of  spinal 
nerves ;  aal  and  dal,  ascending  and  descending  infero-lateral 
tracts;  ct,  cerebellar  tract;  go,  column  of  Goll ;  d,  comma- 
shaped  tract;  PMZ,  superior  marginal  zone  ;  pec,  posterior-external 
column.  The  parts  left  black  do  not  undergo  degeneration  ;  the 
ascending  degenerations  are  shaded  with  dots,  the  descending 
with  lines. 

observation,  but  by  experimental  inquiry  and  embryological 
studies.  It  was  found  that  after  division  of  certain  nerves,  or 
injuries  to  certain  parts  of  the  brain  or  spinal  cord,  particular 

*  We  have  hesitated  to  adopt  the  terms  ascending  and  descending, 
as  not  harmonizing  with  the  terms  employed  in  dealing  anatomically 
with  the  cord  of  the  quadruped,  but  it  is  difficult  to  find  a  suitable 
substitute. 


THE  NERVOUS  SYSTEM  403 

tracts  became  degenerated  either  in  a  forward  or  backward 
direction.  By  this  and  other  means,  it  was  ascertained 
that  certain  paths  or  tracts  exist  in  the  white  matter  of  the 
cord,  connecting  the  brain  with  the  cord  and  rice  versa. 
It  must  not  be  supposed  that  the  function  of  an  ascending 
or  descending  tract  is  necessarily  entirely  exerted  in  the 
direction  given  to  it  by  its  name  ;  the  tracts  are  called 
ascending  or  descending  according  to  the  direction  taken  by 
the  degeneration. 

The  following  are  the  main  tracts  in  the  cord  (see 
Fig.  90)  : 

Descending  Tracts.  Ascending  Tracts. 

Crossed  pyramidal  tract.  Direct  cerebellar  tract. 

Direct   pyramidal    tract — column  Median  superior  tract — column  of 
of  Tiirck  (only  found  in  man  and  Goll. 

anthropoid  apes).  Lateral  superior  tract — column  of 
Infero-lateral  descending  tract.  Burdach. 

Infero-lateral  ascending  tract. 

These  paths,  known  to  various  observers  by  somewhat 
different  names,  are  distributed  between  the  superior, 
lateral,  and  inferior  columns.  The  tracts  are  not  found 
throughout  the  entire  length  of  the  cord,  and  those  descend- 
ing diminish  in  size  from  the  head  to  the  tail,  those  ascend- 
ing diminish  from  tail  to  head. 

The  crossed  pyramklal  tract  is  large  in  man  but  small  in 
the  dog  (Fig.  91);  it  is  in  connection  with  the  motor  region 
of  the  brain,  and  its  great  size  in  man  appears  to  bear  a 
distinct  relation  to  the  development  of  the  motor  region  of 
the  cerebrum.  The  fibres  which  form  it  arise  in  the  motor 
areas  of  the  cerebrum  and  find  their  way  to  the  bulb 
(medulla),  where  they  cross  at  the  pyramids  to  the  opposite 
side  of  the  cord,  and  descend  in  a  region  external  to  the 
superior  cornu. 

The  infero-lateral  descending  tract  arises  from  the  mid- 
brain :  it  travels  down  the  same  side  of  the  cord. 

The  direct  cerebellar  tract  arises  from  the  cells  of  Clarke's 
column  ;  the  fibres  composing  it  are  very  large  and  run 
headward  to  end  in  the  cerebellum. 

26—2 


404     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

The  median  miperior  tract  occupies  a  position  on  the 
superior  part  of  the  cord  on  either  side  of  the  median 
fissure ;  it  is  a  sensory  tract  and  is  affected  in  locomotor 
ataxia  of  the  human  subject.  The  fibres  composing  it  are 
small  and  the  tract  terminates  at  the  bulb  ;*  degeneration 
of  this  tract  follows  division  of  the  superior  spinal  roots, 
and  the  fibres  composing  it  have  their  trophic  centre  in 
the  ganglion  of  these  roots,  being  cell  processes  from  the 
cells  of  the  ganglion. 

The  lateral  superior  tract  is  made  up  of  the  fibres  of  the 


Fig.  91. — Diagram  to  illustrate  the  Eelative  Size  of  the  Crossed 
Pyramidal  Tract  (Py)  in  the  Dog,  Monkey,  Man  (Foster, 
after  Sherrington). 

In  B.  Py'  is  an  outlying  portion  of  the  pyramidal  tract  separated  from 
the  rest  by  the  cerebellar  tract.  Py-d  in  A  is  the  direct  pyramidal 
tract  only  present  in  man. 

sensory  roots ;  some  of  these  fibres  plunge  into  the  grey 
matter  at  once  and  end  there,  others  pass  in  the  preceding 
tract,  the  median  superior,  and  run  for  a  great  distance 
forward,  many  even  reaching  the  bulb. 

The  infero-lateral  ascending  tract  runs  the  entire  length 
of  the  surface  of  the  cord ;  the  origin  of  it  is  not  clearly 
known  ;  degeneration  cannot  be  brought  about  by  dividing 
the  superior  roots,  so  that  its  origin  must  be  in  the  cord. 
It  runs  head  ward  to  terminate  in  the  cerebellum. 

It  has  not  been  found  possible  to  divide  the  whole  of  the 

*  The  word  '  bulb  '  is  used  throughout  to  indicate  the  medulla 
oblongata. 


THE  NEKVOUS  SYSTEM  405 

white  matter  into  tracts  ;  even  after  all  the  above  have 
been  defined,  there  is  still  much  left  unaccounted  for. 

When  the  various  tracts  in  the  spinal  cord  reach  the 
bulb  they  undergo  change  in  form,  position,  and  distri- 
bution, in  order  that  they  may  arrive  at  the  various  parts 
of  the  brain  to  which  they  are  proceeding.  Some  tracts, 
however,  are  known  to  pass  unbroken  through  the  bulb, 
viz.,  the  pyramidal  tract  from  the  cerebrum  and  the  cere- 
bellar tracts  ;  all  the  others  are  broken  up. 

Afferent  and  Efferent  Paths  in  the  Cord. — By  experimental 
inquiry  certain  paths  have  been  made  out  in  the  cord 
whereby  impressions  are  transmitted  from  the  centre  to 
the  periphery  and  from  the  periphery  to  the  centre.  We 
have  previously  spoken  of  the  ascending  and  descending 
tracts,  as  representing  the  direction  in  which  degenerations 
travel  along  given  portions  of  the  cord. 

By  the  superior  columns  of  the  cord  such  impressions  as 
temperature,  pressure,  and  muscular  sense  are  conveyed  to 
the  cerebrum,  running  along  the  same  side  of  the  cord 
by  which  they  entered,  and  when  reaching  the  bulb 
crossing  over  to  the  opposite  side  (Fig.  92). 

In  the  lateral  columns  painful  sensations  are  transmitted, 
and  it  is  supposed  that  on  entering  the  cord  a  large  number 
of  the  fibres  cross  over  to  the  opposite  side,  so  that  both 
sides  of  the  cord  are  transmitting  painful  impulses.  Those 
fibres  which  do  not  decussate  in  the  cord  do  so  in  the  bulb. 

The  whole  of  the  afferent  fibres  entering  the  cord  do  not 
reach  the  brain,  many  of  them  enter  the  grey  matter  and 
terminate  by  breaking  up  around  cells,  and  in  this  way  the 
afferent  fibres  contract  connections  with  many  of  the  spinal 
segments  in  front  of  it. 

In  the  direct  cerebellar  tract,  impulses  believed  to  be  con- 
nected with  the  maintenance  of  the  body  equilibrium  are 
transmitted  to  the  brain ;  the  whole  of  the  fibres  composing 
this  tract  do  not  reach  the  cerebellum,  some,  as  just  indi- 
cated, terminating  in  the  cord  itself.  All  voluntary  motor 
impulses  originate  in  the  cerebrum  and  travel  direct  to  the 
bulb  ;  here  they  cross  over  and  run  down  the  opposite  side 


Fig.  92. — Diagram  of  the  Afferent  and  Efferent  Paths  passing 
TO  and  from  the  Brain  by  the  Cord  (Sherrington). 

L,  Left,  R,  right ;  cbm,  cerebrum  ;  cbm,  cerebellum  ;  mo,  medulla  oblon- 
gata containing  the  decussation  of  p,  the  pyramidal  tract,  and  of 


THE  NEEVOUS  SYSTEM  407 

/,  the  fillet ;  the  decussation  of  /  should  really  be  a  little  higher 
instead  of  a  little  lower  than  that  of  p ;  «g,  nucleus  gracilis 
(GoU's) ;  oT,  optic  thalamus  ;  pvc,  the  posterior  vesicular  column, 
or  column  of  Clarke  ;  sp  g,  spinal  ganglion  ;  cg,  median  posterior 
colunm  (GoU's) ;  dc,  direct  cerebellar  tract. 
Tlie  arrows  show  the  direction  of  the  impulses.  A  centripetal  impulse, 
say  from  the  skin,  passes  up  the  afferent  nerve,  through  the  spinal 
ganglion,  and  enters  the  superior  colunms  of  the  cord  ;  it  may 
pass  to  the  cerebrum  direct  via  the  medulla  by  cg,  the  median 
posterior  column,  which  crosses  in  the  bulb  and  so  gains  the 
opposite  side  of  the  brain  :  or  the  impulse  may  pass  by  dc,  the 
cerebellar  tract,  to  the  cerebellum,  entering  it  on  the  same  side,  and 
from  here  crossing  over  to  the  opposite  cerebral  hemisphere.  A 
centrifugal  impulse  originates  in  the  cerebral  cortex,  gains  the 
pyramidal  tract,  passes  through  the  bulb  to  the  opposite  side  of 
the  cord,  enters  the  cells  in  the  inferior  cornu  of  the  grey  matter, 
and  passes  out  of  this  as  the  inferior  spinal  nerve. 


of  the  cord,  travelling  by  the  crossed  pyramidal  tract  to 
the  multipolar  cells  of  the  inferior  cornu  of  the  grey 
matter,  from  which  the  motor  nerves  arise  (Fig.  92).  These 
efferent  fibres  are  the  longest  in  the  cord,  for  unlike  the 
afferent  fibres  they  have  few  connections  with  spinal  seg- 
ments, and  practically  run  direct  from  their  origin  to  their 
termination. 

It  will  be  observed  that  all  sensory  impulses  enter  the 
brain  on  the  side  opposite  to  their  origin,  whilst  all  motor 
impulses  leave  the  brain  on  the  opposite  side  to  that  to  which 
they  are  distributed,  so  that  injury  to  a  motor  area  of  the 
right  brain  leads  to  a  left- sided  body  paralysis. 

In  the  lateral  columns  of  the  cord  both  vaso-motor  and 
sweat  nerves  travel ;  decussating  in  the  cord  they  enter  the 
grey  matter  of  the  opposite  inferior  cornu,  and  pass  out 
with  the  motor  nerves  from  the  spinal  cord. 

Reflex  Action. — Nerve  fibres  do  not  under  natural  circum- 
stances generate  impulses,  they  transmit  them  but  without 
modifying  them  ;  modification  can  only  occur  in  nerve 
centres,  such  as  the  brain  and  spinal  cord,  and  these  centres 
always  consist  largely  of  nerve-cells,  of  which  the  nerve- 
fibres  leaving  or  entering  the  centre  are  simply  processes  or 
branches.  Dealing  at  present  solely  with  the  spinal  cord, 
it  may  be  described  not  as  one  long  centre,  but  a  series  of 
centres  lying  end  to  end,  each  capable  to  a  greater  or  less 


408     A  MANUAL  0¥  VETERINARY  PHYSIOLOGY 

extent  of  acting  independently  of  its  neighbour,  and  each 
centre  possessing  its  afferent  and  efferent  roots. 

In  these  segments  of  spinal  cord,  complex  acts  can  be 
initiated  by  the  arrival  of  simple  centripetal  impulses  ;  such 
acts  may  be  carried  out  without  any  assistance  from  the 
brain,  for  they  can  readily  be  demonstrated  in  an  animal 
where  the  brain  has  been  destroyed.  These  acts  are  known 
by  the  name  of  reflex,  from  which  we  must  not  infer  that 
a  centripetal  impulse  is  simply  reflected  into  an  efferent 
channel,  but  rather  that  a  centripetal  impulse  reaches  the 
cord,  and  passing  into  the  grey  matter  stimulates  the 
ganglionic  cells  which  generate  the  efferent  impulse. 

The  structures  necessary  for  a  simple  reflex  act  are 
(1)  an  afferent  nerve  to  convey  the  impression  to  a  nerve 
centre  ;  (2)  a  nerve  centre  in  which  the  outgoing  impulses 
are  generated ;  (3)  an  efferent  channel  for  their  trans- 
mission. More  complex  acts  may  need  more  afferent 
nerves,  a  larger  number  of  excitable  centres,  and  a  greater 
number  of  efferent  fibres. 

A  classical  example  of  a  reflex  act  is  the  drawing  up 
of  the  leg  when  the  foot  is  pinched  in  a  frog  from  which 
the  brain  has  been  entirely  removed.  Depending  upon  the 
degree  of  pressure  applied  to  the  foot,  it  draws  up  either 
one  leg  or  both,  i.e.,  the  reflex  movements  are  unilateral  or 
symmetrical,  according  to  the  number  of  ganglionic  centres 
in  the  cord  which  have  been  stimulated.  Still  greater 
violence  applied  to  the  foot  of  this  brainless  frog  will  affect 
a  larger  number  of  centres  further  forward  in  the  cord,  so 
that  the  fore-limbs  may  share  in  the  reflex,  and  this  is 
known  as  irradiation  ;  still  further  excitation  may  produce 
convulsive  movements  of  the  entire  body,  known  as  general 
action. 

The  brainless  frog  reacts  more  regularly  to  this  experi- 
ment than  one  possessing  a  brain,  which  is  evidence  that 
the  brain  is  capable  of  exercising  a  controlling  influence  or 
inhibitory  effect  over  reflex  actions. 

One  very  prominent  feature  of  a  reflex  act  is  its 
apparently  purposeful   character ;    turning  once   more   to 


THE  NERVOUS  SYSTEM  409 

.  the  brainless  frog,  if  an  acid  be  applied  to  the  skin  of  the 
flank  the  foot  endeavours  to  remove  the  source  of  irritation. 

In  the  dog  very  characteristic  reflex  actions  occur  after 
division  of  the  cord,  such  as  those  of  walking,  galloping, 
micturition,  and  defitcation ;  tickling  the  skin  causes  the 
animal  to  scratch  the  part  with  the  hind  foot. 

The  higher  we  ascend  in  the  animal  scale  the  less  easy 
is  it  to  obtain  evidence  of  free  spinal  reflexes — viz.,  reflexes 
which  take  place  without  any  guidance  from  the  brain.  This 
may  perhaps  be  due  to  a  more  constant  influence  exercised 
over  them  by  the  brain.  Still,  locomotion  is  often  essentially 
a  reflex  act,  and  is  very  complicated  ;  for  instance  the  tactile 
and  muscular  centripetal  impulses  required,  the  exact  group- 
ing of  muscles,  and  the  regulation  of  the  degree  and 
rapidity  of  contraction,  would  appear  at  first  sight  to  need 
the  supervision  of  the  highest  centres  in  the  brain,  but 
such  is  not  the  case ;  a  pigeon  will  fly  after  decapitation. 
If  a  horse  had  to  think  of  every  step  he  had  to  take  he  would 
soon  be  worn  out  and  blunder.  That  the  higher  centres  do 
at  times  come  into  play  is  shown  by  the  judgment  which 
the  horse  exercises  when  jumping,  viz.,  the  proper  distance 
to  take  ofl'  at,  the  amount  of  muscular  contraction  required 
to  lift  the  body,  and  the  needful  height  to  which  it  should 
be  raised,  etc.,  are  all  evidence  of  this. 

By  a  Co-ordinate  Movement  is  meant  one  in  which  the 
contraction  of  various  related  groups  of  muscles  is  so 
adjusted  that  the  extent  of  their  contraction,  and  every- 
thing necessary  for  a  perfect  movement,  is  present  and 
faithfully  carried  out.  This  co-ordination  of  movement 
we  have  seen  may  occur  even  without  the  assistance  of 
the  brain,  and  we  have  alluded  to  the  complex  co-ordinate 
movements  of  locomotion  as  an  example  of  this.  In  the 
spinal  cord,  therefore,  not  only  reflex  but  co-ordmate  move- 
ments are  generated ;  even  the  crossed  or  diagonal  move- 
ments of  locomotion  in  quadrupeds  are  of  this  nature,  and 
are  carried  out  by  the  spinal  cord.  Movements  which  are 
irregular  and  purposeless,  or  m  any  way  fail  to  co-ordinate, 
are  termed  inco-ordinate. 


410     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  Stepping  Reflex. — As  an  example  of  reflex  action 
that  of  stepping  may  be  considered.  When  in  the  dog  the 
spinal  cord  has  been  severed  in  the  hinder  part  of  the 
cervical  region  and  the  '  shock '  from  the  transection  has 
passed  off,  reflex  walking  is  observable.  The  walking  move- 
ment includes  alternate  flexing  and  straightening  of  the 
limb.  The  forward  movement  of  the  hind  leg  in  taking  a 
step  is  produced  by  flexion  at  the  hip,  and  to  prevent  the 
foot  brushing  against  the  ground  as  the  leg  swings  forward 
flexion  occurs  at  the  stifle  and  hock  so  as  to  somewhat 
raise  the  foot.  The  limb  is  then  straightened  again,  so 
that  the  foot  may  reach  the  ground  and  bear  the  weight  of 
the  body.  In  order  to  prevent  the  limb  doubling  up  under 
this  burden  the  extensor  muscles  which  support  the  patella 
joint  and  hock  from  bending  have  to  contract  with  sufficient 
power.  Stiffened  by  the  contraction  of  these  muscles  the 
limb  serves  as  a  prop  to  carry  the  body.  While  the  foot 
rests  on  the  ground  the  body  moves  forward  so  that  in  due 
course  the  hip  advances  in  front  of  a  vertical  drawn  upward 
from  the  foot.  The  extended  hind  limb  at  this  time  is 
sloped  somewhat  backward  as  well  as  downward.  When 
this  posture  is  reached  the  extensor  muscles  are  thrown 
into  further  action,  and  give  the  limb  a  push  off  from  the 
ground,  propelling  the  body  forward.  The  hind  limb  thus 
makes  its  contribution  to  the  progression  forward  of  the 
body.  In  galloping  this  extensor  thrust  is  very  marked, 
and  is  given  by  both  hind  legs  together,  instead  of  alter- 
nately as  in  walking  and  running. 

In  this  reflex,  spinal  stepping,  we  may  study  first  the 
flexion  of  the  limb  which  occurs  in  the  forward  movement 
of  the  step.  Flexion  similar  but  more  pronounced  can  be 
easily  excited  in  the  spinal  dog*  by  exciting  the  skin  of  the 
foot  electrically.  Though  the  flexion  occurs  at  hip,  patella 
joint,  and  hock  together,  it  will  be  simpler  to  confine  our 
examination  to  the  flexion  at  one  of  these  joints  only,  for 

*  '  Spinal  dog '  is  the  term  used  for  a  dog  in  which  the  reflexes  are 
entirely  spinal,  owing  to  the  brain  having  been  destroyed,  or  the  cord 
having  been  cut  off  from  the  brain. 


THE  NERVOUS  SYSTEM  411 

what  occurs  in  the  muscles  of  each  of  the  three  joints  is,  so 
far  as  concerns  us  now,  the  same.  The  chief  muscles  which 
flex  the  stifle  are  the  semitetidinosus  and  biceps  of  the  back 
of  the  thigh.  The  electric  stimulation  of  the  skin  of  the 
foot  is  found  to  throw  these  muscles  into  contraction,  and, 
with  them,  also  the  psoas  muscles  (flexors  of  the  hip)  and 
the  tibialis  anticus,  etc.  (flexors  of  the  hock).  But  this  is 
only  part  of  what  happens.    At  the  same  time  as  the  flexors 


:Re 


Di.\GEAM  OF  A  Reflex  Arc  (Sherrington). 


dd,  dd',  Dendrites ;  pk,  pk'.  perikaryon  ;  ax,  ax',  axones ;  sy,  synapse, 
dd  +  pk  +  ax  =  neurone  ;  (dd  +  pk  +  ax)  +  sy  +  (dd'  +  pk'  +  ax')  =  con- 
ductor ;  Re,  Re  =  receptors  (epithelial)  ;  Ef,  Ef  =  effectors  (muscle). 

The  parts  within  the  dotted  line  lie  in  the  grey  matter  of  the  nerve 

centre. 

of  the  stifle  contract,  the  extensor  muscles  of  the  stifle,  the 
vasti  and  crureus,  are  relaxed.  In  order  to  understand 
how  this  comes  about  it  is  necessary  to  refer  to  a  principle 
of  construction  of  the  nervous  system  which  has  been  termed 
*  the  principle  of  the  common  path.'' 

The  structures  along  which  the  nervous  impulse  in  a 
reflex  action  runs  constitute  what  is  called  a  rejlex  arc. 
A  reflex  arc  is  a  chain  of  nerve  cells  (Fig.  93).  Each  nerve 
cell  consists  of  three  parts :  (1)  a  cell  body  containing  the 


412     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

nucleus  and  called  iheperihariion  ,•  (2)  one  or  more  (usually 
many)  branches  from  the  perikaryon,  called  dendrites,  which 
conduct  impulses  to  the  perikaryon ;  (3)  one  branch  from 
the  perikaryon  which  conducts  impulses  away  from  the 
perikaryon,  and  this  branch  is  called  the  a.rone.  The 
whole  nerve  cell  thus  composed  of  these  three  parts  is 
termed  a  neurone.  The  neurones  forming  a  reflex  arc 
follow  each  other  end  to  end  like  links  in  a  chain.  In  the 
chain  the  neurones  composing  it  are  joined  in  such  a  way 
that  the  axone  of  one  neurone  meets  the  dendrites  of  the 
next  neurone,  and  these  junctions  of  the  axone  of  one 
neurone  with  the  dendrites  of  the  next  are  of  such  a  nature 
that  conduction  of  impulses  from  one  neurone  to  the  next 
occurs  in  one  direction  only,  that  is  from  axone  to  dendrite, 
and  not  backwards  from  dendrite  to  axone.  These 
junctions  between  neurones  are  termed  synapses  (see 
Fig.  93). 

The  first  link  of  each  reflex  chain  is  a  neurone  which 
starts  in  a  receptor  organ,  e.g.,  a  sense-organ.  A  receptive 
field,  e.g.,  an  area  of  skin,  is  always  analysable  into 
receptive  points,  and  the  nerve-path  of  the  reflex  always 
starts  from  a  receptive  point  or  points.  A  single  receptive 
point  may  play  reflexly  upon  quite  a  number  of  different 
effector  organs.  It  may  be  connected  through  its  reflex 
path  with  many  muscles  and  glands  in  various  parts.  Yet 
all  its  reflex  arcs  spring  from  the  one  single  shank,  so 
to  say  ;  that  is,  from  the  one  afferent  neurone  that  conducts 
from  the  receptive  point  at  the  periphery  into  the  central 
nervous  organ.  This  neurone  dips  at  its  deep  end  into  the 
great  central  nervous  organ,  the  cord  or  brain.  There  it 
enters  a  vast  network  of  conductive  paths.  In  this  net- 
work it  forms  manifold  connections.  So  numerous  are  its 
potential  connections  there  that,  as  shown  by  the  general 
convulsions  induced  under  strychnine-poisoning,  its  im- 
pulses can  discharge  practically  every  muscle  and  effector 
organ  in  the  body.  Y^et  under  normal  circumstances  the 
impulses  conducted  by  it  to  this  central  network  do  not 
irradiate  there  in  all  directions.     Though  their  spread  over 


THE  NEEVOUS  SYSTEM  413 

the  conducting  network  does,  as  judged  by  the  effects, 
increase  with  increase  of  stimulation  of  the  entrant  path, 
the  irradiation  remains  limited  to  certain  lines.  Under 
weak  stimulation  of  the  entrant  path  these  lines  are  sparse. 
The  conductive  network  affords,  therefore,  to  any  given 
path  entering  it  some  communications  that  are  easier  than 
others.  This  canalization  of  the  network  in  certain  direc- 
tions from  each  entrant  point  is  sometimes  expressed, 
borrowing  electrical  terminology,  by  saying  that  the  con- 
ductive network  from  any  given  point  offers  less  resistance 
along  certain  circuits  than  along  others.  This  recognizes 
the  fact  that  the  conducting  paths  in  the  great  central 
organ  are  arranged  in  a  particular  pattern.  The  pattern 
of  arrangement  of  the  conductive  network  of  the  central 
organ  reveals  something  of  the  integrative  function  of  the 
nervous  system.  It  tells  us  what  organs  work  together  in 
time  relationship.  The  impulses  are  led  to  this  and  that 
effector  organ,  gland,  or  muscle  in  accordance  with  the 
pattern. 

At  the  commencement  of  every  reflex  arc  is  a  receptive 
neurone,  extending  from  the  receptive  surface  to  the  central 
nervous  organ  (see  Fig.  93).  That  neurone  forms  the  sole 
avenue  which  impulses  generated  at  its  receptive  point  can 
use  whithersoever  may  be  their  distant  destination.  That 
neurone  is,  therefore,  a  path  exclusive  to  the  impulses 
generated  at  its  own  receptive  points,  and  other  receptive 
points  than  its  own  cannot  employ  it. 

But  at  the  termination  of  every  reflex  arc  we  find  a 
final  neurone,  the  ultimate  conductive  link  to  an  effector 
organ,  gland,  or  muscle.  This  last  link  in  the  chain, 
e.g.,  the  motor  neurone,  differs  in  one  important  respect 
from  the  first  link  of  the  chain.  It  does  not  subserve 
exclusively  impulses  generated  at  one  single  receptive 
source  alone,  but  receives  impulses  from  many  receptive 
sources  situate  in  various  regions  of  the  body  (see  Fig.  93). 
It  is  the  sole  path  which  all  impulses,  no  matter  whence 
they  come,  must  travel  if  they  would  reach  the  muscle- 
fibres   which    it   joins.      Therefore,    while    the    receptive 


414    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

neurone  forms  a  inivate  path  exclusively  for  impulses  of 
one  source  only,  the  final  or  efferent  neurone  is,  so  to  say, 
a  public  path,  common  to  impulses  arising  at  any  of  many 
sources  in  a  variety  of  receptive  regions  of  the  body.  The 
same  effector  organ  stands  in  reflex  connection  not  only 
with  many  individual  receptive  points,  but  even  with  many 
various  receptive  fields.  Reflex  arcs  arising  in  manifold 
sense-organs  can  pour  their  influence  into  one  and  the 
same  muscle.  A  limb-muscle  is  the  terminus  ad  quem  of 
nervous  arcs  arising  not  only  in  the  right  eye  but  in  the 
left,  not  only  in  the  eyes  but  in  the  organs  of  smell  and 
hearing  ;  not  only  in  these,  but  in  the  otic  labyrinth, 
in  the  skin,  and  in  the  muscles  and  joints  of  the  limb  itself 
and  of  the  other  limbs  as  well.  Its  motor  nerve  is  a  path 
common  to  all  these. 

Reflex  arcs  show  therefore  the  general  feature  that  the 
initial  neurone  is  a  private  path  exclusive  for  a  single 
receptive  point,  and  that  finally  the  arcs  embouch  into 
a  path  leading  to  an  effector  organ,  and  that  this  final  path 
is  common  to  all  receptive  points  wheresoever  they  may  lie 
in  the  body,  so  long  as  they  have  any  connection  at  all 
with  the  effector  organ  in  question.  Before  finally  converg- 
ing upon  the  motor  neurone,  the  arcs  usually  converge  to 
some  degree  by  their  private  paths,  embouching  upon  inter- 
nuncial  paths  common  in  various  degree  to  groups  of 
private  paths.  The  terminal  path  may,  to  distinguish  it 
from  internuncial  common  paths,  be  called  the  final  common 
path  (see  Fig.  93).  The  motor  nerve  to  a  muscle  is  a  collec- 
tion of  such  final  common  paths. 

A  result  is  that  each  receptor  being  dependent  for  com- 
munication with  its  effector  organ  upon  a  path  not  ex- 
clusively its  own  but  common  to  it  with  certain  other 
receptors,  that  nexus  necessitates  successive  and  not 
simultaneous  use  of  the  common  path  by  various  receptors 
using  it  to  different  effect. 

The  Scratch  Reflex. — Good  opportunity  for  study  of  this 
correlation  between  reflexes  is  given  in  the  '  scratch  reflex.' 
When  the  spinal  cord  has  been  transected  in  the  neck,  this 


THE  NERVOUS  SYSTEM 


415 


reflex  in  a  few  months  becomes  prominent.  Stimuli  applied 
within  a  large  saddle-shaped  field  of  skin  (Fig.  94)  excite  a 
scratching  movement  of  the  leg.  The  movement  is  rhythmic 
flexion  at  hip,  stifle,  and  hock.  It  has  a  frecjuency  of 
about  four  per  second.  The  stimuli  provocative  of  it  are 
mechanical,  such  as  rubbing  the  skin,  or  pulling  lightly  on 
a  hair.  The  nerve-endings  which  generate  the  reflex  lie  in 
the  surface  layer  of  the  skin,  about  the  roots  of  the  hairs. 
A  convenient  way  of  exciting  these  is  b}-  feeble  faradization. 
Prominent  among  the  muscles  active  in  this  reflex  are 


Fig.  94. — The  Scratch  Reflex  (Sherrington). 

The  '  receptive  field,'  as  revealed  after  low  cervical  transection,  a 
saddle-shaped  area  of  dorsal  skin,  whence  the  scratch  reflex  of  the 
left  hind  limb  can  be  evoked.     Ir  marks  the  position  of  the  last  rib. 

the  flexors  of  the  hip.  If  we  record  their  rhythmic  con- 
traction we  obtain  tracings  as  in  Fig.  96.  A  series  of  brief 
contractions  succeed  one  another  at  a  certain  rate,  whose 
frequency  is  independent  of  that  of  the  stimulation.  The 
contractions  are  presumably  brief  tetani.  The  stimulus  to 
the  hair-bulbs  of  the  shoulder  throws  into  action  a  lumbar 
spinal  centre,  innervating  the  hip-flexor,  much  as  the  bulbar 
respiratory  centre  drives  the  spinal  phrenicus  centre.  In 
the  case  of  the  respirator}-  muscle  the  frequency  of  the 
rhythm  is,  however,  much  less. 


416    A  MANUAL  OF  VETEEINAEY  PHYSIOLOGY 

The  reflex  is  unilateral :  stimulation  of  the  left  shoulder 
evokes  scratching  by  the  left  leg,  not  by  the  right.  In  the 
lateral  column  of  the  spinal  cord  loiu/  fibres  exist  directly 
connecting  the  spinal  segments  of  the  shoulder  with  the 
spinal  segments  containing  the  motor  neurones  for  the 
flexor  muscles  of  the  hip,  and  "knee,  and  ankle.  We  thus 
arrive  at  the  following  reflex  chain  for  the  scratch  reflex : 
(i.)  The  receptive  neurone  (Fig.  95,  &a),  from  the  skin  to 
the  spinal  grey  matter  of  the  corresponding  spinal  segment 
in  the  shoulder.  This  is  the  exclusive  or  private  path  of 
the  arc.     (ii.)  The  long  descending  proprio-spinal  neurone 


Fig.  95. — Spinal  Arcs  involved  in  Scratch  Reflex  (Sherrington). 

Diagram  of  the  spinal  arcs  involved  in  Fig.  94.  l,  receptive  or  afferent 
nerve  path  from  the  left  foot ;  r,  receptive  nerve-path  from  the 
opposite  foot ;  sa,  s/3,  receptive  nerve-paths  from  hairs  in  the 
dorsal  skin  of  the  left  side ;  fc,  the  final  common  path,  in  this 
case  the  motor  neurone  to  a  flexor  muscle  of  the  hip ;  pa,  p^, 
proprio-spinal  neurones. 

(Fig.  95,  pa),  from  the  shoulder  segment  to  the  grey 
matter  of  leg  segments.  (iii.)  The  motor  neurone 
(Fig.  95,  Fc),  from  the  spinal  segment  of  the  leg  to  the 
flexor  muscles.  This  last  is  the  Jinal  common  path.  The 
chain  thus  consists  of  three  neurones.  It  enters  the  grey 
matter  twice,  that  is,  it  has  two  neuronic  junctions,  two 
synapses.     It  is  a  disynaptic  arc. 

Now  if,  while  stimulation  of  the  skin  of  the  shoulder  is 
evoking  the  scratch  reflex,  the  skin  of  the  hind  foot  is 
stimulated,  the  scratching  is  arrested.  Stimulation  of 
the  skin   of    the  hind  foot  causes  the  leg  to    be   flexed, 


THE  NERVOUS  SYSTEM  417 

drawing  the  foot  up.  The  drawing  up  of  the  foot  is  effected 
by  strong  tonic  contraction  of  the  flexors  of  hock,  stifle, 
and  hip.  In  this  reaction  the  reflex  arc  is  (i.)  the  receptive 
neurone  (Fig.  95,  l)  (nociceptive)  from  the  foot  to  the 
spinal  segment;  (ii.j  perhaps  a  short  intraspinal  neurone; 
and  (iii.)  the  motor  neurone  (Fig.  95,  fc)  to  the  flexor 
muscle,  e.g.,  of  hip.  Here,  therefore,  we  have  an  arc 
which  embouches  into  the  same  ^final  common  imth  as  sa. 
The  motor  neurone  fc  is  a  path  common  to  it  and  to  the 
scratch  reflex  arcs  ;  both  arcs  employ  the  same  effector 
organ,  a  hip  flexor. 

The  channels  for  both  reflexes  finally  embouch  upon  the 
same  common  path.  The  flexor  effect  specific  to  each 
differs  strikingly  in  the  two  cases.  In  the  scratch-reflex 
the  flexor  effect  is  an  intermittent  contraction  of  the  muscle  ; 
in  the  foot-reflex  it  is  steady  and  maintained.  The  accom- 
panying tracing  (Fig.  96)  shows  the  result  of  conflict  between 
the  two  reflexes.  The  one  reflex  displaces  the  other  from 
the  common  path.  There  is  no  compromise.  The  scratch 
reflex  is  set  aside  by  that  of  the  nociceptive  arc  from  the 
foot.  The  stimulation  which  previously  sufficed  to  evoke 
the  scratch  reflex  is  no  longer  effective,  though  it  is  con- 
tinued all  the  time.  But  when  the  stimulation  of  the  foot 
is  discontinued  the  scratch  reflex  returns.  In  that  respect, 
although  there  is  no  enforced  inactivity,  there  is  inhibition. 
There  is  interference  between  the  two  reflexes,  and  the  one 
is  inhibited  by  the  other.  Though  there  is  no  cessation  of 
activity  in  the  motor  neurone,  one  form  of  activity  that 
was  being  impressed  upon  it  is  cut  out  and  another  takes 
its  place. 

Suppose,  again,  during  the  scratch  reflex,  stimuli  are 
applied  to  the  foot,  not  of  the  scratching  but  of  the  opposite 
side  (Fig.  95,  r).  This  stimulation  of  the  foot  causes  flexion 
of  its  own  leg  and  extension  of  the  opposite.  If,  when  the 
left  leg  is  executing  the  scratch  reflex,  the  right  foot  is 
stimulated,  the  scratching,  involving  as  it  does  the  left 
leg's  flexors,  is  cut  short.  This  inhibition  of  the  flexor 
scratching   movement    occurs   sometimes   when    the    con- 

27 


418 


A  MANUAL  OF  VETEEINAEY  PHYSIOLOGY 


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THE  NERVOUS  SYSTEM  419 

traction    of    the    extensors    is    minimal    or    hardly    per- 
ceptible. 

It  is  obvious  from  this  that  the  final  common  path,  fc, 
to  the  flexor  muscle  can  be  controlled  by,  in  addition  to 
the  before-mentioned  arcs,  others  that  actuate  the  extensor 
muscles,  for  it  can  be  thrown  out  of  action  by  them.  The 
final  path,  fc,  is  therefore  common  to  the  reflex  arcs,  not 
only  from  the  same  side  foot  (Fig.  95,  l)  and  shoulder 
sliin  (Fig.  95,  sa,  s/3),  but  also  to  arcs  from  the  opposite 
foot  (Fig.  95,  r),  in  the  sense  that  it  is  in  the  grasp  of  all 
of  them.  In  this  last  case  we  have  a  conflict  for  the 
mastery  of  a  common  path,  not,  as  in  the  previous  instance, 
between  two  arcs,  both  of  which  use  the  path  in  a  pressor 
manner  although  difl'erently,  but  between  two  arcs  that, 
though  both  of  them  control  the  path,  control  it  difl'erently, 
one  in  a  pressor  manner  heightening  its  activity,  the  other 
in  a  depressor  manner  lowering  or  suppressing  its  activity. 

We  said  that  the  scratch  reflex  is  unilateral.  If  the  right 
shoulder  be  stimulated,  the  right  hind-leg  scratches ;  if  the 
left  shoulder  be  stimulated,  the  left  hind-leg  scratches. 
If  both  shoulders  be  stimulated  at  the  same  time,  one  or 
the  other  leg  scratches,  but  not  the  two  together.  The 
one  reflex  that  takes  place  prevents  the  occurrence  of  the 
other.  The  reason  is,  that  although  the  scratch  reflex 
appears  unilateral,  it  is  not  strictly  so.  Suppose  the  left 
shoulder  stimulated.  The  left  leg  then  scratches.  If  the 
right  leg  is  then  examined  it  is  found  to  present  slight, 
steady  extension  with  some  abduction.  This  extension  of 
the  leg  which  accompanies  the  scratching  movement  of  the 
opposite  leg  contributes  to  support  the  animal  on  three 
legs,  while  it  scratches  with  the  fourth. 

Suppose  now  we  stimulate  the  left  shoulder,  evoking  the 
scratching  movement  of  the  left  leg,  and  that  the  right 
shoulder  is  at  the  same  time  appropriately  and  strongly 
stimulated.  This  latter  stimulus  often  inhibits  the  scratch- 
ing movement  in  the  opposite  leg  and  starts  it  in  its  own.  In 
other  words,  the  stimulus  at  the  right  shoulder  not  only  sets 
the  flexor  muscles  of  the  leg  of  its  own  side  into  scratching 

27—2 


420    A  MANUAL  OF  VETEKINAEY  PHYSIOLOGY 

action,  but  it  inhibits  the  flexor  muscles  of  the  opposite  leg. 
It  throws  into  contraction  the  extensor  muscles  of  that  leg. 
In  the  previous  example  there  was  a  similar  co-ordination. 
The  motor  nerve  to  the  flexor  muscle  is  therefore  under 
the  control  not  only  of  the  arcs  of  the  scratch  reflex  from 
the  homonymous  shoulder,  but  of  those  from  the  crossed 
shoulder  as  well.  But  in  regard  to  their  influence  upon 
this  final  common  path,  the  arcs  from  the  homonymous 
shoulder  and  the  opposite  shoulder  are  opposed. 

Experiments  show  that  this  inhibition  does  not  take  place 
in  the  motor  nerve  itself.  Many  circumstances  connect 
it  with  the  place  where  the  converging  neurones  come 
together  in  the  grey  matter  at  the  commencement  of  the 
common  path.  The  field  of  competition  between  the  rival 
arcs  seems  to  lie  in  the  grey  matter,  where  they  impinge 
together  upon  the  final  or  motor  neurone.  That  is  equiva- 
lent to  saying  that  the  essential  seat  of  the  phenomenon 
is  the  synapse  between  the  motor  neurone  and  the  axone- 
terminals  of  the  penultimate  neurones  that  converge  upon 
it.  There  some  of  these  arcs  drive  the  final  path  into  one 
kind  of  action,  others  drive  it  into  a  different  kind  of 
action,  and  others  again  preclude  it  from  being  activated  by 
the  rest. 

We  are  now  in  a  posilion  to  return  to  the  flexion  at  the 
stifle  in  the  reflex  act  of  stepping.  We  see  that  the  same 
stimulus  which  excites  the  motor  neurones  of  the  flexors 
to  discharge  motor  impulses  into  those  muscles,  causes  the 
motor  neurones  of  the  antagonistic  muscles,  the  extensors 
of  the  knee,  to  cease  discharging  impulses,  and  keeps  them 
prevented  from  discharging  impulses.  The  stimulus  sets  up 
an  intraspinal  excitation  of  the  motor  neurones  of  the  flexor 
muscles  and  an  intraspinal  inhibition  of  the  motor  neurones 
innervating  the  extensor  muscles. 

When  the  flexion  phase  of  the  act  of  stepping  has  been 
passed  through,  the  leg  extends  again,  perhaps  by  its  own 
weight,  perhaps  by  return  of  activity  in  the  motor  neurones 
of  the  extensor  muscles  which  had  been  inhibited.  In  due 
course  the  foot  reaches  the  ground.     When  it  does  so  the 


THE  NERVOUS  SYSTEM  421 

weight  of  the  body  gradual)}'  comes  upon  it,  and  soon 
presses  the  sole  of  the  foot  ^Yith  its  full  force  against  the 
ground.  A  stimulus  is  thus  given  to  nerve-endings  in  the 
sole.  This  stimulus  can  be  imitated;  for  instance,  by 
pressing  against  the  sole  of  the  foot  with  a  finger.  This, 
in  the  spinal  dog,  even  when  the  animal  ia  lying  on  its  side, 
excites  a  strong  reflex  extension  of  the  limb,  the  '  extensor 
thrust.'  Just  such  an  extension  occurs  when  the  foot  is 
pressed  against  the  ground  by  the  weight  of  the  body  in  the 
act  of  stepping.  This  extensor  thrust  gives  the  propulsive 
movement  of  the  body  forward,  which  is  the  contribution 
made  by  the  limb  in  its  reflex  step  toward  the  progression 
of  the  animal.  The  extensor  thrust  is  particularly  marked 
in  the  gallop,  and  is  then  given  by  the  two  hind-limbs 
together,  and  not  alternately,  as  in  walking  and  running. 

The  above  comparatively  simple  acts  form  certainly  only 
a  part  of  the  whole  complex  reflex  which  really  occurs  in 
stepping.  In  such  complex  reflexes  many  stimuli  are  at 
work  together,  and  co-operate  harmoniously  for  a  co- 
ordinate result.  In  walking,  running,  etc.,  probably  very 
important  sources  of  the  reflex  lie  in  the  muscles  and 
joints  of  proximal  parts  of  the  limb — namely,  in  the  joints 
of  the  hip  and  stifle  and  the  great  muscles  acting  on  those 
joints.  These  joints  and  muscles  are  liberally  supplied 
with  aflerent  nerves  conducting  centripetal  impulses  from 
them.  The  importance  of  these  as  sources  of  the  reflex 
of  stepping  is  indicated  by  several  facts.  In  the  first 
place,  a  dog  or  cat  is  found  still  to  walk  well  when  all  the 
nerves  of  all  four  of  the  feet  have  been  severed — not  only  the 
skin-nerves  of  the  feet,  but  all  their  deep  nerves  as  well. 
In  the  second  place,  when  the  spinal  dog  is  lifted  so  that 
its  limbs  do  not  touch  any  solid  support  whatever,  reflex 
walking  and  galloping  are  performed,  although  the  limbs 
are  stepping  wholly  in  the  air.  But,  for  this  reflex  walk- 
ing in  the  air,  it  is  necessary  that  the  limbs  hang  down. 
The  reflex  ceases  if  the  dog  be  inverted,  so  that  gravity  no 
longer  is  acting  on  the  joints  and  muscles  as  it  does  in  the 
position  usually  accompanying  acts  of  stepping.     Further, 


422     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

it  is  probable  that  the  spinal  centres  which  execute  reflex 
walking,  running,  etc.,  receive  much  help  and  direction  from 
afferent  arcs  which  arise  in  the  labyrinth  of  the  ear. 

The  stimuli,  which  are  the  source  of  reflex  walking,  etc., 
arise,  therefore,  almost  certainly  in  many  receptive  organs. 
Individually  the  action  of  each  of  these  may  be  quite  weak. 
They  sum  up  their  effect,  because  their  impulses  converge 
on  the  same  final  common  paths,  the  motor  neurones. 
The  summation  is  probably  largely  the  work  of  these 
motor  neurones.  Their  shape  bespeaks  for  them  the  func- 
tion of  an  organ  for  such  summation.  In  each  motor 
neurone  its  dendrites  converge  to  the  perikaryon  as  a 
meeting  place,  and  there  the  impulses  carried  to  it  by  the 
dendrites  add  their  excitatory  effects  together.  As  the 
synapses  are  places  where  inhibition  and  irreversibility  of 
conduction  are  established  in  the  reflex  arc  and  where 
reflex  arcs  meet,  so  the  perikaryon  seems  a  place  where 
summation  of  impulses  from  various  harmonious  sources 
are  added  together  for  a  conjoint  effect. 

But  the  reflex  acts  carried  out  by  the  cord  are  not 
limited  to  those  affecting  skeletal  muscles ;  the  act  may 
be  a  secretory  or  nutritive  one,  or  involving  the  contraction 
or  relaxation  of  pale  muscle  :  for  example,  the  contraction 
and  dilatation  of  the  bloodvessels  under  the  influence  of 
the  vaso-motor  system,  the  peristaltic  movements  of  the 
intestines,  the  contraction  of  the  bladder  and  uterus,  and 
the  secretions  from  the  various  abdominal  glands,  are  all 
examples  of  reflex  acts.  The  time  occupied  by  a  reflex  act 
varies  dependently  upon  the  strength  of  the  stimulus  and 
the  nature  of  the  reflex  ;  the  sharper  the  stimulus  the  more 
rapid  the  reflex,  the  more  active  the  centre  the  more  rapid 
the  response ;  impulses  which  have  to  cross  the  cord  take 
longer  than  those  which  enter  and  return  from  the  same 
side.  It  is  mainly  during  this  appreciable  delay,  as  measured 
by  delicate  apparatus,  that  the  changes  are  occurring  in  the 
grey  substance  which  lead  to  an  efferent  response.  In  the  dog 
the  time  occupied  by  a  reflex  on  the  same  side  is  estimated 
at  "022  up  to  2-3  seconds,  according  to  circumstances. 


THE  NEEVOUS  SYSTEM  423 

Tendon   Reflexes. — The   muscle  and   tendon   reflexes,  so 
well  known  in  the  human  subject,  have  not,  so  far  as  we 
are  aware,  been  studied  in  the  ungulates ;  nor  do  we  know 
whether  the    existence    of   any  reflexes  has  been  demon- 
strated, if,  perhaps,  we  except  the  immediate  lifting  up  of 
the  foot,  which  generally  follows  pressure  on  the  so-called 
'  chestnut '  found  on  the  inside  of  the  fore-arm  of  the  horse. 
One  of  the  best  known  of  the  tendon-reflexes  in  man  is 
the  knee-jerk,  a  jerking  forward  of  the  leg  when  the  straight 
ligament   of   the  patella  is  struck.      This  is  caused  by  a 
momentary  single  spasm  of  the  extensor  muscles  of  the 
knee,  and  although  often  called  a  reflex  act  cannot  truly  be 
so,  because  the  time  between  the  blow  and  the  jerk  is  too 
short  for  any  reflex  act.     It  is  well  seen  in  the  dog,  cat, 
rabbit,  etc.    Although  not  a  reflex  action  it  is  dependent  on 
the  reflex  tonus  that  is  maintained  in  the  muscles  by  the 
spinal  arcs  connected  with  them ;  if  that  tonus  be  much 
lowered,  as  by  severance  of  the  nervous  reflex  arc,  the  jerk 
can  no  longer  be  elicited.     The  jerk  is  a  good  index  of  the 
condition  of  the  reflex  arc,  and  therefore  of  the  condition 
of  the  activity  or  depression  of  the  segments  of  the  cord 
by   which   the   extensor   muscles   are    innervated.      It   is 
depressed  during  sleep  or  anesthesia,  and  by  antemia  of 
the  cord  ;   it  is  intensified  when  the  cerebral  restraint  is 
removed  from  the  lumbar  spinal  segments  by  diversion  or 
attention  to  another  part,  or  by  severance  of  the  cord  in 
the  dorsal  region.     Another  brisk  '  jerk  '  in  the  dog  is  the 
ischial,  obtained  from  the  hamstring  muscles  by  tapping 
the  tuberosity  of  the  ischium. 

Automatic  Action.  —  Nerve  centres  are  not  as  a  rule 
capable  of  issuing  impulses  which  are  not  the  result  of  an 
afferent  stimulus ;  one  centre  there  is  however  which  seems 
to  do  so.  This  is  the  respiratory  centre  in  the  bulb  (p.  109). 
In  the  same  way  the  tone  of  the  vascular  system,  or  the  force 
which  keeps  the  muscular  wall  of  the  vessel  in  the  neces- 
sary condition  of  constriction,  is  in  part  brought  about  by 
automatic  impulses.  The  tone  of  the  muscular  walls  of 
the  vascular  system  seems  to  be  due  to  tonic  (permanent) 


424     A  MANUAL  OF  VETEEINAKY  PHYSIOLOGY 

actions,  either  of  local  nerve  apparatus  in  the  sympathetic 
system  or  of  the  muscular  coat  itself. 

Special  Centres  in  the  Spinal  Cord. — In  the  cord  certain 
centres  exist,  which  though  ordinarily  under  the  control  of 
a  chief  centre  in  the  bulb,  yet  are  capable  of  carrying  on 
peculiar  reflex  actions  even  after  the  cord  has  been  divided, 
and  thus  separated  from  the  controlling  influence  of  the 
bulb. 

The  cilio-spinal  centre  lies  between  the  cervical  and 
dorsal  portions  of  the  cord  ;  in  it  fibres  originate  which 
through  the  cervical  sympathetic  supply  the  dilator  muscle 
of  the  iris.  Destruction  of  the  region  in  question  causes  a 
contraction  of  the  pupil,  whilst  irritation  of  it  causes  the 
pupil  to  dilate. 

The  ano-spinal  centre,  found  in  the  lumbar  portion  of 
the  cord,  controls  the  act  of  def^ecation  ;  it  would  appear 
to  be  highly  developed  in  herbivora,  which  possess  the 
power  of  bringing  it  into  play  not  only  when  the  body  is  at 
rest  but  during  movement.  The  functions  of  the  ano- 
spinal  centre  are  rather  complex,  inasmuch  as  it  has  not  only 
to  maintain  the  tone  of  the  sphincter,  but  also  to  relax  it 
during  def?ecation,  and  under  the  latter  condition  simul- 
taneously contracts  both  the  wall  of  the  intestine  and  the 
abdominal  muscles. 

The  vesicospinal  centre  also  exists  in  the  lumbar  portion 
of  the  cord,  and  governs  micturition  ;  its  action  is  similar 
to  that  of  the  ano-spinal  centre. 

In  the  lumbar  portion  of  the  cord  other  centres  are 
found,  for  example,  the  erection  centre,  the  (jenito-spinal 
centre  which  contains  the  nervous  apparatus  employed  in 
the  emission  of  semen,  and  the  partiirition  centre. 

Vaso-motor  centres  are  found  throughout  the  cord  ;  they 
are  principally  under  the  control  of  similar  centres  in 
the  bulb,  but  may  act  independently.  Sweat  centres  are 
probably  closely  connected  with  the  vaso-motor  centres. 
Trophic  centres  for  the  nutrition  of  the  tissues  also  exist 
in  the  cord ;  destruction  of  parts  by  ulceration,  or  great 
muscular  wasting,  may  follow  injury  of  the  trophic  nerves. 


THE  NEKVOUS  SYSTEM  425 

The  Functions  of  the  Spinal  Cord  may  be  summarized  as 
follows :  The  cord  is  the  central  seat  of  numerous  reflex 
actions ;  some  of  these  are  intermittent  and  occasional, 
others  permanent  or  tonic,  such  as  the  maintenance  of 
muscular  and  arterial  tone.  There  is  evidence  that  it 
assists  in  co-ordinating  movement,  and  it  is  also  the  path 
by  which  the  brain  and  the  body  are  brought  into  con- 
nection, both  in  an  upward  and  downward  direction  and 
from  side  to  side. 

Cranial  Nerves. 

These  are  divided  into  nerves  of  special  sense,  sensory 
nerves,  motor  nerves,  and  mixed  nerves.  Altogether  they 
make  twelve  pairs,  and  all  but  Nos.  1,  '2  and  8  arise  from 
the  medulla. 

For  nerves  Nos.  1  and  '2  see  Smell  and  Vision,  Chapter  XV. 

Third  Pair,  or  Motor  Oculi,  is  one  of  the  motor  nerves  of 
the  eyeball ;  it  supplies  with  motor  power  all  the  muscles 
(excepting  the  external  rectus  and  the  superior  oblique), 
also  the  muscle  of  the  upper  lid.  Through  its  connection 
with  the  lenticular  ganglion  it  supplies  fibres  to  the  iris 
and  ciliary  muscle ;  it  is  also  connected  at  its  origin  with 
two  other  motor  nerves  of  the  eyeball,  viz.,  the  fourth  and 
sixth  pairs. 

The  deep-seated  origin  of  the  third  pair  is  from  the 
corpora  quadrigemina  and  peduncles  of  the  cerebrum. 
Division  of  the  nerve  causes  the  eye  to  turn  downwards 
and  outwards,  owing  to  the  unbalanced  action  of  the 
superior  oblique  and  external  rectus ;  there  is  also 
depression  of  the  ui)per  lip,  immobility  of  the  eyeball,  and 
dilatation  of  the  pupil.  The  action  of  the  third  pair  will 
be  discussed  again  in  connection  with  the  physiology  of 
sight. 

Fourth  Pair,  or  Pathetic. — The  motor  nerve  of  the 
superior  oblique  muscle  of  the  eyeball ;  it  has  a  deep- 
seated  origin  in  the  valve  of  Vieussens. 

Fifth  Pair,  or  Pars  Trigemini,  resembles  a  spinal  nerve  in 
having  two  roots'  a  motor  and  sensory  ;  and  the  resem- 


426     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

blance  is  carried  still  further  by  the  sensory  root  having 
a  large  ganglion  on  it,  the  Gasserian.  The  motor  root 
arises  from  the  trigeminal  nucleus  of  the  medulla,  and  is 
connected  with  the  cerebral  cortex  on  the  opposite  side. 
The  sensory  fibres  arise  from  the  sensory  trigeminal 
nucleus,  and  can  be  traced  downwards  into  the  grey 
matter  of  the  cord.  This  nerve  also  has  connections  with 
the  nerves  arising  from  the  medulla ;  in  this  way  can  be 
explained  the  extensive  connections  and  varied  reflex  acts 
of  the  fifth  pair. 

There  are  three  divisions  of  the  fifth  pair  of  nerves,  viz., 
the  ophthalmic,  the  superior  maxillary  division,  and  the 
inferior  maxillary  division. 

The  ophtlialmic  division  is  the  smallest  of  the  three 
furnished  by  the  Gasserian  ganglion  ;  it  is  exclusively 
sensory,  supplying  with  sensation  the  structures  over  the 
brow,  the  eyeball,  the  lachrymal  gland,  membrana  nictitans, 
and  the  pituitary  membrane  on  both  sides.  The  superior 
ma.riUar;/  division  is  wholly  sensory  and  supplies  part  of 
the  orbit,  eyelids,  skin,  hard  and  soft  palates,  pituitary 
membrane  of  the  nostrils,  and  teeth  (molars,  incisors,  and 
canine),  whilst  the  terminations  of  the  main  trunk  are 
extended  over  the  face,  upper  lip,  and  nostrils,  by  means 
of  a  considerable  plexus  of  nerves  which  issues  from  the 
infra-orbital  foramen.  The  inferior  maxillary  division  is 
a  mixed  nerve ;  it  supplies  motor  power  to  the  muscles  of 
mastication,  viz.,  the  masseters,  buccal  muscles,  internal 
pterygoid,  part  of  the  tem})oralis,  and  the  mylo-hyoid 
muscle  of  the  tongue.  By  means  of  its  great  lingual 
branch,  which  enters  the  tongue  in  conjunction  with  the 
chorda  tympani  of  the  seventh  nerve,  common  sensation 
is  supplied  to  the  anterior  two-thirds  of  the  tongue. 

Besides  the  above,  sensory  branches  are  supplied  to  the 
teeth  and  lips  near  the  commissures,  and  filaments  to  the 
parotid,  molar,  and  buccal  glands. 

Each  of  these  main  divisions  of  the  fifth  nerve  possesses 
a  ganglion  on  it,  viz.,  the  ophthalmic  on  the  ophthalmic 
branch,  the  spheno- palatine  on  the  superior  branch,  and  the 


THE  NEKVOUS  SYSTEM  427 

otic  ganglion  on  the  inferior  branch.  All  these  ganglia 
receive  branches  of  nerve  from  the  sympathetic  and 
cerebro-spinal  system.  It  is  from  the  ophthalmic  or  ciliary 
ganglion  that  the  ciliary  nerves  of  the  iris  and  ciliary 
muscle  arise,  the  motor  root  of  the  ganglion  being  supplied 
by  the  third  nerve,  and  the  sensory  from  a  branch  of  the 
ophthalmic  of  the  fifth.  The  ganglion  on  the  superior 
branch  is  known  as  the  spheno-palatine ;  it  receives  its 
motor  supply  through  the  Vidian  nerve  from  the  facial,  its 
sensory  roots  being  numerous  and  supplied  by  the  spheno- 
palatine branch  of  the  fifth.  This  ganglion  supplies 
branches  to  the  bloodvessels  of  the  orbit,  and  others  to 
the  palate  through  which  motor  power  is  supplied  to  the 
muscles  of  the  soft  palate.  On  the  inferior  division  of  the 
fifth  is  sometimes  found  a  ganglion  known  as  the  otic,  the 
motor  root  of  which  is  derived  from  the  seventh  pair,  and 
the  sensory  from  the  inferior  branch  of  the  fifth.  This 
ganglion  gives  branches  which  supply  the  tensor  tympani 
of  the  internal  ear,  and  some  branches  to  the  Eustachian 
tube  and  tensor  palati. 

In  the  dog  and  cat  is  found  the  submaxillary  ganglion  ; 
it  is  supplied  by  the  chorda  tympani  of  the  seventh  pair 
with  secretory  fibres  for  the  gland  and  dilator  fibres  for  the 
bloodvessels ;  to  this  ganglion  also  runs  a  branch  of  the 
sympathetic.  All  the  fibres  of  the  chorda  do  not  enter  the 
gland,  some  supply  the  tongue.  A  submaxillary  ganglion 
exists  in  both  dog  and  cat  ;  it  lies  in  the  hilum  of  the  gland 
of  the  same  name. 

Division  of  the  superior  maxillary  division  of  the  fifth  in 
the  horse  (Bell's  experiment)  prevents  the  animal  from 
grasping  food  with  its  lips ;  not  for  the  reason  that  they 
are  deprived  of  motion,  but  owing  to  loss  of  sensibility  the 
animal  is  unaware  of  how  to  take  hold  of  the  food.  The 
relation  of  the  fifth  to  muscular  movements  is  that  it  keeps 
the  muscles  aware  of  the  position  of  objects. 

Complete  section  of  the  fifth  pair  causes  loss  of  sensation 
to  one  side  of  the  face,  lips,  mouth,  and  temple,  part  of 
the  ear,  cornea,  conjunctiva,  nasal  mucous  membrane,  and 


428     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

anterior  two-thirds  of  the  tongue.  There  is  paralysis  of  the 
muscles  of  mastication,  and  the  mouth  becomes  injured  by 
the  teeth  owing  to  loss  of  sensibility ;  the  food  collects  on 
the  paralyzed  side,  where  it  decomposes  and  produces  local 
irritation.  The  animal  also  frequently  bites  its  tongue,  as 
its  position  in  the  mouth  cannot  be  felt.  The  cornea  may 
become  cloudy  and  ulcerates. 

As  an  afferent  nerve  in  reflex  acts,  the  fifth  nerve  is  most 
important ;  without  it  there  would  be  no  closure  of  the  eye 
nor  sneezing,  and  irritation  of  the  conjunctiva  would 
produce  no  tears. 

Sixth  Pair,  or  Abducens,  arises  from  the  floor  of  the  fourth 
ventricle,  and  supplies  the  external  rectus  muscle  of  the 
eye  with  motor  power.  Paralysis  of  this  muscle  causes 
internal  squint. 

Seventh  Pair  (Portio  Dura),  or  Facial. — Arises  from  the 
medulla,  passes  through  the  internal  auditory  meatus  in 
company  with  the  eighth  pair,  which  it  leaves  behind  in  the 
internal  ear,  whilst  the  seventh  nerve  escapes  by  the  aqueduct 
of  Fallopius,  passes  beneath  the  parotid,  and  finds  its  way 
on  to  the  cheek  over  the  external  masseter  muscle,  and  is 
eventually  distributed  to  the  upper  and  lower  lips  and  the 
ahiB  of  the  nostrils.  It  essentially  supplies  the  muscles  of 
expression  and  not  those  of  mastication. 

In  its  course  it  is  joined  by  branches  from  the  fifth  pair 
and  vagus,  and  gives  off  to  the  lingual  of  the  fifth,  as  pre- 
viously mentioned,  a  branch  known  as  the  chorda  tympani, 
supplying  the  front  portion  of  the  tongue  with  taste,  and 
secretory  fibres  to  the  maxillary  gland  and  dilator  fibres  to 
the  bloodvessels.  It  is  really  a  branch  not  of  the  facial 
but  of  the  nervus  intermedius,  the  little  cranial  nerve 
lying  between  the  facial  and  the  nerve  of  the  ear. 

The  facial  is  a  motor  nerve  to  the  muscles  of  the  middle 
ear,  external  ear,  cheeks,  lips,  nostrils,  and  orbicular 
muscle  of  the  eye. 

Division  of  the  seventh  nerve  leads  to  alterations  in  sight, 
taste,  hearing,  smell,  and  facial  expression.  As  it  supplies 
the  muscle  which  closes  the  eyelids  (the  orbicularis  palpe- 


THE  NERVOUS  SYSTEM  429 

brarum),  conjunctivitis  occurs  from  exposure  of  the  eyeball; 
hearing  is  affected  owing  to  paralysis  of  the  muscles  of  the 
internal  ear  ;  smell  is  impaired  due  to  the  paralj^zed  con- 
dition of  the  nostrils ;  taste  is  affected  through  paralysis  of 
the  chorda.  The  expression  of  unilateral  facial  paralysis 
in  the  horse  is  characteristic  ;  the  upper  lip  drawn  to  one 
side,  the  elongated  nostril,  the  pendulous  lower  lip,  the 
escape  of  saliva  and  food  from  the  mouth,  the  vacant  look, 


7    ' 

I'  i 

\'' 

It 

J 

Fig.  97. — Characteristic  Facial  Expression  of  the  Horse  with 
Paralysis  of  the  Seventh  Nerve. 

the  open  eye,  and  the  drooping  ear,  point  clearly  to  the 
extensive  distribution  of  this  nerve. 

Bernard  pointed  out  that  horses  were  suffocated  if 
galloped  after  division  of  both  facial  nerves,  owing  to  the 
fact  that  the  nostrils  were  no  longer  capable  of  dilatation. 

Eighth  Pair,  or  Portio  Mollis, — Arises  by  two  roots,  one 
the  nerve  for  the  special  sense  of  hearing,  the  other  dis- 
tributed to  the  otolith  organs  and  the  semicircular  canals, 
and  assists  through  these  in  maintaining  the  equilibrium 


430     A  MANUAL  OF  VETEKINARY  PHYSIOLOGY 

of  the  body.  Injury  to  the  semicircular  canals  produces 
giddiness,  not  deafness,  and  certain  movements  (termed 
'  pendulum-like  ')  of  the  head  occur  ;  the  direction  in  which 
these  are  made  depends  on  the  orientation  of  the  canal 
which  has  been  injured. 

Ninth  Pair,  or  Glosso-Pharyngeal,  arises  from  the  medulla  ; 
it  is  a  mixed  nerve,  and  supplies  motor  power  to  the 
muscles  of  the  pharynx,  and  sensory  fibres  to  the  posterior 
third  of  the  tongue,  soft  palate,  part  of  pharynx,  and 
anterior  surface  of  the  epiglottis.  It  is  also  a  special  nerve 
of  taste,  supplying  the  posterior  third  of  the  tongue,  and 
having  special  nerve  endings,  known  as  '  taste-bulbs,'  in  the 
circumvallate  papillae. 

Tenth  Pair,  or  Pneumogastric. — This  is  both  a  sensory  and 
motor  nerve.  At  its  origin  in  the  medulla  it  is  intimately 
mixed  up  with  the  ninth,  eleventh,  and  twelfth  pairs  of 
nerves,  and  later  on  with  the  sympathetic.  It  is  the  most 
extensively  distributed  nerve  in  the  body,  supplying  the 
oesophagus,  pharynx,  lungs,  bronchi,  trachea,  heart,  stomach, 
and  intestines ;  hence  its  other  name,  vagus. 

The  sensory  branches  of  the  nerve  are  not  highly 
endowed  with  sensation,  probably  for  the  reason  that  their 
chief  function  as  sensory  nerves  is  as  afferent  channels  for 
reflex  action.  The  motor  fibres  are  derived  from  the  spinal 
accessory  nerve.  In  the  foramen  lacerum  the  vagus  is 
joined  by  the  jugular  ganglion,  and  for  a  very  short 
distance  it  is  intimately  connected  with  the  accessory  nerve ; 
here  it  receives  filaments  from  the  accessory,  sympathetic, 
hypoglossal,  and  two  first  cervical  nerves.  The  vagus  now 
descends  behind  the  guttural  pouch  and  joins  the  cervical 
portion  of  the  sympathetic  nerve,  from  which  results,  in  the 
horse  and  most  other  animals,  a  single  cord  which  passes 
down  the  neck  above  the  carotid  artery ;  as  it  enters  the 
chest  it  separates  from  the  sympathetic.  The  arrangement 
of  the  right  and  left  nerves  is  different ;  the  right  gives  off 
the  right  recurrent  which  passes  around  the  dorso-cervical 
artery,  while  the  main  trunk  terminates  above  the  origin 
of   the   bronchi ;  the  left   gives   oft'  its   recurrent   branch 


THE  NEEVOUS  SYSTEM  431 

opposite  to  the  aorta,  and  also  terminates  on  the  bronchi, 
forming  with  the  right  nerve  the  bronchial  plexus  and 
cesoj)hageal  nerves,  the  latter  passing  to  the  stomach  and 
from  thence  to  the  solar  plexus.  The  various  branches  of 
the  vagus  may  best  be  studied  in  the  order  in  which  they 
are  given  off. 

The  plianjngeal  nerve  originates  at  the  superior  cervical 
ganglion  and  passes  to  the  pharynx,  where  it  forms  with 
the  ninth  pair  the  pharyngeal  plexus.  It  is  a  mixed  nerve, 
and  supplies  the  middle  and  constrictor  muscles  of  the 
pharynx  and  the  cervical  j)ortion  of  the  oesophagus  with 
motor  power. 

The  superior  laryngeal  nerve  supplies  the  mucous  mem- 
brane of  the  larynx  with  remarkable  sensibility,  and  gives 
a  motor  branch,  the  external  laryngeal,  to  the  crico-pharyn- 
geus.  In  most  animals  the  superior  laryngeal  supplies  the 
crico-thyroid  muscle  of  the  larynx  with  motor  power,  but 
in  the  horse  this  is  supplied  by  the  first  cervical  nerve. 
It  is  the  superior  laryngeal  nerve  which  reflexly  excites 
coughing,  the  coughing  centre  being  situated  in  the  medulla; 
further,  it  contains  afferent  fibres  in  connection  with  the 
respiratory  centre,  which  when  stimulated  cause  arrest  of 
respiration  :  they  are  therefore  inhibitory  fibres.  Section 
of  the  superior  laryngeal  causes  pain,  and  produces  in 
dogs  a  deeper  and  hoarser  voice  due  to  paralysis  of  the 
crico-thyroid  muscle,  which  can  no  longer  render  the 
vocal  cords  tense.  The  absence  of  sensibility  in  the  larynx 
allows  food  to  pass  into  the  trachea,  and  thus  produces 
pneumonia. 

The  inferior  laryngeal,  or  recurrent,  is  given  off  from  the 
main  trunk  within  the  chest,  on  the  left  side  winding  around 
the  aorta  from  without  inwards,  and  on  the  right  side 
passing  around  the  dorso-cervical  artery  ;  both  branches 
return  up  the  neck  and  supply  all  the  muscles  of  the 
larynx  (excepting  the  crico-thyroid)  with  motor  power.  The 
recurrents  are  of  great  practical  interest,  as  they  are  aft'ected 
(especially  the  left)  in  that  common  form  of  disease  in  the 
horse  known  as  '  roaring,'  which  is  generally  due  to  paralysis 


432     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

and  atrophy  of  the  muscles  which  dilate  the  laryngeal  open- 
ing (see  p.  126)  After  division  of  both  recurrent  nerves  death 
by  asi)hyxia  is  likely  to  follow.  We  have  however  observed 
complete  bilateral  paralysis  of  the  larynx  in  horses  without 
asphyxia  being  produced.  In  such  cases  it  has  been  shown 
that  the  age  of  the  horse  is  the  saving  factor,  the  rigidity  of 
the  cartilages  })reventing  the  arytenoids  from  completely 
collapsing  over  the  opening  of  the  glottis. 

Division  of  the  recurrent  also  leads  to  a  partial  loss  of 
voice,  and  a  peculiar  cough  is  produced  owing  to  paralysis 
of  the  laryngeal  muscles.  As  the  recurrent  supplies 
sensory  branches  to  the  tracheal  portion  of  the  oesophagus 
and  trachea,  division  causes  loss  of  sensation  in  these 
parts. 

It  is  curious  that  the  recurrent  laryngeal  should  contain 
motor  fibres,  not  only  for  the  dilator  but  also  the  constrictor 
muscles  of  the  larynx ;  it  has  been  observed  that  when  this 
nerve  gets  out  of  order,  it  is  the  dilator  muscles  which  first 
become  paralyzed  and  later  the  constrictors  (p.  125).  Irrita- 
tion of  the  peripheral  end  of  the  recurrent  produces  spasm 
of  the  larynx.  There  are  certain  poisons,  such  as  that  con- 
tained in  Lathyriis  sativus  and  others  of  the  Leguminosse, 
which  appear  to  have  a  special  action  on  this  nerve,  or  at 
any  rate  on  the  larynx,  spasm  of  the  larynx  being  one  of 
the  earliest  symptoms  of  poisoning. 

The  cardiac  branches  of  the  vagus  contain  the  fibres 
which  exercise  a  controlling  and  inhibitory  power  over  the 
heart  (see  p.  45).  They  also  contain  the  depressor  nerve 
which  is  leaving  the  heart  to  run  up  the  neck  with  the 
pneumogastric,  entering  the  medulla  by  means  of  the 
superior  laryngeal  branch  ;  for  the  indirect  action  of  this 
nerve  on  the  heart  see  p.  50.  The  depressor  nerve  is 
present  as  a  distinct  branch  in  the  rabbit  and  cat,  but  in 
other  animals  it  is  mixed  up  with  the  vagus.  Lastly,  the 
cardiac  branches  contain  fibres  from  the  S3^mpathetic  which 
supply  accelerator  fibres  to  the  heart  (p.  49). 

The  pulmonary  branches  supply  both  sensory  and 
motor  branches  to  the  trachea  and   motor   fibres   to  the 


THE  NEKVOUS  SYSTEM  433 

bronchi.  Through  these  branches  impressions  are  trans- 
mitted to  the  medulla  by  which  the  respiratory  centre  is 
regulated.  Through  other  branches  centripetal  impulses 
are  transmitted  to  the  vaso-motor  centre  by  which  the 
general  blood-pressure  is  regulated. 

The  thoracic  oesophageal  branches  supply  the  oesophagus 
with  motor  power,  so  that  division  of  the  vagus  causes 
food  to  accumulate  in  the  lower  part  of  the  tube.  The 
oesophageal  nerves,  after  uniting  in  pairs  in  a  peculiar 
manner,  run  along  the  cesophagus  one  superiorly  the  other 
interiorly,  and  passing  through  the  diaphragm  they  enter 
the  abdominal  cavity.  The  superior  nerve  supplies  the  left 
sac  of  the  stomach  and  enters  the  solar  plexus,  from  which 
it  runs  to  the  intestines  and  other  organs  (p.  206) ;  the 
inferior  nerve  terminates  in  the  walls  of  the  stomach  at 
its  cardiac  or  right  extremity. 

Division  of  both  vagi  in  the  horse  causes  the  breathing 
to  become  much  deeper,  more  prolonged,  and  suffocation 
may  result  owing  to  loss  of  motor  power  in  the  larynx. 
Through  the  absence  of  sensation  in  the  larynx,  trachea, 
bronchi,  and  lungs,  food  is  apt  to  find  its  way  into  the 
respiratory  passages  and  produce  pneumonia.  The  lungs 
likewise  undergo  congestion  owing  to  the  laboured  and 
ditiicult  respiration,  and  the  parts  become  oedematous.  In 
the  horse  the  respirations  have  been  known  to  fall  to  five 
per  minute,  but  the  heart  beats  rapidly  owing  to  the 
unbalanced  action  of  the  sympathetic.  Through  paralysis 
of  the  esophagus  and  stomach  food  collects  in  the  latter, 
and  may  extend  throughout  the  entire  length  of  the 
oesophagus  up  the  neck.  Apparently  engorgement  of  the 
stomach  in  the  horse  is  not  invariably  produced  as  the 
result  of  dividing  both  vagi,  for  some  observers  have  noted 
no  difficulty  in  this  respect.  Experiments  made  by  Colin 
show  that  division  of  the  vagi  paralyzes  the  stomach,  so 
that  poisons  may  remain  there  and  cause  the  animal  no 
inconvenience  as  they  never  pass  into  the  intestine,  and 
thus  cannot  become  absorbed  (see  p.  176).  This  is  a  point 
of  practical  importance,  and  warns  us  how  useless  drugs 

28 


434    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

administered  by  the  mouth  may  be  in  some  digestive 
troubles  of  the  horse,  especially  those  of  the  stomach. 

Eleventh  Pair,  or  Spinal  Accessory,  arise  by  two  roots, 
one  from  low  down  the  cervical  portion  of  the  cord,  the 
other  from  the  medulla.  It  is  essentially  a  motor  nerve, 
but  through  being  intimately  connected  with  the  pneumo- 
gastric  it  also  possesses  sensory  fibres.  The  use  of  this 
nerve  is  to  supply  motor  power  to  the  sterno-maxillaris, 
trapezius,  and  a  portion  of  the  levator  humeri  muscles  ; 
at  its  origin  it  supplies  most  of  the  motor  fibres  found 
in  the  vagus,  and  also  furnishes  the  latter  with  its  cardio- 
inhibitory  fibres.  The  accessory  is  considered  also  to 
possess  an  influence  over  the  larynx ;  division  of  it  pro- 
duces no  difficulty  in  breathing,  as  in  the  case  of  the 
recurrent  laryngeal,  but  it  causes  loss  of  voice  due  to 
paralysis  of  the  motor  fibres  of  the  vagus. 

Twelfth  Pair,  or  Lingual. — The  branches  of  this  nerve 
supply  the  tongue  with  motor  power,  and  fibres  to  the 
muscles  which  depress  the  larynx.  Section  of  the  nerve 
on  both  sides  causes  paralysis  of  the  organ ;  dogs  are 
unable  to  lap,  and  injure  the  protruding  tongue  with  the 
teeth. 

Medulla  Oblongata  or  Bulb. 

Situated  at  the  top  of  the  spinal  cord,  and  forming  the 
connection  between  it  and  the  brain,  is  the  medulla 
oblongata.  It  is  composed  of  white  and  grey  matter,  but 
not  arranged  with  the  regularity  found  in  the  cord ;  the 
columns  of  the  latter  are  continued  into  it,  and  give  rise  to 
certain  columns  in  the  bulb  larger  and  more  prominent 
than  those  of  the  cord.  The  inferior  columns  form  the 
inferior  pyramids  of  the  bulb,  the  superior  form  the 
superior  pyramids,  and  the  lateral  columns  dividing  into 
three  parts  help  to  form  the  restiform  bodies. 

As  the  main  paths  or  highways  in  the  cord  are  either 
going  to  or  coming  from  the  brain,  it  is  interesting  to  study 
briefly  their  distribution  in  the  bulb.  Of  all  the  paths  known 
in  the  cord  only  three  pass  for  certain  through  the  bulb  to 


THE  NEEVOUS  SYSTEM  435 

higher  centres  in  the  brain,  viz.,  the  pyramidal  tract  the 
fibres  of  which  are  descending  to  the  cord  from  their  origin 
in  the  cells  of  the  cerebral  cortex,  and  the  cerebellar  tracts 
which  pass  upward  through  the  medulla  to  reach  the 
cerebellum. 

The  tracts  passing  through  the  bulb  from  the  cerebrum 
decussate  in  the  bulb,  and  in  this  way  account  for  a  right 
brain  lesion  producing  a  left  body  paralysis.  All  the  other 
tracts  in  the  cord  terminate  in  groups  of  cells  in  the  bulb, 
and  act  as  carriers  between  it  and  the  cord. 

The  grey  matter  of  the  cord  does  not  maintain  its  charac- 
teristic appearance  in  the  bulb,  the  inferior  cornua  dis- 
appear, while  the  superior  cornua  enlarge.  Owing  to  the 
decussation  of  fibres  in  the  inferior  pjTamid  the  grey  and 
white  matter  get  mingled  up,  and  nuclei  and  masses  of 
nerve  cells  are  formed  as  the  result ;  from  these  nuclei 
the  cranial  nerves  arise.  This  arrangement  leads  to  con- 
siderable complexity  in  the  grey  matter  of  the  medulla,  and 
a  markedly  intricate  arrangement  of  the  fibres  of  the  white 
substance. 

Centres  in  the  Medulla. — The  various  centres  found  in  the 
bulb  are  of  such  importance  to  life  that  an  injury  to  this 
part  generally  means  instantaneous  death.  The  whole  of 
the  rest  of  the  brain  may  gradually  be  removed  without 
destroying  life,  but  the  medulla  itself  will  not  tolerate  inter- 
ference. The  reflex  and  other  centres  are  so  numerous  and 
widespread,  that  it  is  remarkable  how  the  varied  functions 
carried  out  by  them  can  be  performed  within  such  a  limited 
area.  The  centres  localized  in  the  bulb  are  those  for  mas- 
tication, swallowing,  sucking,  vomiting,  respiration,  phona- 
tion,  coughing,  the  movements  of  the  heart,  bloodvessels, 
and  iris,  the  secretion  of  saliva,  the  glycosuria  centre,  a 
centre  for  the  sweat  glands  and  a  centre  for  shivering. 

Though  all  these  functions  have  been  more  or  less  clearly 
referred  to  the  bulb,  we  must  avoid  falling  into  the  error 
that  a  definite  representation  exists  for  each  of  them  ;  and 
though  the  term  centre  is  employed,  it  is  more  as  a  con- 
venient mode  of  expression,  than  as  absolutely  establishing 

28—2 


43G     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

the  fact  that  any  particular  group  or  groups  of  cells  are 
responsible  for  one  function  more  than  another.  Perhaps 
the  only  exception  to  this  is  the  respiratory  centre,  which 
has  been  defined  with  a  certain  amount  of  exactitude. 

The  mastication  and  sivallowing  centres  lie  in  the  floor  of 
the  fourth  ventricle  ;  they  have  for  their  afferent  nerves  the 
inferior  divison  of  the  fifth,  glosso-pharyngeal,  and  the 
superior  laryngeal  of  the  pneumogastric  ;  whilst  the  motor 
branches  are  in  the  motor  parts  of  the  fifth  for  mastication, 
and  in  the  fibres  of  the  pharyngeal  plexus  of  the  vagus  for 
swallowing.  All  the  muscles  of  mastication,  except  the 
digastricus,  receive  motor  nerves  from  the  inferior 
maxillary  division  of  the  fifth  pair.  It  would  appear  that 
the  reflex  act  of  swallowing  is  excited  by  the  presence  of 
food  in  the  pharynx. 

A  vomitinfi  centre  exists  in  the  bulb,  which  in  the  horse 
and  ruminants  is  certainly  most  imperfectly  developed. 
"We  have  previously  (p.  180)  drawn  attention  to  the  fact 
that  there  is  no  drug  which  has  the  power  of  exciting 
vomiting  in  the  horse  ;  tartar  emetic  has  not  the  slightest 
action,  and  the  effect  of  apomorphia  is  only  to  produce  the 
most  alarming  symptoms  of  cerebral  excitement,  but  no 
attempt  at  vomiting.  In  the  dog  and  pig  the  vomiting 
centre  is  well  developed.  The  afferent  nerves  may  be  those 
of  the  pharynx,  palate,  and  root  of  tongue,  viz.,  the  glosso- 
pharyngeal, or  those  from  the  mucous  membrane  of  the 
stomach,  for  example,  the  vagus  and  sympathetic ;  the  im- 
pression having  been  carried  to  the  bulb  the  efferent  nerves 
are  the  phrenics  for  the  diaphragm,  and  vagus  for  the 
stomach  and  (esophagus.  The  vomiting  centre  may  be 
directly  stimulated  b}'  irritating  the  central  end  of  the  vagus. 

Secretion  of  Saliva. — The  centre  for  this  lies  in  the  floor 
of  the  fourth  ventricle  at  the  origin  of  the  seventh  and  ninth 
pair  of  nerves.  The  afferent  nerves  are  those  of  taste,  viz., 
the  gustatory  branch  of  the  fifth  and  glosso-pharyngeal, 
whilst  the  chorda  tympani  is  not  only  afferent  from  the 
fore  part  of  the  tongue  but  is  also  the  efferent  nerve  to 
the  submaxillary,  and  the  superficial  petrosal  that  to  the 


THE  NERVOUS  SYSTEM  437 

parotid  gland.  Other  centres  in  the  bulb  are  the  cardio- 
inhibitory  and  respiratory  centres. 

For  the  respiratory  centre  see  p.  108. 

For  the  cardio-accelerator  and  cardio-inhihitory  centre  see 
pp.  48,  49. 

For  the  vaso-motor  centre  see  p.  75. 

For  the  diabetic  centre  see  p.  230. 

Functions  of  the  Bulb. — The  bulb  apart  from  the  brain 
cannot  elaborate  sensation  or  voluntary  movement.  It 
forms  a  pathway  to  the  brain  for  the  columns  in  the  spinal 
cord,  and  is  a  conductor  of  centripetal  and  centrifugal 
impulses ;  it  gives  origin  to  all  the  cranial  nerves  but 
those  of  smell,  sight,  and  the  motor  nerves  of  the  eyeball ; 
finally  it  is  the  supreme  reflex  centre  for  the  nerves  govern- 
ing respiration,  circulation,  the  action  of  the  heart,  and  the 
digestive  apparatus  from  the  mouth  to  the  intestine. 

The  Pons  Varolii  conducts  centripetal  and  centrifugal 
impulses  to  and  fro  and  up  and  down  ;  it  connects  the 
cerebrum  with  the  bulb  and  cerebellum,  and  several  of  the 
cranial  nerves  arise  in  connection  with  the  grey  matter  of 
the  various  nuclei  found  in  it.  When  stimulated,  pain  and 
muscular  spasms  are  produced. 

The  Thalarni  Optici  are  connected  with  vision,  but  are 
mainly  supposed  to  be  the  centres  for  tactile  impressions 
which  they  transmit  onwards  to  the  cerebral  cortex  (Fig. 
92,  ot). 

The  Corpora  Striata  are  interesting  clinically  on  account 
of  the  comparative  frequency  with  which  they  are  diseased 
in  the  horse.  They  are  considered  to  be  the  centres  for 
co-ordination  of  motor  impulses  ;  when  they  are  destroyed 
the  animal  has  an  irresistible  tendency  to  move  forwards. 
We  have  certainly  seen  this  latter  symptom  shown  in  the 
horse  in  disease  of  the  corpora  striata,  but  it  is  far  from 
invariable.  It  is  remarkable  how  extensively  the  parts  may 
be  affected  and  pressed  upon  by  tumours  without  symptoms 
being  exhibited  :  the  gradual  progress  of  the  pressure  or 
destruction  may  account  for  this.  The  corpora  striata  are 
also  considered  to  be  concerned  in  heat  production  ;  there 


438    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

appears  to  be  no  doubt  that  experimental  injury  of  these 
bodies  produces  a  high  temperature.  Nothing  is  known  of 
the  mechanism,  but  it  is  supposed  that  impulses  pass  from 
the  corpora  striata  to  the  muscles,  the  result  being  a  great 
increase  in  the  amount  of  heat  produced.  It  is  of  interest 
to  remember  that  the  corpora  striata,  unlike  the  optic 
thalami,  are  shown  by  their  developmental  history  to  be 
really  portions  of  the  cortical  grey  matter. 

Cerebellum. 

In  the  cerebellum  is  found  a  collection  of  fibres  and 
ganglion  cells  in  communication  with  tracts  from  the  spinal 
cord,  bulb  and  cerebrum.  It  is  the  first  piece  of  nervous 
tissue  we  have  studied  where  the  grey  matter  is  externally 
placed  and  not  internally  as  in  the  cord ;  the  surface  being 
folded  and  doubled  in  on  itself  to  a  considerable  extent, 
thus  forming  convolutions. 

The  functions  of  the  cerebellum  are  principally  concerned 
in  the  co-ordination  of  movement,  viz.,  harmony  and 
rhythm  in  muscular  actions ;  it  is  enabled  to  carry  out 
this  function  through  its  connection  with  the  superior 
columns  of  the  cord,  which  keep  the  cerebellum  informed 
of  the  position  of  the  limbs.  There  can  be  no  doubt  that 
in  co-ordinating  muscular  movement,  the  cerebellum  is 
assisted  both  by  the  sense  of  sight,  and  by  the  nerves  from 
the  otolith  organs  and  semicircular  canals  of  the  ear;  an 
animal  walks  with  uncertainty  when  the  eyes  are  covered 
up,  and  disease  of  the  internal  ear  is  a  well-known  cause  of 
vertigo  in  the  human  subject. 

Injury  of  the  cerebellum  produces  no  sensory  dis- 
turbance, but  entails  defects  of  movement.  When  sliced 
away  in  birds  they  lose  the  power  of  flying,  walking,  or 
preserving  their  equilibrium  ;  there  is  no  loss  of  conscious- 
ness or  intelligence,  but  an  inability  to  co-ordinate  the 
skeletal  muscles.  Injury  to  one  of  the  crura  of  the  cere- 
bellum produces  *  forced  movements  '  as  they  are  termed. 
The  animal  rolls  over  and  over  around  the  long  axis  of  the 


THE  NERVOUS  SYSTEM  439 

body,  or  else  circus  movements  or  somersaults  are  per- 
formed. In  dogs  superficial  injury  to  one  of  the  processes 
of  the  cerebellum  causes  only  temporary  disturbance,  whilst 
deep  injury  or  removal  of  a  hemisphere  causes  rigidity  of 
the  legs  and  shaking  of  the  head  ;  more  extensive  injury  is 
followed  b}'  disturbance  of  co-ordination.  The  entire  cere- 
bellum has  been  removed  in  the  dog,  the  animal  living  for 
many  months  ;  in  the  first  instance  spasms  of  the  muscles 
of  the  head,  neck,  and  fore  legs,  and  weakness  of  the  hind 
legs  were  present ;  when  the  eyes  were  closed  standing  was 
impossible.  These  symptoms  gradually  gave  way,  and  the 
animal  was  left  with  a  deficiency  of  muscular  tone,  and  a 
tremor  in  the  muscles  which  increased  on  the  performance 
of  voluntary  movement ;  it  could  swim  but  was  muscularly 
weak,  and  eventually  died  from  marasmus. 

The  cerebellum  influences  movement  by  re-inforcing  the 
activitj'-  of  the  opposite  hemisphere  of  the  cerebrum 
(Luciani),  especially  of  the  '  motor  area.'  The  movements 
produced  by  the  opposite  cerebral  hemisphere  become 
wanting  in  steadiness  and  power  when  half  of  the  cere- 
bellum has  been  removed,  and  the  muscles  innervated  by 
that  hemisphere  are  deficient  in  tone.  No  direct  downward 
connection  of  the  cerebellum  with  the  cord  is  known  to 
exist,  though,  as  previously  mentioned,  the  cord  in  an 
upward  direction  is  connected  with  the  cerebellum. 

Mid  Brain. 

There  is  little  of  importance  to  the  general  student  to  be 
said  about  the  physiology  of  the  mid  brain.  The  anterior 
corpora  quadrigemina  are  concerned  with  the  oldest 
primitive  reflex  centre,  working  from  the  optic  nerve  upon 
the  eyeball  muscles.  The  posterior  corpora  quadrigemina 
are  important  in  the  higher  group  of  vertebrates,  viz.,  birds 
and  mammals  which  have  a  cochlea,  that  is,  a  '  hearing  ' 
ear  besides  an  '  equilibrating '  ear.  The  posterior  quadri- 
geminal  bodies  receive  fibres  from  the  cochlea  nerve,  and 
have  reflex  centres  connected  with  lower  auditory  functions. 


440     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

For  instance,  a  cat  with  the  brain  cut  through  just  in 
front  of  the  posterior  bodies  emits  on  stimulation  of  a  hurtful 
kind  to  the  skin  a  long  angry  vocalized  note.  But  this 
ceases  directly  the  section  lies  behind  the  posterior  bodies. 
So  also  the  '  chloroform  cry '  that  animals  and  men  give 
under  chloroform  goes  on  when  the  brain  is  cut  in  front 
of  the  posterior  bodies,  but  not  when  the  section  is  made 
behind  them. 

Cerebrum. 

The  cerebrum  is  composed  of  grey  and  white  matter,  the 
grey  being  externally  placed  and  thrown  into  convolutions. 
These  convolutions,  though  well  marked  in  the  lower 
animals,  are  by  no  means  so  numerous  as  in  the  man-like 
apes  and  man.  The  use  of  the  convolutions  is  no  doubt  to 
increase  the  surface  of  the  brain,  and  the  deeper  and  more 
complex  they  are,  the  greater,  as  a  rule,  is  the  intelligence 
of  the  animal.  In  the  horse  the  convolutions  are  compara- 
tively very  shallow. 

Use  of  the  Cerebrum. — In  the  grey  matter  of  the  cerebrum 
is  located  the  seat  of  sensation,  reasoning,  and  will.  The 
white  matter  is  simply  the  conducting  paths  along  which 
the  impulses  are  distributed. 

It  is  quite  possible  for  an  animal  to  perform  acts  which 
look  as  if  executed  by  intelligence,  or  to  undertake  move- 
ments which  need  precision,  in  spite  of  the  fact  that  it  is 
without  a  cerebrum.  Some  very  curious  observations  have 
been  made  on  the  frog  in  which  the  cerebral  hemispheres 
have  been  removed.  If  stimulated  the  frog  springs,  if 
thrown  into  the  water  it  swims,  if  placed  on  its  back  it 
recovers  its  normal  position,  and  if  stroked  it  croaks.  All 
these  actions  would  indicate  the  presence  of  consciousness, 
but  such  is  not  so  ;  the  frog  without  its  cerebral  lobes  will 
remain,  unless  stimulated,  in  one  position  until  it  dies,  it 
appears  to  possess  no  power  of  spontaneous  movement,  or 
power  of  will.  A  remarkable  experiment  performed  on  a 
frog  m  this  condition  consists  in  placing  it  on  a  board 
which    is   gradually  brought   from  the  horizontal   to  the 


THE  NERVOUS  SYSTEM  441 

vertical  position  ;  during  the  movement  the  animal  crawls 
up  the  inclined  plane,  and  when  the  board  is  vertically 
placed  it  sits  on  the  top  perfectly  balanced  ;  as  the  board 
is  lowered  to  the  opposite  side  from  which  it  was  raised 
the  creature  descends.  It  is  only  during  the  time  the 
board  is  being  raised  or  lowered  that  the  frog  moves,  but 
the  movements  are  executed  with  precision.  It  is  evident 
that  these  acts  which  strike  one  as  being  intelligent  are 
really  reflex,  and  are  executed  by  the  spinal  cord,  the  animal 
being  absolutely  unconscious  of  what  is  going  on,  and  it 
may  be  amongst  even  the  higher  animals  that  some  acts 
regarded  as  volitional  are  in  reality  reflex. 

Motor  and  Sensory  Areas. — So  far  as  we  are  aware  no 
observations  have  been  made  on  the  motor  and  sensory 
areas  of  the  cerebrum  of  Ungulata,  but  the  dog  has  on  this 
point  been  carefully  examined. 

The  dog's  brain  is  marked  by  two  fissures  known  as  the 
sulcus  cruciatus  (Fig.  98,  S),  and  fossa  Sylvii  (Fig.  98,  F). 
Between  these  fissures  are  arranged  four  primary  convolu- 
tions I,  II.,  III.,  and  IV.  (Figs.  98  and  99).  In  the  anterior 
part  of  the  fourth  or  superior  convolution  are  found  from 
before  backwards — 

(a)  The  motor  areas  for  the  muscles  of  the  neck  (Fig. 
98 — 1).  (b)  The  motor  areas  for  the  extensors  and 
abductors  of  the  fore-leg  (Fig.  98—2).  (c)  The  motor 
areas  for  the  elevation  of  the  shoulder  and  extension  of 
fore-limb  movements  as  in  walking  (Fig.  98—8).  (d)  The 
motor  areas  for  the  flexors  and  rotators  of  the  fore-leg 
(Fig.  98 — 3).  (e)  The  motor  areas  for  the  muscles  of  the 
hind-leg  (Fig.  98 — 4).  (/)  The  motor  areas  for  the  retrac- 
tion and  abduction  of  the  fore-leg  (Fig.  98—7).  (r/)  The 
motor  areas  for  the  lateral  switching  movements  of  the 
tail  (Fig.  98—6).  Close  to  No.  2  area  (Fig.  98)  is  one  d 
(Fig.  99),  stimulation  of  which  causes  the  eye  to  turn  to  the 
opposite  side,  opens  the  eyelid  and  dilates  the  pupil.  In 
the  third  convolution  is  situated  an  area  9,  9,  9  (Fig.  99), 
stimulation  of  which  controls  the  movement  of  the  orbicu- 
laris muscle,  produces  an  upward  movement  of  the  eyeball. 


442     A  MANUAL  OF  VETEPJNAEY  PHYSIOLOGY 


Fig.  98. 


To  Illustrate  the  Motor  Areas  in  the  Brain  of  the  Dog 
(Landois  and  Stirling). 

Fig.  98,  cerebrum  of  the  dog  from  above.  Fig.  99,  cerebrum  from  the 
side.  I.,  II.,  III.,  IV.,  the  four  primary  convolutions  ;  S,  sulcus 
cruciatus ;  F,  sylvian  fosse  ;  o,  olfactory  lobe  ;  p,  optic  nerve. 
The  positions  of  the  areas  are  described  in  the  text. 


THE  NEEVOUS  SYSTEM  443 

and  a  narrowing  of  the  pupil ;  behind  this  is  e,  e,  c,  an  area 
which  represents  vision. 

In  the  second  convokition  is  an  area  a  a  (Fig.  99),  which 
produces  retraction  and  elevation  of  the  angle  of  the  mouth 
with  partial  opening  of  it.  Behind  this  is  <:'  c,  stimulation 
of  which  retracts  the  mouth  owing  to  the  action  of  the 
platysma  ;  then  an  area  c^  which  like  9  (Fig.  99)  causes 
elevation  of  one  angle  of  the  mouth  and  of  one  half  the 
face  until  the  eye  is  partly  closed.  Behind  this  is  J  f  f, 
which  is  the  auditory  centre. 

In  the  first  convolution  is  the  oral  centre  {b,  Fig.  99), 
stimulation  of  which  opens  the  mouth,  protrudes  and 
retracts  the  tongue,  while  the  dog  not  unfrequently  howls. 

All  these  centres  have  been  indicated,  but  it  is  necessary 
to  remember  that  though  in  area  they  may  be  as  large 
as  a  pea,  yet  to  an  extent  they  overlap.  The  higher 
the  animal  is  in  the  scale  the  greater  the  complexity 
observed  in  the  areas,  as  for  instance  in  the  monkey,  where 
the  skilled  movements  of  the  hands  and  feet  are  largely 
represented  in  the  cortex.  The  size  of  an  area  bears  no 
relation  to  the  size  of  the  part  supplied,  but  does  bear  a 
relation  to  the  complexity  of  movement  which  the  part  is 
intended  to  produce.  Thus  the  thumb  area  in  the  cortex 
of  the  monkey  is  relatively  larger  than  the  shoulder  or  hip 
area. 

The  effect  of  removing  the  motor  areas  differs  according 
to  the  animal ;  in  the  monkey  it  results  in  permanent  motor 
paralysis  of  hand  or  foot,  but  not  of  parts  with  less  skilled 
movement,  e.g.,  shoulder  or  knee.  In  the  dog  paralysis  is 
not  necessarily  produced,  and  it  has  been  supposed  that 
the  basal  ganglia  are  capable  in  this  animal  of  taking  on 
the  duties  of  the  cortex.  The  destruction  which  has  been 
observed  at  times  in  the  cortex  of  the  horse  is  commonly 
unaccompanied  by  any  symptoms  until  shortly  before 
death. 

Strong  stimulation  of  the  motor  areas  produces  epilepsy. 
By  observing  the  groups  of  muscles  first  affected  and 
knowing  the  region  of  the  cortex  to  which  they  are  related, 


444     A  MANUAL  OF  VETERINAKY  PHYSIOLOGY 

it  is  possible,  certainly  in  man,  to  localize  with  considerable 
exactitude  the  seat  of  the  trouble.  Removal  of  the  anterior 
or  frontal  convolutions  in  the  dog  leads  to  unilateral  motor 
and  sensory  paralysis,  from  which  the  animal  recovers 
with  the  exception  that  there  is  loss  of  muscular  sense. 
If  the  operation  be  performed  on  both  sides  there  is  an 
exaggeration  of  the  symptoms,  and  the  animal  becomes 
vicious.  Removal  of  the  posterior  or  occipital  lobes  leads 
to  blindness,  no  loss  of  motion  or  of  muscular  sense,  and 
the  dog  remains  obedient  but  sluggish.  Removal  of  a  large 
mass  of  cerebral  cortex  causes  the  animal  to  become 
intensely  stupid,  it  walks  slowly,  the  head  hangs  down, 
sensibility  is  diminished  ;  the  dog  sees  but  cannot  compre- 
hend, it  howls  from  hunger  and  eats  until  its  stomach  is 
full,  it  exhibits  no  sexual  excitement,  and  becomes,  in  fact, 
an  eating,  complex,  reflex  machine. 

Colin  draws  attention  to  the  difficulty  in  producing 
paralysis  experimentally  in  the  horse  from  lesions  of  the 
hemispheres.  Neither  the  artificial  production  of  a  clot  in 
the  falciform  sinus,  nor  the  introduction  of  pieces  of  lead 
the  size  of  a  pea  into  the  convolutions,  gave  rise  to 
hemiplegia.  This  quite  bears  out  what  we  know  to  be  a 
clinical  fact,  that  it  is  possible  for  horses  to  have  in  their 
lateral  ventricles  tumours  the  size  of  an  egg  without  pro- 
ducing any  disturbance.  We  have  seen  such  cases,  the 
tumours  being  of  variable  size,  and  the  clinical  history 
has  never  given  more  than  a  few  days'  illness,  though 
the  growths  must  have  been  forming  for  a  considerable 
period. 

The  Circulation  in  the  Brain  is  peculiar  ;  the  veins  or 
so-called  sinuses  are  enclosed  in  very  rigid  membranous 
walls  formed  by  the  dura  mater ;  the  blood  is  driven 
through  these  not  only  by  the  force  from  behind,  but  by 
the  aspiratory  effect  produced  by  inspiration  (see  also 
p.  82). 

Covering's  of  the  Brain. — The  dura  mater  is  a  dense 
fibrous  membrane,  which  acts  the  part  of  a  protective 
covering   for  the  brain ;  between  it  and  the  arachnoid  a 


THE  NERVOUS  SYSTEM  445 

lymphatic  space  known  as  the  subdural  exists.  The  arach- 
noid contains  but  few  vessels  and  no  nerves,  and  covers  the 
extremel}^  vascular  pia  mater ;  between  these  is  formed  the 
subarachnoid  space,  which  contains  the  subarachnoid  or 
cerebral  fluid. 

Cerebral  Fluid. — The  subarachnoid  space  communicates 
with  the  ventricles  of  the  brain,  the  lymph  in  it  is  also 
shown  to  be  in  communication  with  the  perivascular  spaces 
of  the  cerebral  vessels  and  the  Ij^mphatic  spaces  in  the 
perineural  covering  of  nerves.  Through  the  fourth  ventricle 
it  communicates  with  the  central  canal  of  the  spinal  cord, 
and  there  is  also  a  connection  between  the  cerebral  spaces 
and  those  formed  on  the  exterior  of  the  cord.  The  sub- 
dural and  to  an  extent  the  subarachnoid  fluid  communi- 
cates with  the  smuses  of  the  dura  mater.  The  cerebral 
fluid  is  secreted  by  the  pia  mater  and  choroid  plexus.  The 
use  of  this  cerebral  fluid,  which  normally  in  horses  amounts 
to  80  or  90  grains,  is  to  equalize  the  pressure  on  the  brain, 
afford  protection  to  the  latter,  and  through  the  manner  in 
which  the  organ  is  suspended  inside  the  skull  by  the  dura 
mater,  to  save  it  from  jar  and  concussion ;  both  cerebrum 
and  cerebellum  half  float  on  water-cushions.  "Withdrawal 
of  the  cerebral  fluid  leads  to  convulsions,  and  an  increase 
in  the  amount  may  cause  coma  owing  to  the  pressure  it 
exercises. 

Movements  of  the  Brain. — When  the  brain  is  exposed  it 
rises  and  falls  during  each  respiration,  rising  with  expira- 
tion and  falling  during  inspiration ;  the  cause  of  this  is  the 
respiratory  rise  and  fall  of  blood  pressure.  Alterations  in 
the  volume  of  the  brain  have  been  observed  ;  the  brain 
expands  under  a  rise  in  pressure  of  the  systemic  arteries, 
such  as  is  produced  by  stimulating  the  central  end  of  the 
sciatic.  Ether  and  particularly  strj'chnin  causes  a  con- 
siderable expansion ;  chloral  hydrate  and  especially  chloro- 
form cause  a  marked  contraction.  No  vaso-motor  fibres 
have  been  discovered  acting  in  the  brain. 


446     A  MANUAL  OF  YETEEINAKY  PHYSIOLOGY 

The  Sympathetic  System. 

An  extensive  system  of  nerves  exists  in  the  body,  the 
function  of  which  is  mainly  to  sup[)ly  the  bloodvessels, 
viscera  and  glands.  At  one  time,  owing  to  its  peculiar  dis- 
tribution, the  sympathetic  system  was  regarded  as  distinct 
from  the  cerebro-spinal ;  this  is  now  known  to  be  incorrect ; 
the  two  are  intimately  connected. 

The  sympathetic  is  composed  of  nerves  and  ganglia  ;  the 
nerve  fibres  are  remarkable  for  their  fineness  and  are  both 
medullated  and  non-meduUated ;  the  ganglia  consist  of 
multipolar  cells  and  nerve  fibres.  The  numerous  processes 
belonging  to  the  cells  serve  to  increase  the  number  of 
tracts  along  which  impulses  travel,  so  that  these  are  able 
to  pass  out  in  several  directions.  There  is  no  evidence 
that  these  ganglia  can  originate  impulses,  but  they  serve 
to  transmit  nerve  impulses.'  Until  lately  there  was  nothing 
to  show  that  they  were  capable  of  performing  a  reflex  act, 
but  this  would  now  appear  to  be  possible,  although  in  a 
peculiarly  simplified  way. 

Medullated  nerves  by  passing  through  a  sympathetic 
ganglion  lose  their  medulla,  and  Langley  has  shown  that 
nearly  all  the  nerve  fibres  entering  a  ganglion  terminate  in 
the  nerve  cells  of  that  ganglion,  though  some  pass  through 
without  communicating  with  the  cells.  Nicotin  applied  to 
a  ganglion  paralyzes  the  cells  but  not  the  nerve  fibres. 
By  this  method  of  inquiry,  which  is  due  to  Langley,  it  is 
possible  to  demonstrate  what  nerve  fibres  do  and  what  do 
not  end  in  the  various  sympathetic  ganglia.  The  number 
of  fibres  in  a  nerve  is  increased  by  passing  through  a 
ganglion,  and  further,  the  ganglion  exercises  a  nutritive 
effect  over  such  of  the  nerve  fibres  as  are  branches  from 
the  cells  of  the  ganglion. 

Gaskell  has  shown  that  the  extensive  sympathetic  system 
is  capable  of  classification  into  three  groups :  (1)  Vertebral 
gaiKjlia,  which  run  on  either  side  of  the  vertebral  column 
practically  throughout  its  length.  Below  and  in  connection 
with  these  are  the  large  nervous  plexuses  and  ganglia  of 


THE  NERVOUS  SYSTEM  447 

the  chest  and  abdomen,  such  as  the  cardiac,  solar,  and 
mesenteric  plexuses.  These  are  known  as  (2)  the  Collateral 
ganglia ;  from  these  are  given  off  fibres  which  terminate  in 
the  tissues  supplied  by  them,  and  are  known  as  (3)  Terminal 
ganglia.  On  reference  to  Fig.  85,  p.  394,  this  distribution 
is  shown  in  a  diagrammatic  form,  ^  being  the  vertebral, 
S  the  collateral,  and  h'  the  terminal  ganglia. 

It  is  through  the  vertebral  ganglia  that  the  sympathetic 
is  mainly  brought  into  connection  with  the  cerebro-spinal 
system.  White  medullated  nerve  fibres  run  out  from  the 
spinal  cord,  especially  in  the  dorsal  and  lumbar  regions, 
to  join  the  ganglia  on  the  vertebral  chain ;  this  branch  is 
known  as  the  white  ramus  eonununieans  (V,  Fig.  85).  After 
passing  through  the  vertebral  ganglia  it  loses  its  medulla, 
and  a  branch,  the  grey  ramus  conimunicans  (rv.  Fig.  85), 
leaves  the  ganglion,  returns  to  the  spinal  cord,  and  again 
issues  from  it  in  a  manner  previously  described  (p.  77),  to 
supply  the  bloodvessels  of  the  spinal  cord,  and  those  of 
the  fore  and  hind  limbs  with  constrictor  influence.  Those 
fibres  of  the  white  ramus  which  do  not  return  j)ass 
through  the  vertebral  ganglia,  become  non-medullated,  and 
join  the  collateral  ganglia.  White  rami  are  found  running 
out  from  the  spinal  cord  of  the  dog  from  the  second  dorsal 
to  the  second  lumbar  nerve ;  in  front  and  behind  these 
points  there  are  no  white  but  only  grey  rami.  In  the 
cervical  region,  though  there  is  no  white  ramus  yet  fibres 
run  out  from  the  cord  b}-  means  of  the  spinal  accessory 
nerve;  a  division  of  which  enters  the  vagus  and  supplies  the 
heart  with  inhibitory  nerves  (Fig.  15,  p.  46) ;  from  the  second 
and  third  dorsal  nerves  white  fibres  are  given  oft'  which  pass 
through  vertebral  sympathetic  ganglia,  and  finally  reach 
the  heart,  exercising  an  augmentor  effort  (Fig.  15).  In  the 
cervical  sympathetic  fibres  are  found  supplying  constrictor 
influence  to  the  bloodvessels  of  the  head  and  neck,  dilator 
fibres  for  the  iris,  fibres  causing  the  eyelids  to  open,  the 
eyeball  to  come  forward,  and  the  third  eyelid  to  be  re- 
tracted in  the  cat,  dog  and  rabbit ;  besides  these  there  are 
sweat  fibres  for  the  head  and  neck,  secretory  fibres  for  the 


448     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

salivary  glands,  and  for  the  glands  in  the  muzzle  of  the  ox. 
In  both  the  ox  and  dog  trophic  fibres  are  found  supplying 
the  muzzle,  and  in  the  horse  there  are  fibres  for  the 
sebaceous  glands  of  the  skin  of  the  ear. 

Arloing  has  shown  that  in  both  the  ox  and  dog  division 
of  the  cervical  sympathetic  has  been  followed  by  a  dry, 
papillated,  and  hypertrophied  condition  of  the  skin  of  the 
nose  and  muzzle,  due  to  damage  to  the  trophic  fibres. 

From  the  spinal  cord  between  the  sixth  and  thirteenth 
dorsal  and  the  first  and  second  lumbar  nerves  in  the  dog, 
the  greater  and  lesser  splanchnic  nerves  are  given  off, 
which  run  to  collateral  ganglia,  the  solar  plexus  ;  from  the 
first  to  the  third  lumbar  nerves  in  the  dog  fibres  are  also 
given  off  which  form  the  inferior  mesenteric  ganglion 
(see  Fig.  55,  p.  206),  From  these  plexuses  fibres  are  given 
off  supplying  the  muscles  of  the  stomach  and  intestines 
with  inhibitory  power,  so  that  stimulation  of  the  splanchnics 
causes  the  viscera  to  cease  moving  (see  p.  205).  The 
splanchnics  are  also  the  chief  vaso-motor  nerves  to  the 
vessels  of  the  abdomen ;  section  of  them  gives  rise  to  great 
dilatation  of  the  vessels  of  the  intestines,  liver,  kidneys,  etc., 
due  to  vaso-motor  paralysis,  and  so  causes  a  great  fall  in 
blood  pressure  ;  stimulation  of  the  peripheral  end  of  the 
divided  nerve  causes  the  vessels  to  contract  and  raises  the 
general  blood  pressure.  The  splanchnics  contain  sensory 
fibres  ;  it  is  through  these  that  abdominal  pain  is  felt. 
For  further  remarks  regarding  the  nerve  supply  of  the 
viscera,  see  p.  205.  The  sympathetic  system  also  furnishes 
the  pilo-motor  fibres  in  the  cat  and  dog  (see  p.  276). 

The  functions  of  the  sympathetic  may  thus  be  summar- 
ized :  This  nervous  system  supplies  the  bloodvessels  with 
constrictor  and  dilator  fibres,  the  viscera  with  motor  and 
inhibitory  fibres,  accelerator  fibres  to  the  heart,  dilator 
fibres  for  the  pupil,  secretory  fibres  for  sweat,  salivary,  and 
sebaceous  glands,  motor  fibres  to  the  muscles  of  the  hair, 
and  fibres  which  exercise  an  effect  on  the  nutrition  of  a  part. 

Psychical  Powers. — In  attempting  to  define  to  what  extent 
the  faculty  of  reasoning  exists  in  animals,  we  are  treading 


THE  NERVOUS  SYSTEM  449 

on  distinctly  controversial  ground.  Probably  this  question 
can  only  be  positively  answered  in  the  affirmative  for  two 
animals,  viz.,  the  elephant  and  the  dog.  With  the  horse 
the  moral  sense  is  very  small ;  we  do  not  think  he  knows 
he  is  doing  anything  wrong  when  he  kicks  his  stable  down 
once  or  twice  a  week,  or  when  he  '  runs  away,'  but  he  does 
understand  that  he  should  not  refuse  a  jump,  and  a  horse 
careless  in  his  walk  or  trot  knows  exactly  what  every 
stumble  will  l)e  followed  by,  and  anticipates  matters 
accordingly. 

Strength  of  will  most  animals  lose  as  the  result  of 
domestication.  They  become  mere  reflex  machines  or 
automata,  but  there  are  notable  exceptions,  for  instance 
the  ass,  mule,  and  occasionally  the  horse.  The  so-called 
stupidity  of  the  ass  and  provoking  obstinacy  of  the  mule 
are  not  indications  of  want  of  intelligence,  on  the  other 
hand  they  show  a  determination  of  purpose  and  strength 
of  will,  which  if  these  animals  understood  how  to  combine 
against  man,  would  obtain  for  them  their  complete 
freedom  from  civilization. 

The  majority  of  horses  on  the  other  hand  have  no  great 
strength  of  will ;  they  can  be  rendered  docile  and  tractable, 
they  will  gallop  until  they  drop,  work  at  high  pressure 
when  low  would  suffice,  can  never  apparently  learn  the 
obvious  lesson  that  it  is  the  '  willing  horse '  which  suffers, 
and  that  the  harder  they  work  the  more  they  get  to  do. 
All  this  is  due  to  defective  intelligence  and  a  want  of  the 
higher  faculties ;  they  cannot  reason  like  the  dog  or 
elephant,  and  are  more  flexible  than  the  ass  or  mule.* 

Some  horses  do  show  signs  of  reasoning  and  are  capable 
of  grasping  a  position.  A  load  so  heavy  as  to  be  bej'ond 
the  limit  of  his  power,  or  from  some  other  cause,  has  taught 
him  to  refuse  to  work  ;  to  use  the  familiar  expression,  he 

*  We  are  aware  that  the  majority  of  people  will  not  agree  with  these 
views  of  the  defective  intelligence  of  the  horse,  but  we  are  not  alone  in 
our  judgment;  see  'The  Points  of  the  Horse,'  by  Captain  Hayes, 
whose  experience  amongst  horses  in  all  parts  of  the  world  was  very 
considerable. 

29 


450     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

'jibs,'  he  has  learned  to  disobey,  he  has  learned  his  own 
strength,  and  the  comparative  powerlessness  of  his  master, 
and  this  through  an  exercise  of  reason.  In  otlier  words,  the 
horse  which  refuses  to  wear  himself  out  in  the  service  of 
man  is  one  possessing  too  much  intelligence  and  strength 
of  will  for  a  slave ;  a  '  jibber  '  is  an  intelligent  and  not  a 
stupid  horse. 

As  a  rule  the  intelligence  and  affection  of  the  horse  only 
exist  in  books  and  the  imagination  of  those  w'ho  have  the 
least  to  do  with  him  ;  whatever  region  of  the  brain  affection 
is  located  in,  it  does  not  occup}'  much  space  in  the  equine. 
Taking  the  dog  as  the  standard  to  judge  by,  it  may  be  said 
with  the  greatest  truth  that  the  large  majority  of  horses 
have  no  affection  whatever,  either  for  their  ow'n  kind 
(excluding  maternal  affection)  or  for  human  beings.  Two 
strange  horses  cannot  as  a  rule  be  put  together  without 
disagreeing,  and  no  one  ever  heard  of  a  horse  pining  away 
through  the  prolonged  absence  of  his  master  !  The  often 
quoted  example  of  a  horse  jumping  over  a  man  on  the 
ground  rather  than  treading  on  him  is  an  act  misunder- 
stood ;  it  is  true  the  horse  jumps  over  the  man,  but  he  does 
so  because  he  is  taught  to  jump  over  every  obstacle,  and 
the  man  on  the  ground  might  for  all  he  knows  be  a  bush. 
In  other  words  it  becomes  largely  a  reflex  action,  and  only 
to  a  very  limited  extent  a  volitional  act. 

If  the  horse  possesses  but  little  affection  it  is  compensated 
for  by  cherishing  no  resentment ;  he  will  kick  his  friend  as 
readily  as  a  foe,  or  in  many  cases  his  groom  with  as  much 
cheerfulness  as  a  perfect  stranger ;  to  all  his  hard  life  and 
the  abominable  cruelties  of  domestication  he  shows  no  sign 
of  resentment ;  water  and  feed  him,  and  give  him  a  place  to 
lie  in,  and  he  forgets  the  past  in  his  anxiety  for  the  present. 
He  is  a  peculiar  mixture  of  courage  and  cowardice ; 
physical  suffering  he  can  endure,  no  animal  bears  i^ain 
better  ;  when  his  blood  is  up  nothing  is  too  big  or  too  wide 
for  him  in  the  hunting  field,  and  he  has  a  keen  enjoyment 
for  both  chase  and  race  in  spite  of  the  punishment  they 
may  entail.     But  the  same  horse  is  frightened  out  of  his  life 


THE  NEEVOUS  SYSTEM  451 

by  a  piece  of  paper  blowing  across  the  road,  or  at  his  own 
shadow,  and  an  unusual  sight  or  a  heap  of  stones  on  the 
side  of  the  road  has  cost  many  a  man  his  life.  No  animal 
is  more  readilv  seized  with  panic,  and  this  spreads  amongst 
a  body  of  horse  like  an  electric  shock. 

The  horse  has  an  excellent  memory  for  locality,  probably 
nearly  equal  to  that  of  the  dog  or  cat ;  he  never  forgets  a 
road,  and  automaton-like,  if  he  has  once  stopped  at  any 
place  on  it,  he  wants  to  stop  at  the  same  place  next  time 
no  matter  how  long  the  interval  may  be  between  the 
visits. 

Eeasoning  power  in  the  majority  of  horses  is  small ; 
an  animal  runs  away  because  he  is  seized  with  panic,  or 
his  spirits  are  bubbling  over,  but  with  few  exceptions  dis- 
tinct acts  of  reasoning  are  rare.  Of  this  we  daily  see 
examples  in  our  infirmaries ;  horses  injured  in  the  most 
severe  manner  through  their  own  struggles  when  placed  in 
a  little  difficulty,  such  as  a  head  rope  around  the  leg,  or  an 
inability  to  rise  when  down  owing  to  being  too  close  to  the 
wall,  or  some  trifling  circumstance  of  this  kind,  where  if  he 
employed  any  reasoning  powers  he  would  remain  quiet 
until  released,  instead  of  which  he  behaves  like  a  lunatic, 
inflicting  in  a  short  time  injuries  which  may  \a,y  him  up 
for  months.  Or  take  the  case  of  a  horse  which  gets  his 
tail  over  the  reins  when  being  driven  ;  instead  of  lifting  the 
tail  in  response  to  the  exertions  of  the  driver  he  draws  it 
closer  down  to  his  quarters,  gripping  the  reins  as  in  a  vice, 
and  is  so  astonished  and  frightened  at  the  new  state  of 
things  that  he  becomes  uncontrollable.  We  can  hardly 
point  to  a  single  act  in  the  horse  in  which  the  powers  of 
reasoning  are  clearly  brought  into  play,  unless  it  be  that 
he  knows  punishment  follows  refusal  to  obey,  and  often 
learns  to  '  jib.' 

The  horse  is  very  conservative,  he  likes  nothing  new  nor 
any  departure  from  his  ordinary  mode  of  life  ;  he  will 
starve  himself  for  days  rather  than  take  a  new  feeding 
grain,  and  he  dislikes  a  change  of  stable  or  a  new  place. 
His  gregarious  instincts  are  proverbial ;  he   frets   at   the 

29—2 


452     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

absence  of  his  companions,  and  if  used  to  work  amongst  a 
body  of  horses,  as  in  cavalry,  he  will  take  any  degree  of 
punishment  rather  than  leave  them  for  five  minutes. 
During  the  absence  of  his  companions  he  neighs,  sweats, 
paws  with  the  fore  legs,  and  almost  screams  with  delight 
on  rejoining  them,  not  because  he  loves  them,  but  because 
he  dislikes  being  alone. 

Finally,  his  predominant  feature,  and  the  feature  of  all 
animals  below  adult  man,  is  the  childishness  present 
throughout  life  ;  probably  the  absence  of  care,  worry  and 
anxiety  may  account  for  this.  The  horse  will  play  all  day 
with  a  piece  of  rope,  or  nibble  his  neighbour  persistently ; 
even  the  oldest  horses  when  '  fresh  '  will  perform  the  antics 
of  a  foal,  and  imitation  amongst  them  is  so  great  that  if  one 
of  a  string  of  horses  being  led  along  happens  to  kick  out, 
this  repeats  itself  all  along  the  line  as  if  by  preconceived 
arrangement. 

Sydney  Smith  defined  the  difference  between  reason  and 
instinct  as  follows  :  '  If  in  order  to  do  a  certain  thing 
certain  means  are  adopted  to  effect  it,  with  a  clear  and 
precise  notion  that  these  means  are  subservient  to  that  end, 
the  act  is  one  of  reason ;  if,  on  the  other  hand,  means  are 
adopted  subservient  to  an  end,  without  there  being  the 
least  degree  of  consciousness  that  these  means  are  sub- 
servient to  the  end,  then  the  act  is  one  of  instinct.' 
Morgan*  believes  that  between  instinct  on  the  one  hand, 
and  reason  on  the  other,  we  may  insert  as  a  middle  term 
'  intelligence,'  while  Romanes  and  others  use  the  word 
'intelligence'  as  synonymous  with  'reason.'  Morgan 
defines  instinct  as  a  motor  response  to  a  certain  stimulus, 
i.e.,  a  reflex  act,  but  one  accompanied  by  consciousness. 
Animals  come  into  the  world  endowed  with  this  innate 
capacity  for  motor  response ;  but  these  instincts  are  not 
quite  perfect,  they  need  training  and  experience,  and  their 
instructor  is  '  intelligence.'  Intelligence,  according  to  this 
observer,  does  not  imply  a   conscious   knowledge   of   the 

*  Fortnightly  Beview,  August,  1893.  It  is  from  Professor  Morgan's 
paper  that  we  have  obtained  the  views  of  Sydney  Smith  and  Romanes. 


THE  NERVOUS  SYSTEM  453 

relation  between  the  means  employed  and  the  end  attained  ; 
such  a  conscious  knowledge  would  be  reason. 

We  are  asked,  in  other  words,  to  regard  animals  as  simply 
reflex  machines,  their  brain  being  very  little  higher  in  the 
scale  than  their  spinal  cord,  and  for  some  animals  such 
a  position  probably  meets  their  case,  but  certainly  not  for 
all.  If  we  accept  Morgan's  definition  of  instinct  and  intelli- 
gence, it  offers  no  reasonable  explanation  why  dogs  fight, 
and  why  they  \Porry  cats  ;  why  a  horse  so  inclined  will  turn 
his  quarters  towards  another  as  he  passes  and  rapidly  let 
both  hind  legs  fly  in  the  direction  of  his  objective ;  nor  will 
it  explain  why  a  horse  will  use  his  fore  legs  to  strike  when 
he  knows  his  hind  legs  cannot  reach  the  object  of  his  irrita- 
tion. It  is  absolutely  impossible  to  believe  that  such  acts 
imply  no  conscious  knowledge  of  the  relation  between  the 
means  employed  and  the  end  attained. 

The  higher  animals  are  capable  of  a  limited  amount  of 
reasoning ;  with  some  it  is  even  well  developed,  with  others 
it  is  extremely  imperfect.  The  elephant  and  dog  occupy 
the  top  of  the  scale,  the  ox  and  sheep  the  bottom,  the  horse 
comes  midway.  We  do  not  see  how  to  separate  reason  from 
intelligence,  but  there  is  no  difiiculty  in  separating  them 
from  instinct. 

Animals  are  born  with  such  complicated  reflex  acts  as 
walking,  galloping,  jumping,  etc.,  so  highly  developed  that 
they  are  employed  at  once.  No  member  of  the  human 
family  has  been  seen  to  walk  and  run  about  a  few  hours 
after  leaving  the  womb,  for  both  brain  and  spinal  cord  are 
incompletely  developed,  and  the  acts  have  to  be  learned. 
This  is  not  so  with  animals  (excepting  the  dog  and  cat) : 
foals,  calves,  lambs,  goats,  etc.,  are  born  prepared  to  feel 
their  feet  at  once,  they  require  no  teaching  and  no 
imitation,  their  senses  are  perfect,  they  can  recognize  their 
mother  or  a  stranger,  can  see,  smell,  hear ;  in  fact  they 
have  nothing  to  learn,  for  they  are  born  with  as  much 
intelligence  as  their  parents,  and  only  difler  from  them  in 
one  respect,  and  that  is  they  are  born  wild,  and  so  have  to 
learn  confidence.  Domestication  and  obedience  are  not 
properties  transmitted  from  parent  to  offspring. 


CHAPTER    XY 

THE  SENSES 

Section  I. 

Sight. 

The  delicate  structures  composing  the  eye  receive  a  very 
thorough  protection  by  the  anatomical  arrangement  of  the 
parts.  The  orbital  cavity,  for  example,  is  nearly  sur- 
rounded by  incomplete  bony  walls,  and  the  layers  of  fat 
within  it  assist  the  muscles  in  protecting  the  globe  and  the 
optic  nerve.  The  eyelids  sweep  the  cornea  and  protect  the 
part  from  dust  and  exposure,  the  tears  keep  the  face  of  the 
cornea  brilliant ;  the  memhrana  nictitaiis  moves  particles 
of  solid  matter  which  would  otherwise  produce  irritation, 
and  the  eyeball  can  be  retracted  to  a  considerable  extent  to 
assist  it  in  withdrawing  from  injury.  The  size  of  the  orbit 
is  such  that  ordinary  blows  inflicted  upon  the  eye  are 
expended  on  the  margin  of  the  orbital  cavity,  and  not  on 
the  eyeball  itself,  so  that  the  risk  of  serious  injury  is  far 
less  from  large  than  from  small  bodies.  The  shape  of  the 
eyeball  is  not  (in  the  horse)  quite  spherical,  the  vertical 
axis  is  greater  than  the  horizontal,  and  the  posterior  face 
of  the  eyeball  is  distinctly  flatter  than  the  anterior. 

Structure  of  the  Eye. — Issuing  from  the  back  of  the  eye- 
ball, very  low  down  and  inclined  to  the  temporal  side  of 
the  globe,  is  the  ojjtic  nerve,  which  after  describing  a 
peculiar  curve  upwards,  runs  in  the  substance  of  the 
retractor  muscle  to  enter  the  cranium  through  the  optic 
foramen.  This  curve  in  the  optic  nerve  (Fig.  100)  is  neces- 
sitated by  the  horizontal  movements  of  the  eyeball ;  if  the 
eye  looks  backwards  the  curve  is  increased,  whereas  if  it 

454 


THE   SENSES  455 

looks  forwards  the  "  slack  '  is  taken  out  of  the  nerve  and  the 
curve  entirely  disappears.  The  optic  or  second  cranial 
nerve  has  a  deep-seated  origin  in  the  corpora  quadrigemina, 
and  a  representation  in  the  occipital  region  of  the  cerebral 
hemispheres  (see  Fig.  99).  The  fibres  forming  the  optic 
nerve  decussate,  those  from  the  left  brain  passing  to  the 
right  eye  and  vice  versa.  In  some  animals  this  decussation 
is  complete,  such  as  the  horse,  sheep,  and  pig  ;  but  in 
others  a  certain  number  of  fibres  decussate,  whilst  ,some 
enter  the  nerve  on  the  same  side  of  the  brain  as  that  on 
which  they  originate  ;  this  is  the  arrangement  in  the  dog, 
cat,  rabbit,  monkey,  and  man.  This  partial  decussation  is 
considered  in  man  to  play  an  important  part  in  the  pro- 
duction of  sympathetic  ophthalmia,  the  inflammatory 
trouble  running  along  the  optic  nerve  to  the  commissure, 
and  so  finding  its  way  to  the  other  eye.  It  is  quite  certain 
that  in  the  horse,  where  the  decussation  is  complete, 
sympathetic  ophthalmia  from  an  injury  is  unknown. 
Division  of  the  optic  nerve  causes  no  pain  but  only  the 
sensation  of  light ;  stimulation  of  the  nerve  causes  flashes 
of  light  to  be  perceived  by  the  brain — in  fact  the  optic  nerve 
conveys  nothing  but  impulses  which,  when  they  reach  the 
brain,  give  rise  to  that  altered  state  of  consciousness  known 
as  vision.  Moreover,  as  we  shall  presently  point  out,  the 
place  where  the  optic  nerve  enters  the  eye  is  blind. 

The  globe  of  the  eye  is  anteriorly  made  up  of  a  trans- 
parent convex  surface  known  as  the  cornea,  whilst  the 
remainder  of  its  walls  are  opaque  and  formed  by  the 
sclerotic,  choroid,  and  retina.  The  sclerotic  is  the  tunic  on 
which  the  strength  of  the  eyeball  depends,  the  choroid  may 
be  regarded  as  that  which  principally  attends  to  the  vascular 
supply,  while  the  retina  is  the  sensitive  expansion  of  the 
optic  nerve  on  which  the  picture  is  imprinted,  and  thus 
gives  rise  to  sensory  impressions. 

The  shape  and  tension  of  the  eyeball  is  maintained  by 
means  of  its  humours,  which  are  known  as  the  aqueous 
and  vitreous.  The  aqueous  humour  occupies  the  space 
between  the  cornea  and  the  lens.    It  is  a  watery  fluid,  poor 


45G     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

in  solids,  and  is  in  reality  lymph.  It  is  constantly  being 
secreted,  probably  by  the  ciliary  processes,  and  as  con- 
stantly carried  away  by  the  lymphatic  channels  with  which 
it  communicates  through  the  spongy  rujamentum  jtecti- 
natum ;  these  channels  empty  themselves  into  the  anterior 
system  of  veins.  If  the  anterior  chamber  be  experi- 
mentally evacuated  it  is  refilled  in  about  twenty-four 
hours.  The  use  of  the  fluid  it  contains  is  to  maintain 
the  convexity  of   the   cornea ;  after  death  the  process  of 


Pig.  100. — Vertical  Section  of  the  Eye  of  the  Horse,  Natural 

Size. 

c,  Cornea ;  I,  lens ;  i,  iris  ;  cp,  ciliary  process ;  Jp,  ligamentum  pec- 
tinatuni  ;  cl  in,  position  of  ciliary  muscle  ;  si,  suspensory  ligament 
of  lens;  on,  optic  nerve  showing  its  curve. 

drainage  still  appears  to  occur,  though,  of  course,  there  is 
no  reproduction,  the  result  being  that  in  a  day  or  two  the 
cornea  flattens  through  loss  of  the  ai^ueous  humour. 

The  vitreous  humour  is  a  viscid,  tenacious  material,  con- 
tained within  the  hyaloid  membrane,  which  permeates  its 
substance.  The  vitreous  contains  mucin  and  a  very  small 
percentage  of  solids.  The  use  of  this  fluid  is  to  maintain 
the  intra-ocular  pressure,  by  which  the  proper  tension  of 
the  globe  is  brought  about.  The  whole  of  the  vitreous 
chamber   is  rendered  dark    by  the  liberal    application  of 


THE   SENSES  457 

pigment,  with  the  exception  of  a  surface  above  the  optic 
nerve  which  is  brilliant  and  iridescent  in  appearance,  and 
is  known  as  the  tapetum  luciduni. 

Between  the  two  humours  a  diaphragm  is  situated  known 
as  the  iris,  which  regulates  the  amount  of  light  passing 
into  the  eye,  and  behind  this  is  a  focussing  arrangement  or 
lens.  The  cornea,  lens,  and  humours  constitute  the  refract- 
ing apparatus  of  the  eye. 

By  means  of  the  muscles  of  the  eye  the  globe  is  given  a 
considerable  range  of  movement,  and  in  addition  it  can  be 
retracted  within  the  orbital  cavity ;  further,  these  muscles 
afford  some  protection  to  the  optic  nerve. 

The  similarity  in  construction  between  the  eye  and  the 
apparatus  known  as  a  camera  is  very  marked ;  both  have 
a  refracting  surface  anteriorly  placed,  a  diaphragm  to  cut 
off  superfluous  rays  of  light,  an  arrangement  for  focussing, 
a  dark  chamber  in  which  is  placed  a  sensitized  surface, 
and  on  which  a  reduced  and  inverted  image  of  the  picture 
is  impressed. 

Though  we  have  thus  briefly  run  over  the  leading 
features  of  the  eye,  yet  there  are  certain  of  these  structures 
which  need  some  detailed  description  if  we  are  to  under- 
stand clearlj'  the  phenomena  attending  perfect  vision. 

The  Cornea  in  most  animals  is  circular  in  outline,  in 
the  horse  it  is  somewhat  oval ;  when  viewed  from  the 
front  and  divided  into  two  halves  by  a  vertical  line,  it  is 
distinctly  larger  on  its  nasal  than  on  its  temporal  side.  It 
is  a  very  tough,  non-vascular  membrane,  richly  supplied 
with  nerves,  and  nourished  by  the  lymph  which  freely 
circulates  in  it.  It  may  be  regarded  as  the  chief  refractive 
apparatus  of  the  eye.  When  viewed  from  the  side  the 
cornea  is  seen  to  be  convex ;  measurement  shows  that  in 
the  majority  of  horses  the  curvature  of  the  cornea  taken  in 
its  horizontal  and  vertical  meridians  is  not  exactly  the 
same  as  it  would  be  supposing  its  surfaces  were  accuratelj^ 
spherical.  The  excess  of  curvature  of  one  meridian  of  the 
cornea  over  that  of  the  surface  at  right  angles  to  it  pro- 
duces a  defect  in  vision  which  is  known  as  astigmatism ; 


458    A  MANUAL  OF  VETEEINAKY  PHYSIOLOGY 

the  meridian  in  the  horse  which  is  nearly  always  the 
flattest  is  the  horizontal. 

The  Lens  is  composed  of  various  onion-like  layers  of 
different  refractive  powers.  In  shape  it  is  bi-convex,  the 
convexity  of  its  posterior  face  being  greater  than  that  of 
the  anterior.  It  is  held  in  its  place  by  a  capsule  which 
really  suspends  the  lens  in  the  eye,  the  capsule  receiving 
attachment  to  some  long  processes  behind  the  iris  known 
as  the  ciliary  jjrocesses.  In  the  horse  the  lens  is  in  contact 
with  the  ciliary  processes,  in  most  other  animals  there  is  a 
small  space  between  the  two.  The  lens  possesses  inherent 
elasticity,  which  admits  of  its  surface  undergoing  an 
alteration  in  shape,  so  as  to  be  flatter  at  one  time,  more 
convex  at  another.  This  alteration  in  shape  occurs  through 
the  ready  manner  in  which  the  lens  by  its  elasticity  yields 
to  the  pressure  exercised  on  it  through  its  capsule,  so 
that  if  the  tension  of  the  capsule  be  relaxed  the  lens 
bulges,  or  if  the  tension  be  increased  it  flattens.  In 
this  way  the  eye  is  focussed  or  accommodated  to  various 
distances,  a  subject  which  will  be  dealt  with  presently. 

The  Iris  is  a  curtain  with  a  hole  in  the  centre  called  the 
pupil.  The  shape  of  the  pupil  varies  in  different  animals; 
in  the  dog  it  is  circular,  in  the  horse,  sheep,  ox  and  cat 
elliptical ;  in  the  latter  animal  the  elliptical  slit  is  placed 
vertically,  in  the  others  horizontally.  The  iris  is  mainly  a 
collection  of  bloodvessels  and  muscular  fibres,  the  whole 
being  heavily  coated  with  a  brown  pigment  in  the  horse, 
though  occasionally  this  is  wanting,  giving  it  a  bluish- 
white  streaky  appearance,  as  in  the  so-called  '  wall-eyed ' 
horse.  In  the  ox  and  dog  the  iris  is  a  brighter  brown  than 
in  the  horse,  while  in  the  sheep  it  is  brownish-yellow.  The 
muscular  fibres  of  the  iris  are  commonly  described  as 
circular  and  radiating  ;  a  contraction  of  the  circular  muscle 
contracts  the  pupillary  opening,  a  contraction  of  the  radia- 
ting fibres  dilates  it.  Langley  and  Anderson  from  their 
observations  on  the  cat,  dog,  and  rabbit,  have  proved  that 
a  dilator  muscle  to  the  iris  exists  ;  this  question  was  for  a 
long  time  in  dispute.     It  is  now  accepted  that  dilatation  of 


THE   SENSES  459 

the  pupil  is  due  to  the  influence  of  a  dilator  muscle  and 
inhibition  of  the  circular  muscle. 

The  nerve  supply  to  these  circular  and  radiating  fibres  is 
not  the  same ;  the  circular  fibres  are  supplied  with  motor 
power  through  the  third  cranial  nerve,  whilst  the  dilator 
muscle  is  supplied  by  the  sympathetic.  The  latter  fibres 
emerge  from  the  spinal  cord  at  the  first  three  thoracic 
spinal  nerves,  from  a  part  known  as  the  cilio-spinal  centre ; 
from  here  they  travel  up  the  neck  in  the  cervical  sympa- 
thetic, and  reach  the  iris  through  the  ciliary  ganglion.  If 
the  third  nerve  be  divided  the  radiating  muscular  fibres 
of  the  iris  contract  under  the  unbalanced  action  of  the  sym- 
pathetic, and  thus  dilate  the  pupil ;  if  the  sympathetic  be 
divided  the  pupil  contracts  under  the  unbalanced  action  of 
the  sphincter  fibres. 

Stimulation  of  the  retina  by  light  is  the  natural  method 
by  which  alterations  in  the  size  of  the  pupil  are  brought 
about,  the  act  being  reflex ;  in  a  brilliant  light  the  pupil 
contracts,  in  a  low  light  it  dilates.  In  the  horse  this  is 
not  strictly  true ;  in  direct  sunlight  the  pupil  of  this  animal 
is  a  mere  narrow  chink,  but  in  ordinary  daylight  it  barely 
responds,  or  if  it  does  contract  it  is  so  little  as  not  to 
materially  reduce  the  size  of  the  pupil.  Even  when  the 
light  is  concentrated  on  the  eye,  either  by  means  of  a 
mirror  or  a  lens,  the  iris  practically  remains  unchanged. 
Owing  to  this  fact  the  eye  of  the  horse  can  be  examined 
by  the  ophthalmoscope  without  the  use  of  atropin,  or  even 
without  artificial  light,  in  fact,  under  artificial  light  the 
pupil  dilates.  There  are  certain  drugs  which  dilate  the 
pupil  such  as  atropin  and  cocain,  and  others  which  con- 
tract it,  for  example,  morphia  and  eserin.  It  is  curious 
to  observe  in  the  horse  that  although  the  pupil,  when 
normally  contracted,  is  elliptical,  yet  when  it  is  dilated  by 
atropin  it  becomes  circular ;  the  chief  radiating  fibres 
would  therefore  appear  to  be  above  and  below  and  but  very 
few  on  the  sides.  Eversbusch*  has  studied  the  structure 
of  the  iris  of  the  horse,  and  states  that  the  elongated  form 

*■  Zeitschrift  fur  Vergleichende  Augenlieilhunde,  Heft  1,  1882. 


400     A  MANUAL  OF  YETERINAEY  PHYSIOLOGY 

of  the  pupil  is  due  to  the  presence  of  an  accessory 
apparatus  on  the  posterior  surface  of  the  iris,  which  he 
calls  the  llcianientum  inhibitorium ;  through  this  ligament 
the  sides  of  the  iris  are  not  pulled  in  by  the  contraction 
of  the  sphincter  muscle.  The  long  axis  of  the  pupil  in 
the  horse  is  always  horizontal,  or  practically  so,  no  matter 
what  the  position  of  the  head  may  be  ;  this  is  a  point 
which  will  be  touched  on  again  in  dealing  with  the  muscles 
of  the  eyeball.  The  pupil  of  the  horse  dilates  moderately 
after  the  animal  has  been  galloped  ;  immediately  after  a 
violent  death  it  dilates  widely,  but  in  the  course  of  twenty- 
four  hours  or  so,  it  gradually  contracts  until  the  pupil 
becomes  a  mere  slit. 

In  the  horse  there  exists  on  the  edge  of  the  iris,  at  the 
centre  and  upper  part  of  the  pupil,  one  or  more  large  soot- 
like bodies  known  as  corpora  nigra ;  a  small  one  may  be 
found  on  the  lower  margin  of  the  iris,  but  the  upper  ones 
are  the  most  prominent.  \Yhen  the  pupil  is  strongly  con- 
tracted in  direct  sunlight,  the  centre  of  it  is  entirely  blocked 
out  by  these  pigmentary  masses,  and  divided  into  an  inner 
and  outer  portion.  It  would  appear  as  if  this  caused  an 
imperfect  image  to  be  imprinted  on  the  retina,  and  this 
view  we  at  one  time  held,  but  on  subjecting  the  question  to 
actual  experiment  no  broken  image  was  found  to  result 
from  the  use  of  a  diaphragm  the  centre  of  which  was 
blocked  out.  The  use  of  these  bodies  is  doubtless  to  assist 
in  absorbing  rays  of  light,  but  their  position  in  the  centre 
of  the  pupil  would  not  appear  theoretically  to  be  the  most 
suitable  position,  and  they  must  have  some  other  function. 
The  horse,  as  far  as  we  know,  is  the  only  animal  possessing 
them. 

Li[iamentum  Peetinatum. — Around  the  attached  margin  of 
the  iris,  viz.,  at  the  corneo-scleral  border,  a  peculiar  spongy 
tissue  exists  which  gives  the  iris  at  this  part  a  distinctly 
elevated  rim ;  this  is  known  as  the  ligamentum  peetinatum. 
Eoughly  speaking  it  is  a  rim  of  spongy  iris  traversed  by 
canals,  crevices,  and  spaces,  which  lead  into  the  lymphatic 
system  of  the  eye ;  the  function  of  this  tissue  is  to  carry 


THE   SENSES  461 

off  the  aqueous  humour  as  rapidly  as  it  is  worn  out  and 
replaced,  by  which  means  the  normal  tension  of  the  anterior 
chamber  is  maintained. 

The  Choroid  coat  contains  the  vessels  which  nourish 
the  retina ;  it  possesses  innumerable  nerves,  numerous 
lymphatics,  and  further  it  is  an  elastic  coat.  Anteriorly 
behind  the  iris  it  forms  the  peculiar  folded  structure  known 
as  the  ciliary  processes,  and  in  front  of  this  it  furnishes  the 
tissue  which  is  called  the  iris ;  the  iris  and  ciliary  processes 
are  therefore  part  of  the  choroid  coat.  With  the  exception 
of  one  area  the  whole  of  the  interior  of  the  choroid  is 
covered  with  pigment,  and  the  same  extends  on  to  the 
processes  and  iris.  The  area  which  is  an  exception  lies  on 
the  posterior  wall  of  the  eyeball  above  the  optic  nerve  ;  it 
is  of  a  brilliant  colour,  being  a  mixture  of  green,  yellow,  and 
blue,  and  is  known  as  the  tapetum  lucidum.  This  is  found 
in  both  herbivora  and  carnivora ;  in  the  former  it  is  due  to 
the  interference  of  light  causing  iridescence,  produced  by  the 
arrangement  of  the  connective  tissue  fibres  of  the  choroid, 
and  not  to  the  presence  of  any  pigment ;  in  carnivora  it  is 
due  to  minute  crystals  in  the  cells  of  the  part,  the  crystals 
causing  the  interference.  The  use  of  the  tapetum  is 
generally  supposed  to  be  to  enable  animals  to  see  in  the 
dark ;  this  of  course  is  impossible,  but  it  is  probable  that  the 
presence  of  a  tapetum  may  enable  an  animal  to  see  better 
in  a  dim  light. 

The  Ciliary  Zone  is  a  peculiar  and  important  part  of  the 
eye,  formed  on  the  one  hand  by  the  junction  of  the  cornea 
and  sclerotic,  and  on  the  other  by  the  iris  and  ciliary 
processes.  Between  these  lies  a  muscle  known  as  the 
ciliary,  which  is  firmly  attached  to  the  corneo-scleral 
margin,  and  runs  backwards  into  the  choroid,  where  it  is 
attached.  In  man  the  ciliary  niuscle  consists  of  both 
circular  and  longitudinal  (or  meridional)  fibres ;  in  the 
horse,  and  probably  all  the  lower  animals,  only  meridional 
fibres  exist.  The  muscle  is  composed  of  unstriped  fibres,  and 
its  use  is  to  pull  the  choroid  forward ;  the  object  of  this  will 
be  apparent  when  we  discuss  the  question  of  accommodation. 


462     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

The  Vitreous  humour  is  enclosed  in  the  hyaloid  membrane ; 
anteriorly  this  membrane,  here  known  as  the  Zomde  of 
Zinii,  becomes  dovetailed  into  the  ridges  formed  by  the 
ciliary  processes,  and  enveloping  the  lens  forms  its  sus- 
pensory ligament.  If  the  amount  of  vitreous  humour 
present  is  sufficient  in  quantity,  this  ligament  of  the  lens 
must  always  be  tense,  and  as  it  is  very  inelastic  it  tends  to 


Rod9. 


Cones. 


Fig.   101. — Diagram   of   Structure   of   Eetina   (Bowditch,    after 

Cajal). 

A,  Layer  of  rods  and  cones  ;  B,  external  nuclear  layer  ;  C,  external 
molecular  layer  ;  E,  internal  nuclear  layer  ;  F,  internal  molecular 
layer  ;  G,  layer  of  ganglion  cells  ;  H,  layer  of  nerve  fibres. 


keep  the  lens  flattened ;   we  shall   refer  to  this  again  in 
speaking  of  accommodation. 

The  lietina  lies  within  the  choroid  and  outside  the 
vitreous  humour ;  it  spreads  out  from  the  entrance  of  the 
optic  nerve  of  which  it  is  the  expansion.  Microscopic 
examination  shows  this  membrane  to  be  composed  of  seven 
layers    (Fig.  101),    of   which   the   most   important   is  one 


THE  SENSES  463 

termed  from  its  appearance  the  layer  of  rods  and  cones. 
It  has  been  shown  conclusively  that  these  rods  and  cones 
are  the  essential  elements  of  the  retina,  and  that  wherever 
they  are  absent  the  part  is  insensitive  to  light,  as,  for 
example,  at  the  entrance  of  the  optic  nerve  which  forms 
the  blind  spot.  Though  the  layer  of  rods  and  cones  is 
the  most  important  it  is  not  placed  as  one  would  suppose, 
next  the  vitreous  humour,  but  next  to  the  choroid,  whilst 
the  layer  next  to  the  vitreous  humour  is  composed  of 
nerve  fibres  and  ganglion  cells.  Eays  of  light  have, 
therefore,  in  the  first  place  to  pierce  the  entire  thickness 
of  the  retina  to  arrive  at  the  rods  and  cones  ;  here  they 
give  rise  to  a  nervous  impulse  which  retraces  its  steps  in 
the  retina,  until  it  arrives  at  the  layers  next  the  vitreous 
humour,  from  which  it  is  carried  off  by  the  optic  nerve  to 
the  brain.  In  one  sense  the  most  important  layer  of  the 
retina  is  the  one  composed  of  the  rods  and  cones,  since  it 
effects  the  primary  conversion  of  light-vibrations  into 
visual  impulses.  Each  cone  is  connected  with  a  single 
nerve  cell,  but  there  may  be  several  rods  to  one  nerve  cell ; 
the  cone  is,  therefore,  considered  to  offer  a  more  direct 
conducting  path  than  the  rods. 

Visual  pinple  or  Rhodopsin  is  a  curious  red  pigment 
existing  in  the  eye ;  it  is  found  in  the  rods  but  not  the 
cones  of  the  retina.  This  colouring  matter  is  readily 
decomposed  by  light,  and  is  consequently  always  being 
produced.  It  is  possible  by  keeping  an  animal  in  the  dark 
in  order  to  increase  the  visual  purple,  to  procure  then  a 
picture  on  the  retina  through  its  decomposition  on  exposure 
to  light.  It  is  believed  that  the  vision  of  night-seeing 
animals  is  mainly  brought  about  by  the  rods  in  virtue  of 
their  visual  purple,  while  the  cones  are  adapted  for  day- 
light. Visual  purple  on  the  rods  increases  their  irrita])ility 
in  dim  lights.  At  the  same  time  it  is  quite  certain  that 
visual  purple  is  not  essential  to  vision,  for  there  is  none  in 
the  fovea  of  the  human  eye,  the  area  of  the  most  acute 
vision,  and  none  in  certain  birds,  reptiles,  and  bats.  In 
people  totally  colour-blind  vision  must  be  carried  on  by 


4(34     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  rods,  as  it  is  supposed  that  the  cones  are  the  seat  of 
colour  perception. 

The  entrance  of  the  optic  nerve  within  the  eyeball  is 
spoken  of  as  the  optic  disc  or  papilla  ,■  it  is  a  concave  oval 
surface  surrounded  by  a  white  ring  formed  of  sclerotic.  It 
lies,  in  the  horse,  towards  the  bottom  of  the  eyeball  and 
inclined  to  the  temporal  side.  This  region  is  blind  owing 
to  the  absence  of  rods  and  cones. 

There  is  no  yellow  spot  in  animals;  in  man  this  exists, 
and  the  area  which  it  encloses,  the  fovea,  is  that  of  the 
most  acute  vision.  In  the  fovea  all  the  other  retinal 
layers  but  that  of  the  cones  have  disappeared ;  there 
are  no  rods  in  the  yellow  spot  or  fovea  of  man.  Rep- 
tiles possess  only  cones  in  their  retina,  and  Ijoth  birds 
and  fishes  have  more  cones  than  rods.  A  line  drawn 
through  the  centre  of  the  cornea  to  the  yellow  spot  is 
called  the  visual  axis  of  the  eye.  The  visual  axis  in  man 
does  not  quite  agree  with  the  optic  axis,  viz.,  a  line  drawn 
exactly  through  the  centre  of  curvature  of  each  refractive 
medium.  In  the  lower  animals  we  have  no  means  of 
knowing  whether  the  optic  axis  is  also  the  visual  axis, 
but  from  the  absence  of  the  yellow  spot  it  is  assumed  to  be. 

There  is,  however,  an  area  of  acute  vision  in  the  horse, 
and  the  animal  brings  it  into  play  by  raising  the  head  very 
high,  and  protruding  the  muzzle  so  as  to  render  the  face 
horizontal. 

The  Ophthalmoscope. — We  may  here  describe  in  outline 
the  theory  of  this  instrument,  and  the  appearance  of  the 
picture  presented  by  it.  To  examine  the  eye,  a  mirror 
with  a  hole  in  the  centre  is  applied  to  the  eye  of  the 
observer  so  that  he  can  see  through  the  hole  into  the 
observed  eye  ;  from  a  suitable  source  of  light,  rays  are 
reflected  by  the  mirror  through  the  pupil  on  to  the  retina 
to  be  examined.  When  light  is  thrown  into  the  eye,  the 
rays  are  reflected  back  through  the  pupil  in  the  direction 
in  which  they  entered,  and  pass  through  the  hole  in  the 
mirror  into  the  eye  of  the  observer.  On  looking  at  the 
retina    of    the    horse,    a    brilliantly   coloured    surface    is 


THE   SENSES  465 

illuminated,  the  tints  being  a  mixture  of  yellow,  green,  and 
blue  studded  with  minute  dots  ;  this  coloured  area  is  the 
tapetum  (Plate  I.)-  Examination  shows  this  surface  to  be 
situated  above  the  optic  disc  or  pa})illa ;  the  optic  papilla 
appears  of  a  pinkish  colour,  with  a  slightly  raised  whitish 
margin.  It  is  very  difficult  to  study  the  eye  of  the  horse, 
owing  to  its  frequent  movement,  so  that  only  occasional 
glimpses  of  the  papilla  can  be  obtained.  From  the  optic 
papilla  a  dense  network  of  vessels  may  be  seen  radiating 
but  extending  no  great  distance  from  it ;  this  is  character- 
istic of   the  retina  of   the  horse.     The  remainder  of  the 


Fig.  102. — Direct  Method  of  using  the  Ophthalmoscope  (Stewart). 

Light  falling  on  the  perforated  concave  mirror  M  passes  into  the 
observed  eye  E';  and,  both  E'  and  the  observmg  eye  E  being 
supposed  emmetropic  and  unaccommodated,  an  erect  vii'tual  image 
of  the  illuminated  retina  of  E'  is  seen  by  E. 

fundus  is  purple  or  brown,  but  owing  to  its  extent  very 
little  of  it  can  be  seen.  In  other  animals  the  vessels 
radiating  from  the  disc  are  wider  apart  and  more  regular, 
and  several  of  them  have  received  names  ;  moreover,  the 
arteries  can  be  distinguished  from  the  veins,  which  is  not 
possible  in  the  horse.  It  is  to  be  borne  in  mind  that  the 
view  thus  obtained  of  the  fundus  of  the  eye  is  a  magnified 
image,  both  the  lens  and  vitreous  humour  making  it  appear 
about  three  times  larger  than  normal.  Owing  to  the 
presence  of  the  tapetum  in  the  horse,  a  perfect  examination 
of  the  lens  and  fundus  may  be  made  without  the  aid  of 
artificial  light ;  while  under  the  influence  of  artificial  light 

30 


466     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


the  pupil  dilates  so  much  that  there  is  no  need  for  the  use 
of  atropin. 

Accommodation. — All  rays  of  light  proceeding  from  a  dis- 
tant object  may  be  regarded  as  parallel,  and  all  those  pro- 
ceeding from  an  object  within  '20  feet  of  the  eye  may  be 
regarded  as  divergent.  A  distant  object  is  one  situated  any- 
where between  20  feet  from  the  eye  and  infinity  ;  an  object 
closer  than  20  feet  to  the  eye  is  called  near,  and  this  point 
increases  up  to  4  or  5  inches,  at  which  distance  no  object 
can  any  longer  be  distinctly  seen.     The  nearest  distance 


FAR 


NEAR 


Fig.  103. 


-Diagram  to  illustrate  Accommodation   (Fo.ster  after 
Helmholtz). 

C.P.,  Ciliary  process  ;  I,  iris  ;  Sp.  1.,  suspensory  ligament ;  1. cm.,  longi- 
tudinal ciliary  muscle  ;  c.c.m.,  circular  ciliary  muscle  ;  c.S.,  canal 
of  Schlemm. 

The  left  half  represents  the  shape  of  the  lens  for  viewing  distant  objects, 
and  the  right  half  that  for  viewing  near  objects. 

at  which  objects  can  be  distinctly  seen  is  called  the  near 
point.  Parallel  rays  need  no  focussing  on  the  retina  other 
than  that  provided  by  the  cornea ;  but  rays  from  near 
objects  do  require  focussing  owing  to  their  divergent  nature, 
and  it  is  evident  that  the  nearer  the  object  to  the  eye  the 
greater  the  focussing  required.  This  focussing  is  brought 
about  by  a  phange  in  the  shape  of  the  anterior  surface 
of  the  lens ;  it  becomes  more  convex  for  near  objects,  and 
this  increase  in  convexity  is  due  to  the  ciliary  muscle 
drawing  forward  the  choroid  coat,  and  with  it  the  ciliary 
processes.     By  this  means  the  tension  normally  exercised 


THE  SENSES  467 

through  the  Zonule  of  Zinn  (the  suspensory  ligament  of 
the  lens)  is  relaxed,  and  the  lens  of  its  own  inherent 
elasticity  bulges  forward  and  so  increases  the  curvature  of 
its  anterior  face  (Fig.  103).  A  more  convex  lens  is  a  more 
convergent  one,  and  its  focus  is  therefore  shorter ;  in  this 
way  the  images  of  near  objects  are  brought  to  a  focus  on 
the  retina  and  distinctly  seen,  whereas  if  this  increase  in 
curvature  had  not  taken  place,  the  image  would  have  been 
focussed  behind  the  retina.  The  power  the  eye  possesses 
of  focussing  itself  is  known  as  the  mechanism  of  accommo- 
dation, and  the  explanation  given  above  is  that  of 
Helmholtz ;  it  is  the  one  generally  accepted. 

When  a  candle  is  held  opposite  to  the  eye  three  images 


ABC 

Fig.  104. — Diagr.a.m  of  the  Katoptric  Test. 

A,  From  the  anterior  sui'face  of  the  cornea  ;  B,  from  the  anterior  face 
of  the  lens  ;  and  C,  from  the  posterior  face  of  the  lens. 

of  the  flame  are  seen ;  one  a  very  sharp  bright  one, 
obviously  reflected  from  the  cornea  ;  a  second  much 
duller,  but  also  large,  reflected  from  the  anterior  surface 
of  the  lens ;  and  a  third  very  small,  brighter  than  the 
middle  one,  and  inverted,  reflected  from  the  posterior  part 
of  the  lens  (Fig.  104).  In  a  normal  eye  these  are  seen 
perfectly  and  move  in  a  definite  direction  when  the  candle 
is  moved,  the  inverted  image  passing  in  an  opposite  direc- 
tion to  the  two  erect  images,  and  all  are  equally  visible  at 
any  point  on  the  reflecting  surfaces.  This  phenomenon 
has  been  taken  advantage  of  in  determining  the  clearness 
of  the  media  of  the  eye,  and  though  superseded  by  the 
greater  accuracy  of  the  ophthalmoscope,  it  is  still  a 
valuable  aid ;  in  cataract  one  or  more  of  the  reflections 
becomes  blurred,  and  sometimes  the  image  is  duplicated. 

30—2 


468     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

The  first  and  second  images  are  erect  inasmuch  as  they  are 
reflected  from  a  convex  surface,  but  the  third  image  is 
inverted,  being  reflected  from  the  posterior  surface  of  the 
lens  which  viewed  from  the  front  is  concave.  During  the 
act  of  accommodation  the  relative  position  of  these  images 
alters  ;  the  second  becomes  smaller  or  larger,  and  advances 
nearer  to  or  recedes  from  the  first,  as  the  anterior  face  of  the 
lens  becomes  more  convex  or  flatter  as  the  case  may  be.  This 
observation  affords  the  proof  that  accommodation  is  due  to 
the  varying  convexity  of  the  anterior  surface  of  the  lens. 

Fishes  are  normally  short-sighted,  and  accommodation 
for  a  distant  object  is  effected  with  them  by  moving  the  lens 
towards  the  retina. 

The  ciliary  muscle  is  governed  by  the  ciliary  nerves.  In 
the  human  subject  the  constrictor  fibres  of  the  iris  and  the 
ciliary  muscle  are  paralysed  by  atropin,  but  in  the  cat  (as 
first  pointed  out  by  Lang  and  Barrett*),  the  dog,  and  cer- 
tainly in  the  horse,  there  is  no  evidence  that  any  paralysis 
of  the  ciliary  muscle  takes  place  under  atropin,  though  the 
pupil  dilates.  Under  the  full  effect  of  atropin  all  these 
animals  can  see  objects  quite  close  to  the  eye,  and  this  they 
could  not  do  if  the  ciliary  muscle  were  paralysed. 

Eyes  which  possess  the  power  of  seeing  objects  distinctly 
a  few  inches  from  the  eye  to  infinity  are  known  as 
Emmetropic  (Fig.  105 — 1)  ;  but  all  eyes  do  not  possess  this 
range  of  vision  owing  to  their  shape,  or  more  correctly,  to 
the  length  of  the  eyeball. 

Myopia  or  short  sight  is  due  to  the  eyeball  being  too  long, 
whereby  the  picture  is  formed  in  front  of  the  retina,  and 
only  a  confused  and  blurred  image  falls  on  the  retina 
(Fig.  105—3). 

Our  observations  show  that  the  majority  of  horses  are 
slightly  short-sighted,  f 

Hypermetropia  or  long  sight  is  due  to  the  eyeball  being 

*  '  The  Refractive  Character  of  the  Eyes  of  Mammaha,'  Royal 
London  Ophthahnic  Hospital  Beports,  vol.  xi.,  part  ii. 

t  '  The  Refractive  Character  of  the  Eyes  of  Horses,'  Proceedings 
of  the  Boyal  Society,  No.  334.     1894. 


THE  SENSES 


469 


too  short,  whereby,  though  vision  may  be  perfect  for  distant 
objects,  those  near  at  hand  are  not  distinctly  seen,  the  pic- 
ture being  brought  to  a  focus  behind  the  retina  (Fig.  105 — 2). 
It  is  obvious  that  a  concave  glass  which  scatters  rays  is  the 
remedy  for  myopia,  while  a  convex  lens  which  converges 
them  is  the  appropriate  glass  for  hypermetropia. 


1. 

Emmetropic. 


Hypermetropia. 


M  yopia. 


Fig.  lO.j. — Diagram  of  ax  Emmetropic,  Hypermetropic  and  Myopic 
Eyk,  to  illustrate  where  the  Focal  Point  exists  (Kirke). 

In  2  tbe  short  eyeball  causes  the  focus  to  form  behind  the  retina ;  in  3 
the  long  eyeball  causes  the  rays  to  come  to  a  focus  in  front  of  the 
retina. 

Astigmatism  is  another  error  of  refraction,  due  to  irregu- 
larities in  the  curvature  of  the  cornea  or  lens,  generally  the 
former.  The  effect  of  this  condition  is  that  the  rays  of 
light  passing  through  one  meridian  of  the  eye  are  brought 
to  a  focus  earlier  or  later  than  those  passing  through  the 
meridian  at  right  angles  to  it.  The  horse  is  very  commonly 
astigmatic ;  the  horizontal  is  generally  the  meridian  of  least  ■ 
curvature,  and  corresponds  to  the  long  diameter  of  the  pupil. 


470    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Errors  of  Refraction. — In  the  following  table  is  given  the 
proportion  of  eyes  affected  with  errors  of  refraction  among 
54  horses. 

Of  100  eyes  (54  horses)  : 

51  were  myopic  and  astigmatic. 

2  were  hypermetropic  and  astigmatic. 

6  were  affected  with  mixed  astigmatism. 
39  were  affected  with  myopia. 

1  was  hypermetropic. 

1  was  emmetropic. 

The  amount  of  error  of  refraction  is  as  a  rule  small,  the 
chief  visual  defect  being  myopia  with  or  without  astigma- 
tism. The  number  of  astigmatic  horses  is  remarkable. 
According  to  Lang  and  Barrett's  observations,*  the  cow 
would  appear  to  be  hypermetropic,  and  the  eye  also  suffers 
from  astigmatism.  In  dogs  and  cats  the  refraction  closely 
approaches  emmetropia.  In  nearly  all  the  wild  animals  ex- 
amined by  these  observers  the  refraction  was  hypermetropic. 

The  Movements  of  the  Eyeball  are  brought  about  by  means 
of  the  ocular  muscles ;  in  this  way  the  globe  of  the  eye  can 
be  rapidly  turned  in  any  direction.  But  the  movements 
are  somewhat  complex,  for  in  some  of  the  lower  animals, 
for  example  the  horse,  the  eyes  are  laterally  placed  in  the 
head,  so  that  vision  is  commonly  single-eyed  and  not 
binocular  as  in  man.  The  eye  that  is  viewing  an  object 
situated  to  one  side  and  moving  to  and  fro  is  being  followed 
in  this  muscular  movement  by  the  eye  which  does  not  see ; 
the  movements  are  conjugate,  but  this  only  occurs  so  long 
as  monocular  vision  is  practised.  If  both  eyes  be  directed 
to  an  object  situated  to  the  front  binocular  vision  becomes 
possible,  and  now  the  movements  are  no  longer  conjugate 
but  opposite,  for  while  the  left  eye  is  inclined  to  the  right 
the  right  eye  is  inclined  to  the  left.  Another  complication 
in  the  ocular  muscles  is  due  to  the  movement  of  the  head  ; 
it  was  first  pointed  out  by  Lang  and  Barrett,  that  in  the 
rabbit  and  guinea-pig  no  matter  what  position  the  head 
occupied  the  pupil  was  always  kept  vertical.     If  the  head 

*  Op.  cit. 


THE  SENSES 


471 


of  the  horse  or  ox  be  raised  or  depressed  to  the  fullest 
possible  extent,  the  mazzle  being  at  one  time  on  the  ground, 
at  the  next  high  in  the  air,  it  will  be  found  that  the  eye- 
balls  rotate  like  a  wheel,  so  that  the  pupil  is  still  kept  hori- 
zontal ;  if  it  were  not  for  this  the  pupil  in  the  uplifted  head 
would  be  vertical  and  in  the  depressed  head  oblique.  When 
the  head  is  elevated  the  eyeball  becomes  depressed  to  such 
an  extent  that  the  sclerotic  shows  largely  above,  while  the 
cornea  partly  disappears  beneath,  the  lower  eyelid.  When 
the  head  is  depressed  to  the  ground  no  more  sclerotic  shows 
than  when  it  is  in  the  ordinary  position ;  the  probable 
cause  of  this  will  be  mentioned  presently. 


■  Sup,:  Rectus, 


Fig.  106. 


E.\:tr:  Reccus. 
inf:0bliaue. 

-The  Muscles  of  the  Left  Eyeball  of  the  Horse  viewed 
FROM  THE  Temporal  Side. 


The  muscles  of  the  eyeball  (Fig.  106)  are  seven  in  number, 
viz.,  four  recti,  two  oblique,  and  one  retractor.  The  use  of 
the  recti  is  clear  enough,  they  rotate  the  eye  in  four  direc- 
tions, outwards,  inwards,  upwards  and  downwards.  The 
two  oblique  muscles  rotate  the  eye  in  opposite  directions 
around  its  anterior-posterior  axis  ;  when  the  superior 
oblique  contracts  it  pulls  the  temporal  side  of  the  eyeball 
upwards,  and  if  it  were  not  counteracted  by  the  inferior 
oblique  it  would  continue  to  contract  until  the  pupil  became 
vertical  like  that  of  the  cat ;  the  inferior  oblique  pulls  the 
temporal  side  of  the  eyeball  downwards,  in  other  words 


472     A  MANUAL  OF  VETEEINAEY  PHYSIOLOGY 

these  oblique  muscles  produce  a  torsion  of  the  globe  or 
swivel  rotation,  and  their  action  is  regulated  from  the 
semicircular  canals  of  the  internal  ear  (see  p.  502).  The 
retractor  partly  withdraws  the  eye  in  its  socket. 

The  nerves  supplying  the  muscles  of  the  eyeball  with 
motor  power  are  the  third  pair  to  all  excepting  the  external 
rectus  and  superior  oblique,  the  external  rectus  being 
supplied  by  the  sixth  pair  or  abducens,  and  the  superior 
oblique  by  the  fourth  pair  or  pathetic.  So  that  we  have 
three  pairs  of  cranial  nerves  supplying  seven  muscles. 
The  orbicularis  palpebrarum,  which  closes  the  eyelids,  is 
supplied  by  the  seventh  nerve,  while  the  muscle  which 
raises  the  upper  lid  derives  its  nerve  supply  from  the  third 
pair. 

The  chief  movements  of  the  eyeballs  are  backwards  and 
forwards,  corresponding  to  the  directions  described  as  out- 
wards and  inwards  in  man.  During  these  movements  it  is 
evident  that  the  external  rectus  of  one  eye  is  acting  in 
conjunction  with  the  internal  rectus  of  its  fellow,  and  such 
is  always  the  case  in  monocular  vision.  Animals  with  the 
eyes  laterally  placed  have,  however,  the  power  of  monocular 
and  also  of  binocular  vision,  but  the  latter  is  only  produced 
by  an  internal  squint,  and  the  movements  of  the  muscles 
are  now  no  longer  conjugate,  for  both  internal  recti  are 
acting  together  (Fig.  107).  Sometimes,  then,  the  group  of 
muscles  employed  in  moving  the  eyeballs  is  the  same  in 
each  eye,  at  other  times  it  is  not.  The  torsion  produced 
by  the  superior  and  inferior  oblique  muscles  is  of  value 
in  the  binocular  vision  of  animals,  and  in  the  vertical 
movements  of  the  head.  When  the  muzzle  is  raised,  as 
previously  described,  the  superior  oblique  muscle  revolves 
the  eyeball  in  its  socket  until  the  pu[)il  is  horizontal ;  the 
explanation  of  the  cornea  partly  disappearing  under  the 
lower  lid,  and  the  sclerotic  showing  extensively  above, 
appears  to  be  due  to  a  conjugate  action  of  the  inferior 
rectus  muscle  whenever  the  superior  oblique  is  so  employed. 
The  inferior  oblique  is  mainlj^  employed  with  the  internal 
rectus  in  pulling  the  eyes  inwards  for    binocular  vision, 


THE  SENSES  473 

also,  as  mentioned  above,  for  maintaining  the  horizontal 
pupil  when  the  head  is  depressed  or  raised. 

Monocular  and  Binocular  Vision. — When  a  horse  directs 
both  eyes  to  the  front  (Fig.  107)  he  produces  a  well-marked 
double  internal  squint,  and  is  then  capable  of  binocular 
vision.  The  eyes  are  rotated  inwards  and  slightly  upwards 
by  the  combined  action  of  the  inferior  oblique  and  internal 
rectus ;  the  pupils  are  not  perfectly  horizontal  but  nearly 
so,  and  the  pupillary  opening  is  brought  so  far  to  the 
front  that  the  inner  segment  of  the  cornea  and  iris  entirely 


.%■■* 


Fig.  107. — The    Position    of   the    Head   and   Eyes   in   Binocular 

Vision. 

disappear  beneath  the  inner  canthus.  In  no  other  position 
than  this  has  the  horse  binocular  vision,  viz.,  single  vision 
resulting  from  the  employment  of  a  pair  of  eyes,  and  it  is 
curious  to  observe  that  the  condition  of  eye  which  gives  a 
horse  single  vision  causes  in  man  double  vision.  Animals 
with  their  eyes  situated  on  the  lateral  side  of  the  head  are 
capable  of  exercising  monocular  vision  for  all  objects  placed 
to  one   side  of   them  and  even    behind  them  (Fig.  108)  ; 


474     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

monocular  would  appear  to  be  for  them  as  perfect  as 
binocular,  but  on  this  point  it  is  difficult  to  judge.  It  is 
certain  that  in  the  horse  when  the  attention,  either  from 
alarm  or  interest,  is  jmriknlarhi  directed  to  an  object,  it 
is  viewed  with  hoih  eyes,  the  head  being  held  very  high, 
and  the  ears  '  pricked  '  and  turned  to  the  front.  In  this 
position  it  is  evident  the  most  sensitive  area  of  the  retina 
is  exposed,  but  there  is  no  fovea  as  in  man.  A  horse  can 
see  an  object  on  the  ground  immediately  under  his  nose, 
and  is  able  to  see  when  grazing ;  this  is  because  his  face 


Fig.  108. — Diagram  illustrating  the  Extent   to  which  a  Horse 

CAN    SEE    BEHIND   HiM. 

With  the  head  straight  to  the  front  he  can  see  out  of  the  'tail '  of  both 
eyes.  By  the  least  inclination  of  the  head,  as  in  Fig.  108,  a  large 
visual  field  behind  him  may  be  covered. 

narrows  below  the  eyes.  When  looking  at  an  object  near 
to  him  on  the  ground,  he  prefers  to  get  his  head  low  down 
in  order  to  see  it ;  but  when  looking  intently  at  a  distant 
object,  he  gets  his  head  as  high  as  possible  with  the  face 
inclining  to  the  horizontal. 

Ordinary  equine  vision  is  monocular,  yet  the  right  eye 
blinks  when  an  attempt  is  made  to  strike  the  left,  though 
it  cannot  possibly  see  what  is  going  on,  and  in  the  same 
way  the  right  pupil  contracts  when  the  left  is  exposed  to 
sunlight.      In  man  binocular  vision    is   perfect,  and   the 


THE  SENSES 


475 


explanation  afforded  is  that  any  part  of  one  retina  corre- 
sponds to  the  same  part  of  its  fellow ;  so  that  if  the 
retinas  be  laid  over  one  another,  the  left  portion  of  one 
will  lie  exactly  over  the  left  portion  of  the  other,  and  their 
upper  and  lower  parts  will  equally  correspond ;   but  the 


Fig.   109. — Diagram    illustrating  Correspokdixg    Points   in   the 
Human  Eye  (Foster). 

z'  x'  y'  are  points  in  the  ri»ht  eye  corresponding  to  z  x  y  in  the  left 
eye;  v.l,  visual  axis.  The  two  figures  above  illustrate  the  corre- 
sponding points  on  the  retina  described  in  the  text. 

temporal  side  of  one  eye  does  not  correspond  to  the 
temporal  side  of  its  fellow,  but  to  the  nasal  side.  In 
Fig.  109,  the  two  circles  represent  the  two  retinas  divided 
into  quadrants,  L  being  the  left  and  R  the  right  eye ;  a 
and  c  in  the  left  eye  correspond  to  a'  c'  in  the  right  eye, 


47G     A  MANUAL  OF  YETEEINAEY  PHYSIOLOGY 


and  b  and  d  in  the  left  correspond  to  b'  and  d'  in  the 
right  eye ;  but  the  optic  nerve  o  is  in  the  left  segment 
of  one  eye,  and  the  right  segment  of  the  other.  When 
the  two  images  of  an  object  fall  on  corresponding  points 
of  the  retina  of  man,  vision  is  binocular  and  only  one 
object  is  seen ;  thus,  if  the  rays  fall  on  the  right  side  of 
one  retina,  they  must  fall  on  the  right  side  of  its  fellow. 
This  is  shown  in  Fig.  109,  v.l  from  x  to  x,  and  x  to  x'  are 

A 


Fig.   110. — DiAGKAM  showing  Horizontal  Section    of  the    Head 

PASSING    THROUGH    BOTH    EYEBALLS,    TO    ILLUSTRATE    CORRESPOND- 
ING Points  in  the  Eetina  of  the  Horse. 

X  X,  The  frontal  bones  ;  p  p,  portion  of  malar  bone  entering  into  the 
formation  of  the  outer  rim  of  the  orbit ;  s,  t  he  nasal  septum. 
Rays  of  light  proceeding  from  A  are  seen  by  both  eyes,  being 
imprinted  on  the  temporal  side  of  each  retina  at  a ;  rays  from  B 
are  seen  at  b  in  the  left  eye,  but  are  not  seen  w  ith  the  right  eye ; 
in  the  same  way  rays  from  C  are  imprinted  at  c  in  the  left  eye, 
but  cannot  be  seen  with  the  right  eye. 

the  two  visual  axes ;  if  the  object  y  x  z  be  looked  at,  z  in 
each  case  falls  to  the  left  of  the  visual  axis,  and  y  to  the 
right,  viz.,  on  corresponding  points,  by  which  means  the 
object  is  seen  as  a  single  one.  Owing,  then,  to  the  manner 
in  which  the  human  eyes  are  placed  in  the  head,  and  the 
convergence  of  axes  of  the  eyeball,  a  ray  of  light  from  any 
point  is  imprinted  upon  the  same  side  of  the  retina  in  both 
eyes,  and  we  see  the  object  not  as  a  double  image,  but 


THE   SENSES  477 

as  a  single  one.  This  explanation  does  not  apply  to  the 
herbivora ;  no  matter  how  greatly  the  eyes  may  be  con- 
verged in  order  to  see  an  object,  the  rays  of  light  do  not 
fall  on  the  same  side  of  the  retina,  but  on  opposite  sides 
of  it.  The  diagram  (Fig.  110)  will  make  this  point  clear. 
The  outer  part  or  temporal  side  of  the  retina  in  the  horse 
corresponds  with  the  temporal  side  of  the  opposite  eye  ; 
while  the  nasal  side  cannot  correspond  with  the  nasal  side 
of  its  fellow,  as  it  is  not  possible  for  a  ray  of  light  from  an 
object  to  strike  both  nasal  sides  at  one  time  (Fig.  110). 

Cartilago  Nictitans. — The  retractor  muscle  of  the  eye 
withdraws  the  eyeball  within  the  orbit,  and  the  pressure 
thus  produced  within  the  cavity  forces  the  cartilago  nictitans 
forward,  so  that  it  may  be  made  to  sweep  nearly  the  whole 
corneal  surface.  The  reason  why  the  cartilage  is  pressed 
forwards  is  due  to  the  fact  that  though  naturally  curved, 
it  becomes  flattened  and  straightened  out  by  the  pressure 
caused  by  retraction  and  so  shoots  forward  ;  when  the 
pressure  is  removed  it  retires  through  its  own  elasticity, 
and  becomes  curved  once  more. 

On  the  cartilage  of  some  animals  is  a  small  gland  termed 
the  Harderian  ;  its  use  is  to  prepare  an  unctuous  secretion, 
probably  of  a  protective  nature.  In  the  eyelids  are  found 
numerous  glands,  the  Meibomian,  which  furnish  an  oily 
secretion,  and  prevent  the  overflow  of  tears. 

The  Tears  are  secreted  by  the  lachrymal  gland  which  is 
placed  on  the  upper  surface  of  the  eyeball ;  they  find  their 
way  into  the  conjunctival  sac  by  numerous  small  tubes. 
The  tears  pass  through  the  narrow  puncta  into  the  lachry- 
mal sac,  and  so  into  the  nostril ;  once  in  the  sac  the  descent 
to  the  nostril  is  readily  understood,  but  it  is  not  clear  why 
the  tears  prefer  passing  through  a  narrow  slit  in  the  eyelid 
to  running  over  the  side  of  the  face ;  probably  the  only 
explanation  is  the  unctuous  secretion  mentioned  above. 
The  use  of  the  tears  is  to  keep  the  conjunctiva  moist  and 
polished,  and  to  wash  away  foreign  bodies. 

The  Eyelashes  of  the  horse  are  peculiar.  Those  on  the 
lower  lid  are  very  few  and  fine,  whilst  on  the  upper  lid  they 


478     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

are  abundant,  and  exist  not  as  a  single  l)ut  as  a  doul)le 
row  ;  the  rows  cross  each  other  Uke  a  treUis-work,  but 
without  interlacing ;  these  eyelashes  are  very  long  and 
strong  (Fig.  111).  A  few  protective  hairs  grow  from  the 
brow  and  below  the  lower  eyelid,  in  some  horses  they  are 
4  or  5  inches  in  length ;    they   appear  to  be  in  connec- 


FiG.  111.— The  Eye  of  the  Horse. 

tion  with  nerve  terminations,  for  their  delicacy  to  the  sense 
of  touch  is  remarkable.  The  function  of  these  hairs  is 
doubtless  protective,  and  they  give  the  eyes  warning  of  danger. 

Physiological  Optics. — When  a  ray  of  light  enters  the  eye  it  has  to 
pass  through  four  surfaces,  and  including  the  air  four  media.  There 
are  two  surfaces  to  the  cornea,  anterior  and  posterior,  and  two  surfaces 
to  the  lens,  anterior  and  posterior  ;  each  of  these  surfaces  differs  in 
curvature.  As  media  there  are  the  aqueous  and  vitreous  humours  and 
the  crystaUine  lens ;  the  latter  is  further  comphcated  by  not  being  of 
the  same  refractive  index  throughout.  The  formation  of  an  image  in 
such  a  complex  optical  system  would  be  difficult  to  understand,  were  it 
not  possible  to  construct  theoretically  from  it  a  simplified  eye,  or,  as  it 
is  known,  a  schematic  eye.  The  basis  of  its  construction  is,  that  so 
long  as  a  complex-  system  has  its  surfaces  and  media  '  centred,'  that  is 
symmetrically  disposed  around  the  optical  axis,  it  is  possible  to  deal 


THE  SENSES 


479 


with  it  as  if  it  consisted  of  two  surfaces  and  two  media,  viz.,  the 
schematic  eye,  and  even  to  simphfy  it  still  further  to  one  surface  and 
two  media,  the  reduced  eye,  the  media  in  the  latter  being  air  and 
water.  In  such  a  simple  optical  system  it  is  readily  possible  to  trace 
the  paths  taken  by  the  rays  of  light,  and  so  understand  the  formation 
of  an  image  on  the  retina  of  the  eye. 

Cardinal  Points. — The  most  simple  optical  system  which  can  be 
devised  has  an  optic  axis  (0  A,  Fig.  112),  viz.,  a  line  passing  through  its 
centre  perpendicular  to  its  refractive  surface  (a  p  b) ;  on  the  optic  axis  is 
situated  the  centre  of  curvature  of  the  refracting  surface,  this  centre  is 
known  as  the  nodal  2)oint  n.     All  rays  of  hght  which  strike  the  refrac- 


FiG.    112. — The    Cardinal    Points   of   a   Simple    Optical    System 

(Foster). 

O  A,  Optic  axis;  a  p  b,  a  curved  spherical  surface;  n,  nodal  point  ; 
Fg,  principal  posterior  focus;  Fj,  principal  anterior  focus  ;  ef,  rays 
proceeding  from  Fj,  rendered  parallel  to  the  optic  axis  ;  p,  the 
principal  point  ;  the  rays  m  d,  0  p,  and  m'  e,  pass  through  the 
nodal  point  n  and  undergo  no  refraction ;  the  rays  c  d,  parallel  to 
the  optic  axis,  are  refracted  and  meet  at  F.,. 

tive  surface  perpendicularly,  such  as  0,  m',  pass  through  the  nodal 
point  and  are  not  refracted ;  all  rays  of  light  parallel  to  the  optic  axis, 
such  as  c  d,  strike  the  refractive  surface  obliquely  and  are  refracted, 
and  the  point  where  they  meet  is  called  the  principal  yosterior  focus, 
F.,.  On  the  optic  axis,  in  front  of  the  refractive  surface,  is  situated  a 
point  Fi  known  as  ih.e  principal  anterior  focus ;  rays  proceeding  from 
this  point  strike  the  surface  obliquely,  and  are  so  refracted  as  to  be 
rendered  parallel  (e  f)  to  the  optic  axis  (0  A).  To  these  must  be 
added  the  principal  point  p,  that  is  the  point  where  the  refracting 
surface  cuts  the  optic  axis. 

These  various  points  are  known  as  the  cardinal  points  of  the  simple 
optical  system  we  have  imagined.    For  a  more  complex  system  such  as 


480     A  MANUAL  OF  VETEEINAKY  PHYSIOLOGY 

the  eye,  even  when  simphfied,  there  are  two  nodal  pomts,  two  principal 
foci,  and  two  principal  points  ;  but  with  the  reduced  eye  where  we  have 
but  one  surface  and  two  media,  the  two  nodal  points  become  one,  and 
the  two  principal  points  one. 

Dioptrics. — In  order  to  be  able  to  calculate  the  position  of  the 
cardinal  points  of  the  eye  certain  data  must  be  known,  such  as  the 
refractive  index  of  the  media,  the  radius  of  curvature  of  each  refracting 


Fig.  113. — The  Cardinal  Poixts  of  the  Eye  of  the  Horse  (Berlin). 

F,  is  the  first  principal  focus,  situate  "7244  inch  in  front  of  the  cornea. 

C  is  the  anterior  principal  point. 

H;  is  the  first  principal  point,  distant  from  the  cornea  '3201  inch. 

H„  is  the  second  principal  point    „  „  „       "3641     „ 

K;  is  the  first  nodal  point  „  „  „       "6693     „ 

K/^  is  the  second  nodal  point         „  „  ,,       "TIS?     ,, 

K^/  to  a  is  the  distance  of  the  retina  from  the  second  nodal  point, 

•8000  inch. 
C  to  Y^,  is  the  distance  from  the  cornea  to  the  second  principal  focus 

(which  Berlin  shows  to  be  behind  the  retina),  1'7594  inches. 

surface,  the  distance  from  the  cornea  to  the  lens,  and  the  thickness  of 
the  latter.  A  very  slight  error  in  the  determination  of  these  may 
produce  a  considerable  error  in  calculation,  so  that  all  measurements 
made  by  us  on  the  frozen  eyes  of  horses  are  rejected  as  wanting  in 
accuracy,  but  as  an  illustration  of  the  measm-ements  of  the  actual  and 
reduced  eye,  those  furnished  by  Berlin*  are  here  given,  though  even 

*  Zeitsclirift  far  Vergleichende  Augenheilkunde,  Heft  1,  1882. 


THE  SENSES 


481 


these  are  not  free  from  error.  According  to  "  Berlin  the  horse  is 
normally  long-sighted,  the  retina  being  in  front  of  the  second  principal 
focus.  What  may  have  been  true  for  the  eye  he  examined  is  not 
universally  true,  for  as  we  have  previously  stated  the  majority  of 
horses  are  shghtly  short-sighted,  therefore  the  point  F^^  will  fall  in 
front  of  the  retina. 

The  simphfied  or  reduced  eye  (Fig.  114),  consisting  of  one  surface 
and  two  media,  gives  for  the  horse,  according  to  Berlin,  the  following 
values : 

Passage  of  Light  through  Lenses. — In  nature  all  rays  of  light  are 
diverging,  but  so  slight  is  the  divergence  of  the  rays   from  distant 


Fig.    114. — The    Cardinal    Points    of   the  Reduced  Eye    of    the 
Horse  (Berlin). 


F^  the  first  principal  focus  is  situated  1'063  inches  in  front  of  the 

cornea. 
F,,  the  second  principal  focus  is  situated  1*427  inches  behind  the 

cornea  (in  the  diagram  it  falls  outside  the  eye,  but  this  is  not 

normal ;  see  above  remarks). 
K  to  rt,  the  distance  from  the  nodal  point  to  the  retina,  1'004  inches. 
H  to  a,  the  distance  from  cornea  to  retina.  l'368o  inches. 

objects,  that  for  the  purposes  of  the  eye  they  are  practically  regarded 
as  parallel.  All  rays  proceeding  from  an  object  situated  at  from 
20  feet  to  infinity  from  the  front  of  the  eye  are  considered  as  parallel 
rays,  all  rays  within  20  feet  from  the  cornea  are  diverging  rays. 
Obviously  the  nearer  the  object  to  the  cornea  the  greater  the  divergence, 
80  that  there  is  more  divergence  in  the  rays  proceeding  from  a  body 
1  foot  from  the  eye  than  in  one  10  feet  from  the  eye ;  conversely,  the 
further  the  object  is  from  the  eye  the  less  divergent  the  rays,  until  we 
reach  that  point  beyond  20  feet  where  the  rays  may  be  regarded  as 
parallel. 

81 


482    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

A  convex  lens  has  two  curved  "surfaces,  and  a  line  drawn  through 
the  centre  of  these  two  surfaces  is  known  as  the  principal  axis  of  the 
lens  (Fig.  115,  m  m).  The  essential  idea  of  a  double  convex  lens  is  that 
it  is  thicker  at  the  centre  than  at  the  edges.  Situated  on  the  principal 
axis  of  a  biconvex  lens  at  a  point  in  its  interior  is  the  optical  centre 
(Fig.  115,  0) ;  any  straight  line  passing  through  the  optical  centre  is 
termed  a  secondary  axis  (Fig.  115,  n  n). 


Fig.  115.  Fig.  116. 

Figures  illustrating  the  Action  of  Lknses  upon  Rays  of  Light 

PASSING   THROUGH   THEM    (LaNDOIS    AND    STIRLING). 

Fig.  115. — Biconvex  lens  ;  O,  optical  centre  ;  m  m,  chief  or  principal 
axis  ;  ot,  n,  secondary  axis. 

'S^h.Qw  imrallel  rays  of  light  (Fig.  116,  a)  pass  through  a  convex  lens 
they  are  refracted  and  brought  to  a  point  /  on  the  opposite  side  of  the 
lens  known  as  the  principal  focus ;  the  only  rays  not  refracted  are 
those  passing  through  the  centre  of  the  lens,  viz.,  those  coinciding  with 
the  principal  or  secondary  axes.  The  converse  of  this  is  also  true,  viz., 
divergent  rays  proceeding  from  the  principal  focus  of  a  lens  /  pass 
through  and  are  rendered  parallel  (Fig.  116). 


Fig.  117. — Rays  of  light  passing  through  a  convex  lens  from  Z  at  a 
point  beyond  the  focus  /,  cross  at  some  point  v,  and  invert  the 
image  (L,andois  and  Stirling). 


The  distance  from  O,  the  optical  centre  of  the  lens,  to  /,  its  principal 
focus,  is  known  as  the  focal  length  of  the  lens.  If  the  divergent  rays 
instead  of  proceeding  from  the  focus  of  the  lens  (Fig.  117,/)  proceed 
from  a  point  I  beyond  the  focus,  then  the  rays  on  passing  through  the 
lens  are  not  rendered  parallel  but  convergent  (as  the  refractive  power 
is  more  than  sufficient  to  render  them  parallel),  and  they  come  to  a 
focus  again  on  the  other  side  of  the  lens  at  the  point  v.     The  distance 


THE  SENSES 


483 


from  the  lens  at  which  they  come  to  a  focus  depends  upon  the  distance 
of  the  luminous  point  from  the  lens  on  the  opposite  side  ;  thus  the 
nearer  the  luminous  point  I  to  the  principal  focus  /,  the  further  will  the 
focus  on  the  opposite  side  recede,  and  vice  versa.  The  two  foci  I  and  v 
are  termed  conjugate  foci,  and  as  wo  have  shown  they  have  a  definite 
relationship. 

If  the  rays  of  light  proceed  from  a  point  L  (Fig.  118),  which  is  nearer 
to  the  lens  than  the  principal  focus  F,  the  lens  is  unable  to  refract 
the  rays  sufficiently  and  they  issue  from  the  opposite  side  divergent 
Fig.  118,  d  d). 

Parallel  rays  of  light  passing  through  a  concave  lens,  instead  of 
being  refracted  to  a  focus  are  bent  so  as  to  become  divergent,  so  that  a 
concave  lens  has  no  real  focus ;  but  if  the  divergent  rays  be  produced 


Fig. 


118. — Raj's  of  light  from  a  pohit  L,  between  the  focus  F  and  the 
lens,  diverge  when  passing  through  a  convex  lens. 


backwards  so  as  to  meet  on  the  principal  axis  of  the  lens,  the  point 
where  they  meet  is  called  the  negative  focus  of  the  lens. 

Spherical  Aberration. — The  rays  of  light  passing  through  a  convex 
lens  are  not  all  equally  refracted,  those  passing  through  the  circum- 
ference being  more  bent  than  those  passing  near  the  centre  ;  the  result 
is  that  the  rays  do  not  all  meet  in  the  same  point,  those  passing 
through  the  circumference  of  the  lens  coining  to  a  focus  earlier  than 
those  passing  near  the  centre.  This  defect,  known  as  '  spherical 
aberration,'  is  remedied  in  the  ej'e  by  the  introduction  of  a  diaphragm 
or  iris,  which  prevents  some  of  the  rays  of  light  from  passing  through 
the  circumference  of  the  lens ;  spherical  aberration  is  further  pre- 
vented by  the  fact  that  the  refractive  index  of  the  central  part  of  the 
lens  is  greater  than  that  of  the  circumference.  Spherical  aberration 
produces  indistinctness  of  vision  by  the  production  of  circles  of  diffusion 
caused  by  those  rays  which  meet  too  early  crossing  each  other  and 
forming  a  circle. 

Chromatic  Aberration  is  due  to  the  decomposition  of  white  light 

31—2 


484    A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

into  its  primary  colours  by  passing  through  a  prism  or  a  convex  lens, 
viz.,  a  spectrum  is  formed.  The  colours  of  the  spectrum  are  differently 
refracted,  the  red  being  the  least  bent,  the  violet  the  most;  when  there- 
fore we  can  see  the  red  distinctly  the  eye  is  not  focusscd  for  the  violet. 
Chromatic  aberration  is  prevented  in  the  eye  by  the  unequal  refractive 
power  of  the  various  media,  and  the  action  of  the  diaphragm  or  iris. 

Formation  of  a  Retinal  Image. — Eays  of  light  falling  on  the 
eye,  as  from  the  arrow  X  0  Y  (Fig.  119),  issue  as  a  pencil 
of  rays  from  every  point  of  the  arrow,  the  pencil  containing 
a  central  ray  known  as  the  principal  ray.  All  principal 
rays  a  a  pass  through  the  nodal  })oint  n  without  undergoing 
refraction,  while  the  rays  h  c,  and  U  c  are  refracted  to  a 


Fig.  119. — Diagram  of  the  Formation  of  a  Eetinal  Image  (Foster). 

a.  Principal  ray  of  the  pencil  of  light  proceeding  from  X ;  a',  principal 
ray  of  the  pencil  of  light  proceeding  from  Y ;  the  principal  rays 
pass  through  the  nodal  point  n  without  being  refracted  ;  the  other 
rays  b,  c  and  b',  c'  are  refracted.  In  this  way  the  arrow  X  Y 
forms  a  smaller  inverted  image  of  an  arrow  on  the  retina  Y  X. 

greater  or  less  extent,  so  that  in  this  way  the  retinal  image 
becomes  inverted,  and  very  much  smaller  than  the  object 
it  represents ;  it  is  a  miniature  though  perfect  representa- 
tion of  the  object  presented  to  the  eye.  The  chief  refrac- 
tion undergone  by  these  rays  is  at  the  anterior  surface  of 
the  cornea ;  doubtless  the  other  media  also  refract,  the 
lens  for  example,  but  an  eye  can  have  very  good  distant 
vision  without  a  lens,  whose  important  function  is  to 
provide  the  means  for  accommodation. 

Theory  of  Vision. — The  change  which  occurs  that  enables 
the  vibratory  ether  to  start  a  nerve  impulse  by  its  action 
on  the  retina  is  unknown.  A  photochemical  theory  based 
on  the  ready  decomposition  of  visual  purple  has  been  pro- 


THE  SENSES  485 

posed.  In  this  view  it  is  suggested  that  the  action  of  light 
on  the  visual  purple  is  allied  to  that  of  light  on  the  photo- 
graphic plate,  and  that  the  chemical  change  thus  set  up  in 
the  retina  excites  a  nerve  impulse  which  is  transmitted  by 
the  optic  nerve  and  tract  to  the  visual  centre  in  the  cortex 
of  the  brain. 

Though  the  retinal  picture  is  so  completely  inverted  that 
the  right  hand  of  the  object  becomes  the  left  of  the  image, 
and  the  top  becomes  the  bottom,  yet  the  mind  does  not 
perceive  the  image  as  inverted,  but  mentally  refers  the 
picture  not  to  the  retina  but  back  to  the  object. 

Turning  once  more  to  Fig.  119,  we  observe  that  the  angle 
X  n  Y  is  equal  to  the  angle  Y  n  X.  The  angle  X  n  Y  is 
spoken  of  as  the  Usual  Angle,  and  all  objects  having  the 
same  visual  angle  form  the  same  sized  picture  on  the 
retina.  By  the  aid  of  the  visual  angle  the  size  of  an 
image  on  the  retina  may  be  calculated,  provided  we  know 
the  distance  of  the  nodal  point  from  the  retina ;  thus  at 
the  distance  of  a  mile,  a  man  six  feet  high  is  represented 
on  the  retina  of  the  horse  by  an  image  ^io  of  an  inch  in 
height,  in  the  human  eye  at  the  same  distance  the  picture 
of  the  man  would  be  ysV^  o^  an  inch,  or  about  the  size  of 
a  red  blood-corpuscle.  The  nearer  the  object  the  larger 
the  image ;  taking  the  six-foot  man  again  at  a  distance  of 
10  yards,  his  height  on  the  retina  of  the  horse  would  be 
i  of  an  inch,  whilst  on  the  retina  of  a  man  it  would  be 
rather  over  ^  of  an  inch. 

Section  IL 
Smell. 

The  nasal  chambers  are  divided  by  a  septum,  and  each 
chamber  contains  the  turbinated  bones.  It  has  been 
observed  that  acuteness  of  smell  is  often  associated  with 
large  and  extremely  convoluted  turlnnates.  By  the  arrange- 
ment of  these  bones  the  nasal  passage  may  l)e  divided  into 
two  channels,  one  which  lies  next  the  floor  of  the  chamber, 
which  from  its  obvious  communication  leads  directly  to  the 


486     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

respiratory  passages,  and  another  channel  which  lies  above 
it  and  leads  to  structures  situated  very  high  in  the  face 
and  nose,  but  with  no  outlet  save  what  is  furnished  it  from 
below.  But  apart  from  this  there  are  differences  in  the 
physical  characters  of  the  mucous  membrane  which  divide 
the  nasal  chamber  into  a  lower  part  through  which  the  air 
travels,  and  an  upper  part  which  is  devoted  to  the  sense  of 
smell ;  the  one  is  known  as  the  respiratory  and  the  other 
the  olfactory  portion.  Both  the  respiratory  and  olfactory 
portions  of  the  nasal  chambers  are  supplied  with  sensation 
by  the  fifth  pair  of  nerves. 

In  the  horse  the  nasal  chambers  are  of  extreme  import- 
ance, inasmuch  as  it  is  the  only  animal  we  are  called  upon 
to  deal  with  which  is  unable  under  ordinary  circumstances 
to  breathe  through  the  mouth ;  the  majority  of  animals 
can  breathe  through  both  nose  and  mouth,  but  owing  to 
the  extreme  length  of  the  soft  palate  in  the  horse  this  is 
under  ordinary  circumstances  impossible.  So  far  as  respira- 
tion is  concerned  the  question  of  the  nostrils  has  been 
dealt  with  (p.  92),  but  the  arrangement  of  that  portion 
devoted  to  the  sense  of  smell  has  yet  to  be  considered. 

From  the  olfactory  tracts  in  the  brain  are  formed  the 
olfactory  lobes,  which  in  some  animals  possess  a  well- 
marked  cavity,  in  others  only  a  canal ;  in  the  cavity  some 
fluid  is  contained  which  communicates  with  the  cerebro- 
spinal, and  notably  in  the  horse  with  that  contained  in  the 
lateral  ventricles.  From  the  olfactory  bulbs  nerve-fibres 
are  given  off  which  penetrate  the  cribriform  plate  of  the 
ethmoid,  and  ramify  over  the  mucous  membrane  covering 
the  upper  portion  of  the  septum,  the  superior  turbinated 
bone,  and  the  upper  third  of  the  superior  and  middle 
meatus.  The  mucous  membrane  of  the  olfactory  region 
differs  from  that  of  the  respiratory  portion  in  being  thicker 
and  of  a  yellowish  tint ;  it  is  in  this  membrane  that  the 
fibres  of  the  olfactory  nerve  are  distributed.  This  nerve  is 
non-medullated,  and  in  the  surface  of  the  membrane  where 
it  terminates  two  or  three  different  kinds  of  cells  are  to  be 
found.     One  known  as  a  rod  cell  is  generally  believed  to 


THE  SENSES  487 

be  the  terminal  cell  of  the  olfactory  nerve,  though  this 
has  also  been  attributed  to  a  cylinder  cell  which  is  likewise 
found  in  the  membrane ;  other  observers  consider  that 
both  cells  are  the  terminal  organs  of  the  olfactory  nerve. 
No  definite  statement  can  be  made  on  this  point,  but 
perhaps  the  balance  of  opinion  is  in  favour  of  the  rod  cell 
being  the  chief  agent  whereby  odours  give  rise  to  nervous 
impulses  which  result  in  smell. 

Before  an  odour  can  affect  the  olfactory  nerves  it  has  to 
diffuse  into  the  higher  cavities  of  the  nasal  chambers,  and 
from  being  gaseous  it  must  become  dissolved  in  the  fluid 
which  bathes  these  surfaces.  We  have  no  idea  of  the 
nature  of  the  particles  which  constitute  an  odour,  but  it 
is  certain  that  before  they  can  make  any  impression  on 
the  olfactory  nerve  endings,  they  must  become  dissolved 
in  the  fluid  covering  the  nerve  terminations,  for  a  dry 
olfactory  surface  is  insensible  to  smell. 

There  are  certain  odours  which  excite  the  olfactory 
organs  more  readily  than  others  ;  thus  flesh,  blood,  and  offal 
have  a  remarkably  stimulating  effect  on  the  carnivora, 
whilst  grass,  grain,  and  vegetable  products  generally, 
stimulate  the  herbivora.  The  odour  of  blood  or  flesh  is 
evidently  repulsive  to  the  herbivora,  and  may  even  cause 
nervousness  and  fright ;  there  are  exceptions  to  this,  for 
we  have  known  a  horse  eat  meat  with  evident  pleasure. 
Some  of  the  herbivora  have  a  remarkably  keen  scent, 
antelopes  and  deer  have  the  power  of  detecting  the  presence 
of  man  even  a  considerable  distance  away,  and  it  is  evident 
that  in  most  animals  the  sense  of  smell  plays  a  more 
important  part  in  their  daily  lives  than  with  ourselves. 
It  is  through  the  sense  of  smell  that  the  male  is  attracted 
to  the  female  during  the  *  cestrous '  season,  and  not  only 
can  the  odour  of  a  female  in  this  condition  be  detected  at 
a  considerable  distance,  but  the  smell  is  evidently  most 
persistent.  The  organ  of  Jacobson,  which  is  well  marked  in 
herbivora,  is  said  to  have  some  connection  with  the  sense 
of  smell.  Cuvier  regarded  it  as  the  means  by  which  the 
herbivora    distinguished    between     poisonous    and    non- 


488     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

poisonous  plants ;  this  is  not  correct,  for  cattle-poisoning 
is  comparatively  frequent,  and  in  certain  parts  of  the 
world,  for  instance,  South  Africa,  is  extremely  common 
among  horses  and  cattle.  Experience  is  a  valuable  factor  ; 
animals  brought  up  on  a  pasturage  containing  poisonous 
plants  frequently  learn  to  disregard  them. 

The  odour  of  a  body  can  be  detected  with  greater  accuracy 
by  *  sniffing  ';  by  this  inspiratory  act  no  time  is  lost  in  diffu- 
sion occurring  between  the  respiratory  and  the  olfactory 
region,  as  the  odoriferous  particles  are  forcibly  drawn  up- 
wards. The  sense  of  smell  rapidly  becomes  blunted,  at 
any  rate  in  ourselves  ;  any  offensive  odour  is  always  most 
marked  when  first  detected. 

By  the  sense  of  smell  animals  have  the  power  of  recog- 
nising their  own  offspring ;  a  cow  which  has  lost  her  calf 
will  yield  milk  for  weeks  to  a  '  dummy '  clothed  in  the  skin 
of  the  dead  calf,  and  she  can  recognise  the  difference  between 
her  '  dummy  '  and  that  belonging  to  another  cow.  If  the 
skin  of  a  young  animal,  kid  for  instance,  be  dressed  with  an 
agent  which  disguises  the  body  smell,  the  mother  is  unable 
to  recognise  her  young.  The  odour  of  food  is  readily 
recognised  by  the  herbivora,  though  to  the  human  senses 
all  the  grains  are  equally  free  from  any  odour  but  that  of 
the  sack  which  contains  them.  Without  tasting  it,  a  horse 
will  refuse  a  grain  he  is  not  familiar  with.  It  is  possible 
that  everything  and  everybody  has  a  distinctive  odour,  at 
least  it  would  appear  to  be  so  from  the  remarkable  manner 
hounds  will  follow  a  scent,  or  a  dog  recognise  his  own 
master  in  the  dark  from  amongst  a  crowd  of  other  persons. 
In  the  case  of  hounds,  the  amount  of  odour  required  to 
stimulate  the  olfactory  organ  must  be  something  too 
infinitesimal  for  expression. 


THE  SENSES  489 

Section  III. 
Taste. 

The  sense  of  taste  is  nearly  though  not  quite  dependent 
upon  the  sense  of  smell.  There  are  certain  substances 
which  cannot  be  distinguished  when  the  nose  is  closed, 
there  are  others  which  can  be  readily  distinguished  by  the 
tongue  alone.  This  has  led  to  a  classification  of  taste 
sensations  of  which  four  qualities  exist,  viz.,  sweet,  bitter, 
acid,  and  salt.  Animals  are  certainly  capable  of  dis- 
tinguishing all  of  these.  It  is  probable  that  each  distinct 
taste  affects  a  particular  part  of  the  tongue  ;  in  man  it  has 
been  shown  that  the  back  part  of  the  tongue  is  sensitive  to 
bitter  tastes,  the  tip  to  sweet  and  saline  tastes,  the  sides  to 
acid  tastes,  ^Yhile  the  middle  portion  of  the  tongue  is  insen- 
sitive to  any  taste.  The  flavour  of  a  substance  is  not 
obtained  by  the  sense  of  taste  alone,  but  by  the  union  of 
the  senses  of  smell  and  taste.  Without  smell  taste  would 
be  nearly  impossible. 

On  the  tongue  certain  papillae  are  found  which  are  inti- 
mately connected  with  the  sense  of  taste,  viz.,  the  filiform, 
fungiform,  and  circumvallate ;  the  latter  are  probably  the 
most  important  in  connection  with  the  sense  of  taste,  but 
the  others  are  most  numerous.  In  both  circumvallate  and 
fungiform  papillae,  but  especially  the  former,  structures 
are  found  known  as  taste  buds,  bulbs,  or  taste  goblets. 
They  are  balloon  or  barrel-shaped  bodies,  the  walls  of 
which  are  formed  of  elongated  cells  resembling  the  staves 
of  a  barrel ;  this  structure  is  open  top  and  bottom ;  the 
nerve  fibrils  enter  below,  whilst  above  is  formed  the  gusta- 
tory pore,  or  opening  into  the  interior  of  the  body  of  the 
cell  by  which  fluid  finds  its  way  in.  Within  the  goblet  or 
barrel  are  other  cells,  processes  from  which  may  be  pro- 
jecting at  the  pore.  It  appears  to  be  essential  to  taste  that 
fluid  should  readily  find  its  way  into  the  pore,  and  as  a 
provision  to  ensure  this  the  papillae  containing  the  buds  are 
situated  close  to  glands.    M'Kendrick  states  that  in  a  single 


490    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

circumvallate  papilla  of  the  ox  1,7G0  taste-goblets  have 
been  counted,  m  the  papilla  foliata  of  the  sheep  and  pig 
9,500,  and  in  that  of  the  ox  as  many  as  80,000  goblet- 
cells.  The  nerve  supplying  these  taste-buds  is  the  glosso- 
pharyngeal, which  is  essentially  the  nerve  of  taste,  and 
mainly  distributed  to  the  posterior  part  of  the  tongue  ;  if 
this  nerve  be  divided  the  taste-bulbs  degenerate.  The 
glosso-pharyngeal  nerve  consists  of  a  medullated  and  non- 
medullated  portion ;  the  former  terminates  in  the  tongue 
in  end  bulbs,  whilst  the  latter  proceeds  to  the  taste-goblets. 
The  goblet  cells  are  not  strictly  limited  to  the  tongue,  but 
have  been  found  in  the  palate,  and  close  to  the  epiglottis ; 
they  have  not  been  found  on  the  anterior  two -thirds  of  the 
tongue,  a  region  which  we  know  to  be  also  possessed  of  the 
sense  of  taste,  and  one  not  supplied  by  the  glosso-pharyngeal 
nerve.  This  area  of  the  tongue  is  su])plied  by  the  gustatory 
branch  of  the  fifth,  and  it  is  to  this  nerve  (which  probably 
receives  its  taste  fibres  from  the  chorda  tympani  of  the 
seventh)  that  the  sensation  of  taste  is  here  imparted.  Sensa- 
tion to  the  tongue  is  supplied  by  the  lingual  branch  of  the 
fifth  pair,  while  motor  power  is  furnished  by  the  hypoglossal 
or  twelfth  pair. 

It  is  necessary  for  the  purpose  of  taste  that  the  substance 
should  be  dissolved ;  this  is  one  of  the  functions  of  saliva, 
and  experiments  on  herbivora  show  that  taste  produces  an 
abundant  secretion  from  the  submaxillary  and  sublingual 
glands,  though  not  from  the  parotid. 


Section  IV. 

The  Cutaneous  Senses  and  Muscle  Sense. 

These  are  pressure,  icannth,  cold  and  pain,  and  nerves 
through  which  these  qualities  are  conveyed  are  known  in 
the  human  subject  to  be  remarkable  for  the  fact  that  they 
are  distributed  in  '  spots '  throughout  the  whole  cutaneous 
surface.  Whether,  as  some  suppose,  there  are  special  nerves 
which  convey  these  sensations  is  not  definitely  known,  but 


THE  SENSES  491 

it  appears  to  be  proved  that  each  of  these  senses  has  its 
own  spots  of  distribution  in  the  skin,  those  for  pain  being 
probably  the  most  superficially  seated  as  well  as  the  most 
numerous,  while  the  warm  spots  are  the  fewest  in  number 
and  the  deepest  seated. 

Temperature  Senses. — Cold  spots  are  more  widely  dis- 
tributed than  warm,  and  exist  in  largest  number  in  the 
clothed  parts  of  the  body.  The  cold  spots  are  sensitive  to 
cold,  the  warm  spots  to  warmth ;  the  fact  that  the  former 
exceed  the  latter  in  distribution  and  number  suggests 
that  it  is  more  necessary  the  body  should  be  made 
acquainted  with  the  fact  that  it  is  cold  than  that  it  is  hot. 
In  fact  the  feeling  of  warmth  or  cold  does  not  depend  upon 
the  temperature  of  the  body  but  the  temperature  of  the 
skin.  It  is  obvious  that  the  observations  made  in  the 
investigation  of  a  temperature  sense  could  only  be  carried 
out  on  man.  There  is  no  reason  to  think  it  does  not 
equally  apply  to  all  animals. 

Pressure  Sense. — This  has  also  a  punctiform  distribution, 
the  spots  being  more  numerous  than  those  of  the  tempera- 
ture sense.  The  special  nerve  endings  connected  with  this 
sense  are  found  in  a  ring  around  the  hair  follicle,  in  which 
position  they  are  obviously  most  favourably  situated  for 
stimulation  through  the  hair  itself.  In  the  hairless  parts 
of  the  skin  special  tactile  corpuscles  are  found,  and  in  the 
horse  special  nerve  endings  are  found  in  the  foot  associated 
with  tactile  sensibility. 

Tactile  sensations  play  a  very  important  part  in  the  lives 
of  animals.  In  the  lips  and  muzzle,  which  correspond  to 
the  fingers  of  the  biped,  are  located  the  touch  organs 
proper  (p.  272)  ;  the  parts  are  endowed  with  exquisite  sensi- 
bility, which  enables  the  animal  to  be  kept  acquainted  with 
the  nature  of  its  surroundings  and  the  character  of  its 
food.  The  long  feelers  or  hairs  growing  from  the  muzzle, 
face  and  brow  of  the  horse  are  in  connection  with  nerves  in 
the  skin,  and  are  valuable  for  tactile  and  consequently  pro- 
tective purposes.  The  tactile  sensibility  of  the  foot,  by 
informing  the  animal  of  the  character  of  the  ground  it 


492     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

is  travelling  over,  is  useful  though  not  absolutely  essential 
in  locomotion  ;  nor  is  the  tactile  sensibility  in  the  foot 
of  the  horse  absolutely  essential  to  its  safety  in  pro- 
gression, as  is  clearly  proved  by  the  results  of  plantar 
neurectomy. 

Pain  Sense  is  the  most  widely  distributed  of  the  cutaneous 
senses.  It  is  distributed  in  spots,  probably  supplied  by 
special  fibres,  though  no  special  nerve  endings  have  been 
determined.  Pain  confined  to  the  surface  of  the  body  can 
be  readily  located,  but  the  localisation  of  interior  pain  is 
difiicult ;  that  of  colic  for  example  is  referred  to  the  abdo- 
minal wall.  It  is  considered  in  man  that  the  explanation 
of  the  difficulty  in  localising  interior  pain  is,  that  the 
segment  of  the  spinal  cord  supplying  the  affected  organ, 
refers  the  pain  to  the  skin  region  of  the  same  spinal 
segment  instead  of  to  the  organ. 

Painful  sensations  are  of  various  characters,  hence  such 
terms  as  stabbing,  boring,  burning,  throbbing,  etc.,  to  ex- 
j)ress  the  impression  imparted.  It  is  presumed  that 
amongst  the  lower  animals  these  different  qualities  of  pain 
exist ;  it  is  quite  certain,  for  instance,  that  the  pain  exhibited 
by  a  horse  during  an  attack  of  colic  is  very  different  from 
that  shown  when  pus  is  forming  in  the  foot.  Pain  may  be 
conveyed  by  channels  which  under  ordinary  conditions 
convey  no  sensation,  especially  is  this  the  case  in  disease. 
The  normal  heart,  liver,  muscles,  bones,  etc.,  may  be 
handled,  pinched,  wounded,  and  cauterized,  without  causing 
much  or  any  sensation,  but  under  the  condition  of  inflam- 
mation they  become  acutely  sensitive,  and  the  same  applies 
to  such  viscera  as  the  intestines,  kidneys,  bladder,  etc.  Of 
the  nature  of  pain  nothing  whatever  is  known. 

Muscle  Sense. 

Sensory  nerve  endings  have  been  found  in  muscle  and 
tendon  (p.  353).  In  the  former  they  are  spoken  of  as  neuro- 
muscular sjnndles;  these  from  their  construction  are  readily 
affected  by  variation  in  the  tension  of  contracting  muscles. 


THE  SENSES 


493 


and  in  this  way  they  keep  the  central  organism  informed 
of  their  condition.  They,  with  the  tendon  endings,  are 
employed  in  judging  active  muscular  movements  ;  passive 
movements  are  determined  by  impulses  passing  to  the  centre 
from  the  sensory  nerve  endings  in  joints,  while  the  positioK 
of  the  limbs  is  known  by  sensory  impressions  which  arise  in 
the  skin  and  subcutaneous  tissue  of  joints.  When  muscle 
sense  is  lost  inco-ordinate  muscular  contractions  occur  ; 
well  seen  in  a  dog  in  which  the  sensory  roots  leading  to  the 
hind  limbs  are  divided.  There  is,  of  course,  under  these 
conditions,  no  loss  of  motor  power,  yet  the  animal,  through 


m.'n.6. 
Fig.  120. — Muscle  Spindle  (Halliburton,  after  Ruffini). 

c,  Sheath  of  the  spindle;  n.tr.,  trunk  of  nerve,  which  sends  fibres 
through  the  sheath  into  the  spindle,  where  they  form  endings 
(pr.e.,  s.e.,pl.e.)  of  various  kinds;  m.n.b.,  bundle  of  motor  fibres 
(Stewart). 

a  loss  of  muscle  sense,  drags  the  limbs  as  if  they  were 
paralysed.  Later  on  the  sensory  impressions  which  should 
pass  from  muscle,  joints,  and  skin,  but  are  unable  to  reach 
the  cord  through  the  roots  being  divided,  are  now  replaced 
by  visual  impressions,  and  the  dog  learns  to  walk  through 
the  medium  of  his  eyes,  but  if  placed  in  the  dark  the  whole 
of  the  pseudo-paralytic  symptoms  return.  From  this  it  is 
evident  that  one  of  the  necessary  conditions  for  perfectly 
controlled  muscular  movements  is  a  muscle  sense  pouring 
in  sensory  impulses  into  the  central  nervous  system,  and 
by  imparting  a  continuous  knowledge  of  the  condition  of 
the  muscles  effecting  their  control. 


494     A  MANUAL  OF  YETERINAKY  PHYSIOLOGY 

Thirst. 

Thirst  is  referred  to  the  pharynx  ;  observations  show  that 
moistening  the  palate  allays  thirst,  while  on  the  other 
hand,  the  filling  of  the  stomach  with  water  through  a 
fistula  does  not  immediately  allay  the  desire  for  fluid. 

The  loss  of  water  caused  by  sweating,  purging,  etc.,  is 
made  good  to  the  blood  by  taking  up  water  from  the 
tissues ;  in  this  way  the  drain  on  the  lymph  may  be  con- 
siderable. It  has  been  supposed  that  the  sensation  of 
thirst  referred  to  the  palate  may  be  brought  about  by  a 
deficiency  of  water  or  lymph  in  the  part.  Little  or  nothing 
is  known  of  the  nervous  apparatus  involved  in  thirst,  nor 
why  dryness  of  the  tissues  should  be  referred  to  the 
pharynx  and  palate.  The  sense  of  thirst  is  generally 
only  lost  in  one  particular  group  of  affections — viz.,  acute 
disorders  of  the  digestive  system  in  the  horse.  No  horse 
suffering  acute  intestinal  or  stomach  pain  will,  as  a  rule, 
drink,  yet  the  dry  condition  of  the  mouth  suggests  that 
thirst  should  be  present. 

Hunger. 

Hunger  is  referred  to  the  stomach.  The  close  approxima- 
tion of  the  stomach  walls  is  not  necessary  for  the  production 
in  all  animals  of  the  sensations  of  hunger,  for  some  of  the 
herbivora  may  be  very  hungry  even  when  the  stomach 
contains  a  moderate  amount  of  food,  the  horse  and  rabbit 
for  example  ;  further,  the  sensations  of  hunger  may  be 
removed  though  the  walls  of  the  stomach  remain  in  opposi- 
tion— viz.,  by  the  introduction  of  nutritive  enemata.  The 
reason  why  the  sensations  of  hunger  are  referred  to  the 
stomach  wall  is  unknown. 

An  animal  deprived  of  its  cerebrum  shows  all  the  usual 
signs  of  hunger,  though  obviously  in  this  case  it  is  an  un- 
conscious exhibition. 


THE  SENSES  495 

Section  Y. 
Hearing. 

The  Nature  of  Sound. — When  a  body  is  made  to  vibrate 
its  vibrations  are  communicated  to  the  adjacent  air  and 
give  rise  in  this  to  waves  which  travel  at  a  definite  rate, 
and  when  the}^  reach  the  ear  so  act  upon  its  structures  as 
to  lead  to  the  sensation  of  sound.  The  vibrations  which 
constitute  the  waves  take  place  to  and  fro  along  the  direc- 
tion in  which  the  wave  is  travelling ;  in  this  sound  differs 
from  light,  whose  vibrations  are  transverse  to  the  direction 
of  propagation. 

In  comparing  one  sound  with  another  we  are  conscious 
of  only  three  possible  differences  between  them  ;  they  may 
differ  in  loudness,  pitch,  and  quality.  Of  these  loudness  is 
dependent  on  the  magnitude  of  the  to-and-fro  motion  of  the 
vibrating  particles  whose  movements  transmit  the  sound  ; 
a  loud  sound  means  a  large  wave.  Pitch,  on  the  other 
hand,  depends  on  the  frequency  of  the  vibrations,  a  high 
note  implying  rapid  vibrations,  or  a  shorter  wave-length. 

Sounds  may  be  simple  or  compound.  The  vibrations  of 
a  tuning-fork  give  rise  to  a  typically  simple  sound,  of 
varying  loudness  or  pitch,  but  possessing  little  quality. 
Now,  most  vibrating  bodies  do  not  give  rise  merely  to  such 
simple  vibrations,  but  set  up  a  variable  series  of  different 
wave-lengths  along  with  their  fundamental  simple  vibra- 
tion. Thus,  most  sounds  consist  of  a  fundamental  tone 
accompanied  by  more  or  less  of  these  other  tones — the 
partial  tones,  overtones,  or  liarmonies,  as  they  are  termed. 
The  quality  of  a  sound  depends  upon  these  partial  tones  ; 
where  they  are  absent  the  tone  is  thin,  where  they  are 
present  they  give  richness,  and  confer  on  it  that  '  character  ' 
which  enables  us  to  recognise  one  musical  instrument  from 
another  by  the  mere  sound  it  emits. 

Those  sounds  which  we  group  under  the  general  term  of 
'  musical '  result  from  the  regularity  of  their  causative  vibra- 
tions and  the   definiteness  in  wave-length   of   the   latter. 


496     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Noise  is  essentially  the  result  of  the  absence  of  this 
regularity  and  definiteness.  We  have  usually  no  difficulty 
in  discriminating  noise  from  musical  sounds,  but  the  one 
may  merge  into  the  other,  as  in  the  case  of  the  noise  of 
street  traffic  when  we  are  near  it,  and  the  musical  hum- 
ming tone  it  produces  when  heard  from  a  distance. 

From  observations  on  the  human  subject  it  has  been 
ascertained  that  the  smallest  number  of  vibrations  audible 
are  about  thirty  per  second,  while  the  average  human  ear 
can  recognise  up  to  30,000  vibrations  per  second.  It  is 
undoubted  that  some  animals  can  recognise  a  smaller 
number  of  vibrations  than  thirty  per  second.  Galton 
shows  that  the  cat  is  capable  of  recognising  sounds  in- 
audible to  the  human  ear. 

External  Ear. — The  vibrations  of  sound  are  collected  by 
a  freely  moving  funnel-shaped  body  or  external  ear ;  it  is 
composed  mainly  of  cartilage,  which  is  curved  and  hollowed 
out  in  such  a  way  as  to  form  a  good  collector,  while  several 
muscles  enable  it  to  assume  considerable  changes  in  direc- 
tion. The  two  chief  directions  taken  by  the  ears  are  back- 
wards and  forwards ;  judging  from  the  behaviour  of  many 
horses  in  carrying  one  ear  backwards  and  the  other  forwards, 
it  would  appear  that  they  are  capable  of  hearing  and  appre- 
ciating sound  in  two  opposite  directions  at  one  and  the 
same  time  ;  we  say  appreciating,  inasmuch  as  something 
more  than  mere  hearing  is  required  for  auditory  judgment. 
The  fannel  formed  by  the  external  ear  leads  somewhat 
indirectly  to  a  canal  known  as  the  external  auditory  meatus  ; 
in  and  around  this  is  found  an  unctuous  secretion,  and 
above  it,  in  the  funnel  of  the  ear,  are  many  hairs  which 
evidently  are  for  the  purpose  of  protection. 

The  increments  of  the  ears  give  evidence  of  what  is 
passing  through  the  mind  of  an  animal.  The  ears  of  the 
horse  are  turned  well  to  the  front  and  closely  pricked — viz., 
the  points  approximated,  when  he  is  attentive,  whether  the 
attention  be  devoted  to  a  something  he  is  alarmed  at  or 
pleased  with.  The  ears  are  laid  back  on  the  poll  in  sour- 
ness of  temper  and  in  vice ;  they  are  moved  rapidly  to  and 


THE  SENSES  497 

fro  when  a  horse  is  anxious  either  from  impending  danger 
or  other  cause ;  one  ear  carried  forward  and  the  other 
backward  or  both  turned  backwards  are  considered  the  sign 
of  a  good  stayer  and  wilhng  worker,  while  drooping  ears 
are  indicative  of  muscle  fatigue  or  debility. 

Whatever  part  those  remarkables  sacs,  the  guttural  pouches 
(confined  solely  to  solipeds),  are  intended  for,  it  is  probable, 
from  their  anatomical  connection,  that  they  take  some  share 
in  the  sense  of  hearing,  perhaps  that  of  supplying  the  need- 
ful amount  of  air  to  the  middle  ear.  The  actual  use  of  the 
guttural  pouches  is  involved  in  obscurity,  but  we  may  pro- 
visionally consider  them  as  part  of  the  middle  ear.  In  man 
acuteness  of  hearing  is  enhanced  by  listening  with  an  open 
mouth  ;  the  fact  that  the  horse  cannot  breathe  through  the 
mouth  may  explain  the  presence  of  these  large  air-sacs 
beneath  the  skull ;  in  other  words,  they  are  probably 
associated  with  acuteness  of  hearing. 

At  one  end  of  the  external  auditory  canal  is  a  piece  of  membrane 
stretched  completely  across  it  known  as  the  Tympanum,  it  separates 
the  external  from  the  middle  ear  (Fig.  121).  The  Middle  Ear  is  on  the 
opposite  side  of  the  tympanum  to  the  external  ear  ;  it  consists  of  a  cavity 
containing  a  chain  of  very  small  bones,  known  as  the  malleus,  incus  and 
stapes,  which  stretch  like  a  bridge  across  the  space  from  the  tympanum 
to  the  third  or  internal  ear.  The  middle,  like  the  external  ear,  is  in 
communication  with  the  external  air,  but  by  means  of  a  passage 
known  as  the  Eustachian  canal  which  opens  into  the  pharynx.  The 
tympanum  has,  therefore,  air  on  both  sides  of  it,  the  object  of  which  is 
to  ensure  that  the  atmospheric  pressure  on  either  side  is  equal,  and  in 
this  way  ensure  its  free  swing.  The  air  finds  its  way  into  the 
Eustachian  tube  during  the  act  of  swallowing,  and  by  the  same  channel 
it  is  conveyed  to  the  guttural  pouches. 

The  Tympanum  is  concave  towards  the  external  ear  ;  in  the  middle 
ear  the  handle  of  the  malleus  is  fixed  to  the  central  bulging  part  of  it, 
and  as  this  bone  articulates  with  the  incus,  and  the  latter  with  the  stapes, 
any  alteration  in  the  shape  of  the  drumhead,  such  as  is  produced  by 
the  vibrations  of  sound,  causes  the  bridge  of  bones  to  move  ;  further, 
their  movement  is  assisted  by  some  small  muscles  which  are  attached 
to  them. 

Ihe  Internal  Ear,  known  as  the  lahyrintli  (Fig.  122),  is  composed  of 
the  semicircular  canals,  the  vestibule, SiuA.  the  cochlea;  these  are  con- 
tained in  a  solid  piece  of  bone  in  which  two  small  foramina  or  windows 

32 


498     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

exist,  one  known  as  the  fenestra  ovalis,  the  other  the  fenestra 
rotunda  ;  the  base  of  the  stapes  or  third  bone  of  the  ear  is  attached 
to  the  membrane  which  covers  the  fenestra  ovalis. 


Fig.  121. — Diagrammatic  Section  of  the  Horse's  Ear. 

1,  External  auditory  canal;  2,  the  tympanum  ;  3,  chain  of  bones  across 
the  middle  ear  ;  4,  the  Eustachian  tube ;  5,  the  internal  ear. 

All  three  parts  of  the  labi/rintli  communicate,  but  it  is  quite  certain 
that  all  three  do  not  take  an  equally  active  part  in  hearing.  The 
evidence  on  this  point  is  clear  so  far  as  the  semicircular  canals  are 


Fig.  122. — The  Labyrinth  (Edmunds). 

The  semicircular  canals  are  to  the  right,  the  cochlea  to  the  left ;  both 
windows  may  be  seen,  the  fenestra  rotunda  being  the  lowermost. 
The  groove  across  the  body  of  the  organ  lodges  the  auditory  nerve. 
The  figure  is  enlarged. 

concerned,  and  some  have  even  included  the  vestibule,  regarding  the 
cochlea  as  the  essential  organ  of  hearing.  The  whole  of  the  internal 
ear  is  lined  by  a  membrane  containing  a  fluid  known  as  the  peri- 
lijmiyli;  this  peri-lymph  has  free  access  to  all  parts  of  the  inner  ear. 


THE  SENSES 


■499 


Within  this  membrane  is  a  membranous  labyrinth,  the  counterpart  of 
the  semicircular  canals  and  vestibule,  and  this  also  contains  fluid 
known  as  endo-hjvrph.  The  membranous  labyrinth  is  composed  of 
two  pouches,  the  saccule  and  utricle  lying  in  the  vestibule;  with  the 
latter  the  membranous  semicircular  canals  are  connected,  while  the 
former  communicates  with  the  middle  canal  of  the  coclilea.  On  both 
utricle  and  saccule  is  an  area  known  as  the  macula  acustica,  on  which 
branches  of  the  auditory  nerve  are  distributed  to  cells  known  as  hair 
cells  ;  similar  areas  exist  in  the  semicircular  canals.  The  hairs  on 
the  cells  project  into  a  mucoid  mass  frequently  containing  crystals 
of  carbonate  of  lime ;  these  crystals  are  known  as  otoliths. 


^eissner's  mewbran^ 


Fibres  of 
Audit  01 

Nerve-'^^WM 


Scala  VesHbuli^^  ^^i£analis  cochleae 

^m-Memi  ran  a 
I   tecioria 

^AUo]  9)/d.  J  .Hair  cells 

1  ^^^^^^   '  4^"Memb  r  an  a 

\      ScaJaTympani    r-^^  baSilaris 

fe>^  ^M^PiUars  of  Ccrli 


Fig.  123. — Diagrammatic  Transverse   Section  of  a  Turn  of  the 
Cochlea  (Stewart). 


The  two  windows  existing  in  the  bony  labyrinth  have  been  men- 
tioned above.  The  base  of  the  stapes  lies  over  one  of  them,  and 
between  the  stapes  and  the  peri-lymph  is  the  membrane  which  Unes 
the  internal  ear.  Every  movement  of  the  tympanum  causes  the  bony 
bridge  to  oscillate,  and  every  oscillation  of  this  thrusts  the  stapes 
against  the  membranous  window,  and  so  sets  up  oscillations  in  the 
peri-lymph  which  are  transmitted  throughout  the  internal  ear. 

The  cochlea  resembles  in  appearance  the  shell  of  a  snail,  its  interior 
being  divided  into  three  spiral  channels  which  wind  their  way  from  base 
to  apex  like  a  circular  staircase.  The  number  of  twists  in  the  cochlea  is 
two  and  a  half ;  the  axis  around  which  these  wind  is  composed  of  soft 
bone,  having  canals  up  which  the  auditory  nerve  travels.  If  a  spiral 
of  the  cochlea  be  cut  across  (Fig.  123)  the  three  canals  it  contains  are 
seen.  These  are  divided  by  septa  ;  one  septum,  known  as  the  lamina 
spiralis,  separates  the  upper  canal  or  scala  vestibuli,  from  the  lower 
one  or  scala  tyvipani.     The  third,  or  middle  canal,  is  of  a  triangular 

32—2 


500    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


shape  and  called  the  cochlear  canal ;  it  contains  the  essential  organs  of 
hearing,  and  lies  between  and  to  the  outside  of  the  other  two.  The 
roof  of  the  cochlear  canal  is  formed  by  a  piece  of  tissue  known  as  the 
membrane  of  Beissner,  whilst  its  floor,  on  which  is  situated  the 
essential  organs  of  hearing,  or  Organ  of  Cord,  is  formed  by  the 
membrana  basilaris,  which  connects  the  outer  wall  of  the  cochlea  to 
the  lamina  spiralis.  The  cochlear  canal  is  the  continuation  of  the 
membranous  labyrinth.  The  upper  passage  of  the  cochlea,  viz.,  the 
scala  vestibuli,  is  continuous  with  the  lymphatic  peri-lymph  space  of 
the  vestibule,  whilst  the  scala  tympani,  or  lower  passage,  ends  at  the 
base  of  the  cochlea  in  a  blind  extremity  in  which  is  a  membranous 


Fig.  124. — Organ  of  Corti  (Barker,  after  Eetzids). 

mb,  Basilar  membrane ;  re,  nerve  fibres  passing  in  to  arborize  around 
the  hair  cells  ;  p,  inner  pillar  of  Corti,  with  its  basal  cell,  b  ; 
2^',  outer  pillar,  with  its  basal  cell,  b' ;  1,  2,  3,  supporting  cells  of 
Deiters  ;  iJ,  Hensen's  supporting  cells  ;  i,  internal  hair  cells  with 
its  hairs ;  e,  external  hair  cells ;  e',  hairs  of  three  external  hair 
cells ;  n,  «',  to  n*,  cross-sections  of  the  spiral  strand  of  cochlear 
nerve  fibres  (Stewart). 

window,  the  fenestra  rotunda,  which  separates  the  scala  tympani 
from  the  cavity  of  the  tympanum.  The  cochlear  canal  terminates 
suddenly  at  the  summit  of  the  cochlea,  and  at  this  point  the  two  scalae, 
which  in  their  windings  have  been  decreasing  in  size  from  base  to  apex, 
meet  and  communicate  by  a  small  opening,  the  helicotrema,  and  the 
fluid  of  the  one  is  thus  in  connection  with  that  of  the  other. 

Organ  of  Corti.  —  This  consists  of  a  triangular  -  shaped  tunnel 
(Fig.  124),  the  base  of  which  rests  on  the  basilar  membrane  ;  the  tunnel 
is  composed  of  certain  rods  arranged  side  by  side,  inclined  from  both 
sides  towards  each  other  and  meeting  superiorly  like  an  inverted  V. 
At  this  point  the  rods,  known  as  the  rods  of  Corti,  fit  into  each  other  in 
a  peculiar  manner.     Flanking  either  side  of  the  tunnel  are  certain  cells 


THE  SENSES  501 

of  two  distinct  kinds ;  those  nearest  to  the  tunnel  are  somewhat  flask- 
shaped,  and  having  hairs  growing  from  their  summit,  are  spoken  of  as 
the  inner  and  outer  hair  cells;  external  to  the  outer  hair  cells  are 
some  tall  conical  cells  known  as  Hensens  cells.  It  will  be  remem- 
bered that  the  auditory  nerve  ascends  the  axis  of  the  cochlea,  giving 
off  fibres  which  in  their  passage  ramify  over  the  lamina  spirahs,  at 
the  outer  edge  of  which  the  above-described  organ  of  Corti  exists ; 
having  reached  this  the  fibres  lose  theu'  medulla,  and  the  naked  axis 
cylinders  pass  into  the  cells  flanking  the  triangular  tunnel,  some  fibres 
crossing  the  tunnel  to  reach  the  cells  on  the  opposite  side.  How 
the  nerve  terminates  in  the  hair  cells — for  it  is  to  these  that  it  is 
distributed — is  unknown,  but  that  the  hah'  cells  are  the  organs  of 
hearing  is  undoubted ;  Hensen's  cells  are  probably  only  of  a  nutritive 
natm*e  and  unconnected  with  auditory  impulses.  This  description  of 
the  organ  of  Corti  is  as  it  presents  itself  in  tranverse  section ;  if,  how- 
ever, we  look  at  the  tunnel  from  above  where  the  rods  from  either  side 
meet,  it  is  observed  that  in  their  union  the  rods  of  the  outer  wall  of  the 
tunnel  fit  into  the  heads  of  the  rods  of  the  inner  wall,  and  the  square- 
ness of  their  heads  is  such  that  the  arrangement  is  very  like  the 
keyboard  of  a  piano. 

Auditory  Sensations. — Any  analysis  of  these  is  hardly 
necessary  in  a  work  dealing  with  the  lower  animals ;  we 
have  no  direct  evidence  that  they  understand  or  appreciate 
the  difference  between  music  and  noise  ;  a  dog  will  howl  at 
one  as  readily  as  another.  At  the  same  time  it  is  certain 
that  animals  can  learn  to  recognise  sounds  and  associate 
them  with  certain  ideas,  as  for  instance  the  commotion  and 
excitement  amongst  the  horses  of  a  regiment  when  the 
trumpet  sounds  *  feed,'  and  again  the  recognition  by  a  dog 
of  its  master's  voice.  Further,  we  have  undoubted  evidence 
that  sounds  which  are  so  feeble  as  not  to  affect  the  human 
ear  are  readily  perceived  by  some  animals,  so  that  the 
acuteness  of  their  sensations  is  greater  than  that  of  our 
own,  though  their  capacity  for  the  enjoyment  of  music  is 
absent  or  extremely  small. 

The  vibrations  set  up  in  the  tympanum  are,  as  we  have 
seen,  communicated  to  the  chain  of  bones,  the  stapes  of 
which,  through  the  fenestra  ovalis,  imparts  a  push  to  the 
peri-lymph  of  the  labyrinth ;  this  fluid  transmits  the  impulse 
through  the  vestibule,  and  from  here  into  the  scala  vestibuli 


502     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

of  the  cochlea.  The  vibrations  ascend  the  spiral  staircase, 
and  set  in  motion  the  membrane  of  Reissner,  which  causes 
the  lymph  in  the  cochlear  canal  to  vibrate ;  when  these 
vibrations  reach  the  summit  of  the  cochlea  they  enter  the 
scala  tympani  through  the  helicotrema.  The  lymj^h  in  this 
canal  is  now  set  in  motion,  with  the  result  that  the  basilar 
membrane,  on  which  the  organ  of  Corti  rests,  is  affected, 
and  the  vibrations  are  ultimately  lost  at  the  blind  extremity 
of  the  canal,  whose  membrane  is  pushed  outwards  at 
the  fenestra  rotunda.  Every  push  inwards  at  the  fenestra 
ovalis  causes,  therefore,  a  push  outwards  at  the  fenestra 
rotunda.  During  the  time  the  vibrations  are  crossing 
the  cochlear  canal  from  one  scala  to  another  the  organ 
of  Corti  is  affected,  and  by  means  of  the  auditory  nerve 
the  impulse  is  conveyed  to  the  brain.  It  is  in  this 
organ  of  Corti,  with  its  nerve  endings,  that  the  complex 
sounds  which  make  up  even  a  single  note  of  music  are 
analysed,  and  this  analysis  was  at  one  time  supposed  to  be 
effected  by  the  rods  of  the  organ,  which  were  believed  to 
vibrate  to  their  own  particular  tone,  in  the  same  way  as  a 
tuning-fork  will  pick  out  its  own  tone  from  sounds  in  its 
vicinity  and  vibrate  to  it.  This  view,  tempting  as  it  is,  is 
negatived  by  the  fact  that  the  rods  of  Corti  do  not  exist  in 
birds,  and  it  has  therefore  been  supposed  that  the  vibrations 
to  the  nerves  terminating  in  the  organ  are  set  up  by  the 
vibration  of  the  basilar  membrane  on  which  the  organ  is 
built,  but  the  question  is  far  from  settled. 

Even  the  function  of  the  vestibule  is  disputed  ;  while 
some  hold  from  analogy  that  it  is  connected  with  auditory 
sensations — through  the  nerves  terminating  in  hair-cells 
which  are  found  on  the  areas  previously  described  as  the 
macula  acustica — others  believe  that  it  is  wholly  devoted 
to  the  perception  of  movements  of  the  body,  by  which 
means  the  animal  is  informed  of  the  extent  and  direction 
of  its  own  movements. 

The  Semicircular  Canals  and  Labyrinth,  though  connected 
with  the  internal  ear  and  sharing  in  common  with  it  the 
nerve  of   hearing,  are   yet  devoted  to  functions  of  quite 


THE  SENSES 


503 


another  kind.  To  the  labyrinth  is  assigned  the  control  of 
the  ocular  muscles  and  the  maintenance  of  the  horizontal 
position  of  the  pupil  (see  p.  472).  It  is  also  engaged  in  the 
obscure  problem  of  muscle  tonus  (p.  364),  and  this  is 
gathered  from  the  fact  that  destruction  of  it  delays  the 
appearance  of  rigor  mortis  on  the  same  side.  Finally, 
the  labyrinth  is  the  means  by  which  the  body  learns, 
or  is  made  acquainted  with  its  position  and  movements, 
and  this  is  effected  by  impulses  proceeding  from  it  to 
the  cerebellum.     To  quote  the  words  of  Sherrington,  the 


Fig.  125. — Diagram  showing  the  Position  Occupied  by  the  Semi- 
circular Canals.     After  Ewald  (Stewart). 

H,  P,  S,  are  three  mutually  rectangular  planes  which  indicate  the 
position  of  the  canals.  In  the  horizontal  plane,  H,  are  found  both 
external  canals ;  in  a  vertical  longitudinal  plane,  S,  are  found 
both  superior  canals;  in  a  vertical  transverse  plane,  P,  both 
posterior  canals  are  placed.  The  plane  of  the  superior  vertical 
canal  of  one  side  is  parallel  to  the  plane  of  the  posterior  vertical 
canal  of  the  opposite  side. 


labyrinth  keeps  the  world  right  side  up  for  the  organism, 
by  keeping  the  organism  right  side  up  to  the  external  world. 
This  function  falls  to  the  semicircular  canals,  the  arrange- 
ment of  which  is  peculiar  and  interesting. 

There  are  three  bony  semicircular  canals,  so  arranged 
that  their  three  planes  are  placed  at  right  angles  to  each 
other,  two  being  vertical  and  one  horizontal  (Fig.  125)  ; 
within  each  bony  canal  is  one  of  membrane,  the  two 
being  separated  l)y  a  fluid  known  as  the  peri-lymph. 
Within  the  membranous  canals  is  also  a  fluid  known 
as  the  endo-lymph,   and  at  certain   parts   of   the   canals 


504     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  vestibular  branch  of  the  auditory  nerve  has  special 
nerve  endings  known  as  hair-cells.  Impulses  set  up  in 
these  hair-cells  are  brought  about  either  by  alterations  in 
the  pressure  of  the  i^eri-lymph,  such  as  occur  in  consequence 
of  movement,  or  by  mechanical  stimulation  produced  by 
grains  of  calcium  carbonate  (otoliths)  found  in  the  labyrinth, 
closely  associated  with  the  nerve  endings.  Impulses  so  set 
up  are  conveyed  to  the  cerebellum,  which  is  the  centre 
dominating  the  co-ordination  of  muscular  movements,  and 
the  movements  necessary  for  equilibration. 

The  semicircular  canals  are  arranged  as  above  described, 
so  that  movements  of  the  body  in  the  three  dimensions  of 
space  may  produce  their  respective  effects  on  the  brain.  It 
is  by  means  of  them  that  an  animal  is  made  acquainted 
with  the  direction  in  which  its  body  is  travelling,  forward 
or  backward,  right  or  left,  up  hill  or  down,  or  in  the  move- 
ments which  occur  in  jumping.  It  is  no  wonder,  consider- 
ing their  extraordinary  importance,  that  these  canals  are 
securely  lodged  within  the  substance  of  the  hardest  bone 
in  the  body. 

If  the  semicircular  canals  be  injured  the  resulting 
phenomena  depend  upon  the  position  of  the  canal  which 
has  been  destroyed.  If  the  horizontal  canal,  the  head 
oscillates  in  a  horizontal  plane  ;  if  the  vertical  canals  be 
damaged  forced  movements  occur  in  a  vertical  plane ; 
standing  and  locomotion  become  impossible.  If  all  three 
canals  are  destroyed  violent  inco- ordinate  movements 
result,  the  animal  turns  somersaults,  the  head  is  twisted, 
the  eyeballs  roll  from  side  to  side,  and  special  measures 
have  to  be  taken  to  prevent  the  creature  killing  itself 
through  its  own  violence. 


CHAPTER  XVI 

THE  LOCOMOTOR  APPARATUS 

The  muscles  are  attached  to  bones,  and  these,  by  their 
movements,  maj^  be  incHned  to  each  other  at  angles  of 
varying  size.  These  angles  are  opened  and  closed  during 
progression,  and  the  mechanical  aid  which  is  introduced 
to  effect  this  is  that  of  the  lever.  The  Lever  is  com- 
posed of  a  power,  fulcrum,  and  weight,  and  according 
to  the  relative  positions  which  these  occupy,  a  lever  is 
spoken  of  as  being  of  the  first,  second,  or  third  order. 

In  a  lever  of  the  first  order  the  power  is  at  one  end,  the 
weight  at  the  other,  and  the  fulcrum  between  the  two. 
The  muscles  which  extend  the  head  act  as  a  lever  of  this 
order,  the  head  being  the  weight,  the  occipito-atloid  articu- 
lation the  fulcrum,  and  the  muscles  of  the  neck  the  power. 
In  extension  of  the  hind-leg  the  gastrocnemii  muscles  are 
the  power,  the  hock- joint  the  fulcrum,  and  the  leg  below 
the  hock  the  weight.  A  lever  of  the  first  order  is  principally 
a  lever  of  extension,  and  exists  all  over  the  body  ;  it  is  also 
a  lever  of  power,  for  if  the  long-arm  be  5  feet,  and  the 
short-arm  1  foot,  a  power  of  1  lb.  at  the  long-arm  will 
support  a  weight  of  5  lbs.  on  the  short-arm.  It  is  to 
be  noted  that  as  a  lever  increases  in  power  it  loses  in 
speediness  of  action. 

In  the  lever  of  the  second  order,  which  is  a  rare  one  in 
the  body,  the  weight  is  placed  between  the  fulcrum  and  the 
power  as  in  a  wheelbarrow,  the  wheel  being  the  fulcrum. 
When  the  leg  is  fixed  on  the  ground  and  the  body  passing 
over  it  a  lever  of  the  second  order  is  formed,  the  ground 

505 


506     A  MANUAL  OF  VETEPJNARY  PHYSIOLOGY 

being  the  fulcrum,  the  triceps  or  gastrocnemii  the  power, 
and  the  body  through  the  elbow  or  hock  joints  the  weight. 

The  third  order  of  lever  is  the  lever  of  flexion.  The  power 
is  placed  between  the  fulcrum  and  weight ;  the  nearer  the 
power  is  to  the  fulcrum,  the  greater  the  flexion  obtained 
for  a  given  expenditure  of  muscular  force.  This  lever  is 
one  for  speed,  and  what  it  gains  in  speed  it  loses  in  power ; 
it  is  therefore  a  wasteful  lever,  but  an  essential  one  in  the 
limbs.  Examples  of  it  in  the  body  are  numerous  ;  in  the 
flexion  of  the  elbow- joint,  the  weight  is  the  leg  below  the 
elbow,  the  power  is  the  flexor  brachii  muscle  at  its  insertion 
into  the  radius,  whilst  the  elbow-joint  forms  the  fulcrum. 
In  the  flexion  of  the  hock  the  power  is  the  flexor  metatarsi, 
the  fulcrum  is  the  hock-joint,  the  weight  being  represented 
by  the  limb  below  the  hock.  The  reason  why  the  third 
lever  is  more  frequent  than  the  others,  is  due  to  the  fact 
that  the  chief  movements  of  the  limbs  are  directed  to 
moving  comparatively  light  weights  through  a  great 
distance,  or  through  a  certain  distance  with  great  pre- 
cision, rather  than  moving  heavy  weights  through  a  short 
distance  (Foster).  As  to  the  weight  to  be  carried,  we  may 
say  that  the  weight  of  the  fore-leg  of  a  cavalry  horse  cut  off 
at  the  elbow  was  found  to  be  17  lbs.  8  ozs. ;  cut  oft"  at  the 
knee,  through  the  upper  row  of  bones,  it  was  found  to 
weigh  7  lbs.  10  ozs. ;  one  fore-foot  with  corona  weighed 
2  lbs.  3  ozs.,  and  the  hind-leg,  cut  off  at  the  hock-joint, 
weighed  10  lbs.  9  ozs. 

Stillman*  points  out  that  the  terms  flexor,  extensor, 
adductor,  and  abductor,  cause  the  chief  function  in  muscles 
to  be  lost  sight  of,  viz.,  the  power  of  propelling  ;  it  is 
necessary,  however,  to  remember  that  propelling  is  not  a 
power  apart  from  flexion  and  extension,  but  the  result  of 
them. 

Co-operative  Antagonism. — As  a  rule,  to  which  there  are 
certain  exceptions,  the  contraction  of  any  group  of  muscles 
is  attended  by  a  contraction  and  not  a  relaxation  of  their 
antagonists.     This  is  described  by  Waller  as  *  Co-operative 

*  '  The  Horse  in  Motion.' 


THE  LOCOMOTOR  APPARATUS      507 

Antagonism.'  The  amount  of  contraction  thus  exhibited 
by  antagonistic  muscles  is  insufficient  to  neutralize  the 
effect  of  the  direct  motors,  but  it  would  appear  that  for  the 
due  performance  of  such  movements  as  flexion,  extension, 
etc.,  the  antagonistic  group  of  muscles  should  offer  some 
slight  opposition.  This  can  readily  be  demonstrated  by 
flexing  the  fingers  and  grasping  the  arm  with  the  opposite 
hand ;  both  extensor  and  flexor  muscles  will  be  felt  to 
harden.  Moreover  the  opposition  of  antagonistic  muscles 
appears  in  many  cases  to  be  essential  to  the  due  performance 
of  movement ;  Waller  quotes  as  an  example  of  this  the  fact 
that  in  lead  palsy  only  the  extensor  muscles  of  the  arm  are 
affected,  yet  the  flexors  are  powerless  to  act. 

The  difference  existing  between  the  articulation  of  the 
fore  and  hind  limbs  with  the  trunk  has  until  recent  years 
been  the  cause  of  considerable  error  being  promulgated. 
It  was  previously  supposed  that  the  muscular  attachment 
of  the  fore-leg  to  the  trunk  indicated  that  the  body  was 
simply  slung  between  the  fore-legs,  the  latter  acting  as 
props  whilst  the  hind-limbs  did  the  work.  Instantaneous 
photography  has  shown  us  that  the  fore-limbs  not  only  act 
as  props  but  as  propellers  of  the  body ;  especially  is  this 
seen  in  the  gallop,  where  by  measurement  it  has  been 
shown  that  one  fore-leg  will  propel  the  body  a  distance  of 
10  feet,  and  in  so  doing  will  raise  it  4  inches  in  height  in 
a  vertical  direction.*  By  means  of  the  fore-legs  also,  the 
horse  is  enabled  in  draught  to  assist  its  hind-legs  in  stopping 
weights. 

Joints  are  formed  wherever  two  bones  come  into  contact. 
Dealing  only  with  those  joints  in  the  limbs  which  are  of 
the  most  practical  interest,  it  is  observed  that  ball-and- 
socket  joints  (as  in  the  hip),  hinge-like  joints  (as  in  the 
hock),  and  gliding  joints  (as  in  the  knee),  are  found;  all 
these  are  coated  with  articular  cartilage  and  lubricated  with 
synovia.  Synovia  is  a  viscid,  yellow,  alkaline  fluid  con- 
taining proteids,  mucin,  and  salts.  The  viscidity  of  synovia 
is  due  entirely  to  the  mucin  it  contains,  and  this  confers  on 
*  Stillmau,  '  The  Horse  in  Motion.' 


508    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

it  its  slippery  nature.  There  is  no  difference  between  the 
synovia  of  joints  and  that  of  burste.  It  is  said  that  the 
amount  of  synovia  in  a  joint  is  greater  in  animals  at  rest 
than  in  those  at  work,  but  the  extra  bulk  appears  to  be  due 
to  an  increase  in  the  watery  material,  whilst  the  proteids 
are  decreased  ;  the  salts,  on  the  other  hand,  especially  those 
of  sodium,  exist  in  a  larger  proportion  than  in  the  synovia 
of  working  animals. 

The  bursse  in  the  limbs  of  the  horse  are  very  important 
structures  ;  they  are  placed  where  the  tendons  pass  through 
bony  channels,  and  without  them  the  rapid  movements  of 
the  limbs  would  be  impossible ;  that  the  strain  on  them 
from  wear  and  tear  is  considerable  we  know  from  practical 
experience. 

Hock-joint. — Solipeds  appear  to  stand  alone  in  having 
the  ridges  of  the  astragalus  placed  obliquely,  instead  of 
vertically  as  in  other  animals ;  the  oblique  ridges  in  the 
horse  occasion  some  considerable  difference  in  the  action  of 
the  limb.  It  is  usual  to  speak  of  a  screw  action  of  the  hock 
produced  by  the  oblique  ridges  of  the  astragalus ;  this 
screw  action,  we  believe,  is  an  entire  misconception.  The 
ridges  on  the  astragalus  do  act  as  a  screw  but  not  on  the 
hock ;  tJie  eject  is  on  the  stijie,  and  produces  that  remark- 
able stitle  action  particularly  well  seen  in  trotters.  If  the 
ridges  on  the  astragalus  turned  the  hock  outwards,  every 
horse  would  travel  as  if  it  were  *  cow-hocked.'  The  leg 
below  the  astragalus  is  carried  directly  forwards ;  when, 
however,  it  comes  to  the  ground,  and  the  body  passes  over 
it,  it  is  not  uncommon  in  some  horses  to  observe  a  con- 
siderable twist  outwards  of  the  hock-joint,  the  toe  being 
turned  in ;  this  is  due  to  the  ascent  of  the  lower  end  of  the 
tibia  on  the  astragalus,  leading  to  the  upper  end  of  the 
tibia  turning  in  the  stijie-joint,  the  result  of  the  leg  being 
extended.  The  object  of  the  stifle  being  turned  outwards 
during  the  flexion  of  the  leg  is  to  clear  the  abdominal  wall, 
and  the  reason  why  solipeds  have  oblique  ridges  on  the 
astragalus  and  ruminants  and  carnivora  vertical  ones,  is 
that  the  ribs  of  the  latter  class  are  short  and  do  not  come 


THE  LOCOMOTOR  APPARATUS      509 

near  the  pelvis  (as  in  the  horse),  and  therefore  the  abdominal 
wall  is  not  in  the  way.  A  spring  or  automatic  flexion 
action  in  the  hock  has  been  described,  such  as  may  readily 
be  observed  in  the  dead  leg,  when  if  the  hock  be  flexed 
slightly  it  either  flies  back  or  completes  its  revolution  with 
a  Jerk.  This  condition  does  not  exist  during  life,  nor  after 
death  until  rigor  mortis  occurs ;  it  is  produced  by  the 
lateral  ligaments  of  the  hock-joint,  and  is  purely  a  post- 
mortem condition. 

The  flexor  metatarsi  muscle  is  remarkable  in  having  a 
tendon  running  its  whole  length,  so  that  from  the  origin  at 
the  femur  to  the  insertion  at  the  front  of  the  hock  there  is 
a  stout  tendinous  cord.  A  somewhat  similar  arrangement 
exists  in  connection  with  the  gastrocnemii  muscles.  When 
the  flexor  metatarsi  acts  the  hock  is  flexed,  but  the  use  of 
the  tendon  running  from  origin  to  insertion  is  not  at  first 
sight  quite  clear.  Chauveau  considers  that  it  automatically 
flexes  the  hock,  but  tendons  are  devoid  of  any  such  power ; 
it  would  appear  that  its  function  is  to  relieve  the  muscle 
when  the  animal  is  standing,  or  sleeps  standing.  When 
muscles  which  perform  flexion  and  extension  are  acting 
together  with  equal  force  no  movement  results ;  such  is 
the  case  when  the  weight  is  on  the  limbs  and  the  animal  at 
rest.  When  a  horse  is  at  rest  his  gastrocnemii  muscles  and 
flexor  metatarsi  are  acting  in  opposite  directions  and  equally  ; 
the  one  is  trying  to  close  the  femoro-tibial  angle,  the  other 
keeping  it  open.  It  is  the  function  of  the  tendinous  portion 
of  the  flexor  metatarsi  and  gastrocnemii  muscles  to  assist 
in  keeping  the  leg  fixed  without  any  great  muscular  effort. 

The  chief  movement  of  the  hock  occurs  between  the  tibia 
and  astragalus.  Though  the  range  of  motion  between 
these  bones  is  considerable,  yet  it  is  not  fully  exercised  in 
all  paces;  it  is  only  in  the  jump  and  gallop  that  the  angle 
formed  between  the  tibia  and  metatarsal  is  closed  to  any 
great  degree.  When  the  joint  is  completely  flexed  in  the 
dead  dissected  limb,  if  we  look  at  the  posterior  part,  viz., 
the  now  uncovered  ridges  of  the  astragalus,  we  find  that 
when    the   joint    is    flexed   to   the    utmost   the    tibia   and 


510     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

astragalus  are  no  lonc/er  in  apposition,  the  tibia  has  partly 
left  the  astragalus  and  a  small  space  exists  between  them. 
To  prevent  flexion  to  a  dangerous  degree  two  stops  are 
placed  on  the  anterior  face  of  the  inferior  extremity  of 
the  tibia,  one  outside,  the  other  inside,  the  outside  being 
the  larger  of  the  two ;  these  stops  come  into  contact  with 
two  rests  on  the  astragalus,  and  in  this  way  we  think  a 
certain  amount  of  jar  may  be  imparted  to  this  bone.  As 
the  inside  stop  comes  into  contact  with  the  astragalus 
slightly  before  the  outside  stop,  we  conceive  it  possible  that 
the  inside  of  the  astragalus  receives  more  concussion  than 
the  outside.  Can  this  help  to  offer  any  explanation  of  the 
position  of  spavin?  Of  the  ridges  on  the  astragalus,  one 
is  narrow,  the  other  broad ;  the  narrow  one  is  the  inside 
ridge,  and  it  runs  completely  down  to  the  surface  which 
articulates  with  the  magnum,  and  sometimes  considerably 
overlaps  it. 

The  movement  in  the  true  hock-joint  is  very  simple  as 
well  as  extensive  ;  but  the  movements  between  the  small 
bones  composing  the  joints  are  complicated.  In  the  first 
instance  they  are  very  limited  ;  the  astragalus  moves  on  the 
magnum,  the  magnum  on  the  medium,  and  the  medium  on 
the  large  metatarsal ;  but  the  amount  of  movement  in  these 
is  not  the  same,  the  movement  between  the  astragalus  and 
magnum  being  the  greatest.  One  might  suppose  that  the 
movement  in  this  part  was  rather  of  a  front  to  rear,  viz., 
to  and  fro  character,  though  the  fact  that  the  ligamentous 
attachment  between  the  bones  is  situated  at  the  central 
part  suggests  that  this  is  probably  not  the  case.  Pathology 
proves  the  correctness  of  the  latter  supposition.  An 
examination  of  the  face  of  these  bones  when  affected  with 
articular  disease  exhibits  well-marked,  sharp,  and  rather 
deep  grooves,  which  run  obliquely  across  the  surface  of 
the  bones,  and  are  better  seen  between  the  astragalus  and 
magnum  than  elsewhere.  The  grooves  are  the  result  of 
friction  during  the  movement  of  the  joint,  and  they  indicate 
that  the  motion  of  these  bones  on  one  another  is  more 
of  the  nature  of  a  rotation.     Again,  these  grooves  show 


THE  LOCOMOTOR  APPARATUS      511 

where  the  greatest  amount  of  pressure  normally  comes  on 
the  bones ;  it  will  always  be  found  that  the  most  extensive 
damage  in  disease  is  on  the  anterior  and  internal  surface, 
and  this  rule  holds  good  whether  it  be  the  astragalus, 
magnum,  medium,  or  head  of  the  large  metatarsal  which 
we  are  examining.  If  a  longitudinal  section  of  the  leg 
from  the  thigh  to  the  fetlock  be  made,  it  is  observed  that 
the  line  of  weight  on  the  bony  column  mainly  falls  through 
the  anterior  part  of  the  hock- joint.  There  can  be  no 
doubt  that  this  pressure  is  removed  by  resting  the  leg, 
viz.,  flexing  the  hock,  and  this  is  probably  the  reason  why 
no  horse  ever  stands  for  any  length  of  time  resting  equally 
on  both  hind-legs. 

The  Stifle  is  the  largest  joint  in  the  body ;  the  cause  of 
its  rotation  has  been  previously  described.  One  function 
of  this  joint  is  that  of  rendering  the  limb  firm  and  rigid 
when  the  foot  is  on  the  ground,  and  this  it  does  by  the  con- 
traction of  the  muscles  inserted  into  the  patella ;  if  the 
latter  bone  be  kept  fixed  on  the  upper  part  of  the  trochlea 
of  the  femur,  no  flexing  of  the  hock  or  stifle  can  occur. 
This  experiment  can  be  readily  tried  on  a  horse  just 
destroyed ;  the  limb  having  been  extended,  the  simple 
pressure  of  the  hand  on  the  crural  muscles  is  suflicient  to 
prevent  the  bending  of  the  hock  unless  considerable  force 
be  employed.  No  bending  of  the  hock  during  life  can  occur 
if  the  foot  be  kept  extended  ;  the  first  movement  in  the 
advance  of  the  leg  and  the  flexing  of  the  hock  and  stifle  is 
that  the  foot  is  flexed.  In  a  certain  surgical  condition,  com- 
monly known  as  dislocation  of  the  patella,  the  limb  is  rigid 
from  the  femur  to  the  metatarsus ;  but,  though  the  foot 
may  be  flexed,  neither  hock  nor  stifle  responds,  owing  to 
the  patella  being  fixed.  We  believe  that  in  the  majority 
of  these  cases  the  patella  is  not  fixed  from  dislocation,  but 
from  cramp  of  the  vasti  muscles.  The  amount  of  move- 
ment in  the  stifle  is  considerable,  and  to  admit  of  it  being 
carried  out  with  perfect  freedom,  the  convex  condyles  of 
the  femur  play  in  cups  formed  of  cartilage  on  the  upper 
surface  of  the  tibia. 


512     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  Hip  is  a  cup  and  ball  joint ;  the  range  of  outward 
movement  obtained  by  it  in  the  horse  is  limited  by  the 
insertion  of  the  ligamentum  teres  (and  pubio-femoral  liga- 
ment) into  the  inner  side  of  the  head  of  the  femur,  and 
not  into  its  centre  as  in  most  other  animals.  This  is  said 
to  be  the  reason  why  the  horse  rarely  '  cow-kicks.'  The 
lengthening  of  these  ligaments  accounts  for  *  cow-hocks  ' 
in  horses. 

The  Shoulder-joint  is  characterized  by  the  considerable 
surface  of  movement  afforded  by  the  humerus  and  the 
small  surface  of  the  scapula,  the  object  being  to  obtain  a 
large  range  of  motion  for  the  humerus. 

The  Elbow  presents  an  articulation  with  ridges  which 
influence  the  turning  outwards  of  the  knee  in  progression ; 
if  the  knees  are  turned  out  too  much  the  leg  below  is 
thrown  in  as  it  is  brought  forward,  and  in  this  way  one 
cause  of  '  brushing  '  and  *  speedy  cutting  '  is  produced. 

The  Knee  consists  of  three  main  joints  and  numerous 
minor  ones ;  the  upper  joint  possesses  the  largest  range  of 
motion,  whilst  the  lower  joint  practically  does  not  open. 
Probably  such  defects  as  '  speedy  cutting '  and  its  opposite 
condition,  '  dishing,'  are  influenced  not  only  by  the  elbow, 
but  by  the  shape  of  the  articular  surfaces  between  the 
radius  and  upper  row  of  bones.  The  radius  is  peculiar  in 
presenting  on  that  articular  surface  next  the  knee  a  concave 
surface  anteriorly  and  a  convex  one  posteriorly ;  these 
form  two  condyles,  of  which  the  inner  is  more  curved  than 
the  outer.  The  outer  condyle  plays  on  the  trapezium, 
cuneiform,  and  lunar ;  the  inner  condyle  plays  solely  on 
the  scaphoid.  When  the  knee  is  flexed  the  influence  of 
the  condyles  is  seen ;  the  concave  articular  surface  of  the 
radius  is  removed  from  the  surface  of  the  bones  of  the 
knee,  and  the  convex  articular  surface  appears  as  the  joint 
grows  wider.  The  inner  condyle  being  larger  than  the 
outer  depresses  the  scaphoid,  so  that  a  very  important 
movement  occurs  between  the  scaphoid  and  lunar.  This 
action  of  the  radius  on  the  scaphoid  throws  the  foot 
slightly  outwards,  probably  with  the  object  of  enabling  it 


THE  LOCOMOTOR  APPARATUS      513 

to  clear  the  opposite  limb.  We  believe  that  an  examina- 
tion of  the  knees  of  '  dishing '  horses  will  show  that 
extreme  curvature  of  the  inner  condyle  of  the  radius  is  the 
cause  of  the  action,  in  the  same  way  that  turned-in  elbows, 
and  alterations  in  the  curvatures  of  the  radius  and 
humerus,  will  probably  account  for  horses  throwing  the 
foot  inwards,  and  thus  '  brushing  '  or  '  speedy  cutting.' 

The  Fetlock  Joint,  owing  to  the  presence  of  the  sesamoid 
bones,  forms  a  yielding  articulation.  In  a  state  of  repose 
the  greater  part  of  the  horse's  weight  is  borne  on  the 
posterior  half  of  the  metacarpal  articulation  and  the 
articular  surface  of  the  sesamoids.  One  great  advantage 
gained  by  the  articulation  of  the  fetlock  being  yielding  is 
the  destruction  of  the  concussion  of  impact  when  the  body 
comes  to  the  ground.  A  similar  condition  is  observed 
in  the  joint  of  the  foot,  for  which  see  the  chapter  devoted 
to  that  subject. 

The  Function  of  the  Suspensory  Ligament  has  been  a 
fruitful  source  of  discussion.  Its  chief  use,  no  doubt,  is  to 
support  the  fetlock;  in  no  other  way  could  a  joint  be 
supported  which  is  placed  in  this  part  of  the  limb,  pos- 
sessed of  so  much  motion,  and  exposed  to  such  concussion. 
Though  ligaments  and  tendons  are  held  to  be  non-elastic, 
yet  we  must  claim  for  the  suspensory  ligament  a  little  more 
elasticity  than  would  be  obtained  if  the  sesamoids  were 
united  by  bony  tissue  to  the  metacarpal,  and  the  pleasant- 
ness and  freedom  from  jar  experienced  in  the  riding-horse 
are  in  part  due  to  the  suspensory  ligaments. 

Stillman  claims  for  the  suspensory  ligament  a  function 
which  he  believes  to  be  demonstrated  by  instantaneous  photo- 
graphy, viz.,  that  it  acts  the  part  of  a  spring,  flexing  the 
fetlock  sharply  when  the  weight  is  taken  off  it,  and  explains 
why  the  dirt  is  thrown  out  of  the  feet  of  a  galloping  horse. 
We  have  no  evidence  of  the  correctness  of  this  statement ; 
the  sharp  picking  up  of  the  foot  from  the  ground  in 
walking  (a  movement  so  rapid  as  almost  to  defy  detection) 
must  rest  with  the  flexor  muscles.  Besides  these  functions, 
the  suspensory  ligaments  assist  the  horse  to  stand  while 

33 


514     A  MANUAL  OF  VETEKINARY  PHYSIOLOGY 

sleeping.  If  the  suspensory  ligament  be  divided,  the 
fetlock  sinks  but  does  not  come  to  the  ground ;  if  the 
perforans  be  divided  a  slight  sinking  of  the  fetlock  is  the 
only  change.  To  bring  the  fetlock  to  the  ground,  both 
flexors  and  suspensory  ligament  must  be  divided,  which 
demonstrates  that  all  three  support  the  weight  while 
standing. 

Function  of  the  Check  Ligaments. — Horses  are  enabled  to 
sleep  while  standing,  and  remain  for  some  considerable  time 
without  lying  down,  by  means  of  a  singular  arrangement 
of  so-called  *  check '  ligaments  which  exists  in  both  fore  and 
hind  limbs ;  we  have  previously  touched  on  this  question 
so  far  as  the  hind-limb  is  concerned.  The  flexor  tendons 
of  the  fore-limb  support  the  weight,  the  extensors  keep  the 
limb  rigid.  In  order  that  the  strain  of  supporting  the 
weight  may  not  be  placed  solely  on  the  muscles  of  the  arm, 
both  flexor  and  extensor  tendons  receive  branches  of 
ligament  from  the  radius  and  metacarpus.  These  are 
attached  to  the  tendons  in  such  a  way  as  to  cut  off  the 
muscles,  at  any  rate,  to  a  considerable  extent,  from  the 
strain  of  standing  in  one  position  for  any  length  of  time.  In 
the  act  of  standing  the  rigidity  of  the  bony  column  of  the 
leg  is  maintained  by  the  extensor  tendons,  each  phalanx 
having  an  extensor  attached  to  it,  viz.,  the  extensor  meta- 
carpi  leading  to  the  large  metacarpal  bone,  extensor  pedis 
to  the  corona  and  pedis  (receiving  also  a  slip  from  the  sus- 
pensory ligament),  and  extensor  suffraginis  to  the  suffraginis. 
This  latter  receives  a  strong  slip  of  ligament  from  the  out- 
side of  the  carpus,  which  entirely  takes  off  the  strain  from 
the  muscle,  and  keeps  the  tendon  taut  during  sleep. 
Further,  the  horse  is  enabled  to  stand  whilst  sleeping  by 
means  of  the  fascia  of  the  arm  and  thigh  ;  both  of  these 
are  attached  to  the  muscles  and  tendons  of  the  part, 
affording  them  considerable  support  of  a  non-muscular 
nature. 

Centre  of  Gravity. — The  centre  of  gravity  at  rest  is  fixed, 
but  during  motion  it  oscillates  from  front  to  rear,  depend- 
ing on  the  position  of  the  body  and  the  pace.     Owing  to 


THE  LOCOMOTOK  APPAEATUS      515 

the  fact  that  more  weight  is  carried  on  the  fore  than  on  the 
hind  legs,  the  centre  of  gravity  lies  nearer  to  the  elbow 
than  the  stifle.  If  a  vertical  line  be  dropped  just  behind 
the  ensiform  cartilage  of  the  sternum,  and  intersected  by  a 
horizontal  one  passing  through  the  lower  part  of  the 
middle  third  of  the  body,  the  point  of  intersection  is  the 
centre  of  gravity  of  the  body  at  rest ;  this  is  the  rule  given 
by  Colin.  We  may  say,  speaking  roughly,  that  the  vertical 
line  passes  about  6  inches  behind  the  elbow,  the  horizontal 
jusfc  below  the  shoulder-joint ;  the  centre  of  gravity  is  where 
these  intersect.  It  is  obvious  that  the  position  of  the 
centre  of  gravity  will  vary  with  different  horses,  but  not  to 
such  an  extent  as  seriously  to  affect  the  truth  of  the  above 
statement.  During  locomotion  the  centre  of  gravity  moves 
to  the  front  and  rear  of  the  normal  at  rest ;  for  example, 
in  jumping  it  is  in  front  of  it  when  the  fore-legs  are 
coming  to  the  ground,  behind  it  when  the  hind-legs  are 
leaving  the  ground ;  it  is  in  front  of  it  during  draught, 
behind  it  during  backing. 

Distribution  of  the  Weight  of  the  Body. — The  fore-legs 
carry  more  weight  than  the  hind,  which  is  perhaps  the 
reverse  of  what  might  be  expected ;  but  if  a  horse  be 
carefully  weighed,  it  is  found  that  the  fore-legs  take  more 
than  one-half  the  body  weight.  The  position  of  the  head 
(which  may  weigh  as  much  as  40  and  50  lbs.)  considerably 
affects  the  weight  on  the  legs.  Thus,  if  the  head  be  raised 
up  when  the  fore-legs  are  weighed,  the  latter  will  be  found  to 
be  carrying  over  20  lbs.  less  weight  than  if  the  head  were 
dependent.  The  practical  application  of  this  fact  is  obvious 
— keep  a  stumbler  well  in  hand.  When  a  man  is  on  the 
horse's  back,  it  is  found  that  66  per  cent,  of  his  weight  is 
carried  on  the  fore-legs,  and  34  per  cent,  on  the  hind ;  the 
amount  of  weight  on  the  fore-legs  is  increased  by  leaning 
forward  in  the  saddle,  and  decreased  by  leaning  back.  An 
explanation  why  fore-legs  are  worn  out  earlier  than  hind 
is  afforded  us  by  what  we  now  know  of  the  physiology  of 
locomotion — viz.,  the  fore-legs  act  as  propellers  of  the 
body,  and  owing  to  their  being  nearest  to  the  centre  of 

33—2 


516     A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

gravity,  they  also  bear  the  largest  share  of  the  weight  of 
the  body  and  the  weight  of  the  rider. 

The  Structure  and  Function  of  the  Limbs  in  Relation  to  the 
Production  of  Lameness. — As  high  up  as  the  shank  we  may 
say  that  there  is  no  practical  difference  in  the  anatomical 
arrangements  of  the  fore  and  hind  limbs,  and  yet  we  know 
how  commonly  the  foot  and  coronet  of  the  fore-leg  are 
affected  with  lameness,  and  how  rarely  in  comparison  the 
hind  one.  In  comparing  the  knee  and  hock  great  differences 
are  observed ;  it  is  true  that  in  both  a  number  of  pieces  of 
bone  enter  into  their  formation,  but  here  the  likeness  ends  ; 
the  small  bones  of  the  knee  have  considerable  movement, 
the  small  bones  of  the  hock  only  a  trifling  amount.  The 
lower  row  of  knee  bones,  so  far  as  movement  is  concerned, 
are  the  nearest  approach  to  the  movement  of  the  small 
bones  of  the  hock,  yet  the  latter  are  frequently  diseased, 
the  former  rarely  affected.  Evidently,  then,  the  presence  of 
small  and  comparatively  immobile  bones  in  the  hock  cannot 
constitute  an  explanation  of  the  frequency  of  hock  disease. 
Does  the  manner  in  which  the  joints  are  flexed  throw  any 
light  on  the  acknowledged  fact  that  knee  disease  is  rare 
and  hock  disease  frequent  ?  It  will  be  observed  that  these 
two  joints  bend  in  opposite  directions ;  the  knee  opens  in 
the  front  when  flexed,  the  hock  opens  at  the  back ;  we  have 
already  given  reasons  for  believing  that  some  injury  may  be 
inflicted  on  the  hock-joint  by  its  method  of  closing. 

Continuing  this  comparison  of  the  fore  and  hind  leg,  it 
may  be  remarked  that  the  stifle  corresponds  to  the  elbow, 
and  the  patella  to  the  ulna ;  during  flexion  of  these  joints 
the  elbow  opens  at  the  back  whilst  the  stifle  opens  in  front ; 
in  other  words,  though  corresponding  joints — the  elbow 
and  the  stifle,  the  hock  and  the  knee — they  do  not  agree  in 
the  direction  in  which  their  movement  is  made.  The  hip- 
joint  corresponds  to  the  shoulder-joint,  and  though  in  the 
hip  all  the  movement  is  done  by  one  bone  instead  of  two, 
yet  the  to-and-fro  movement  is  practically  the  same  in 
each. 

When  the  fore-leg  comes  to  the  ground,  no  matter  what 


THE  LOCOMOTOK  APPARATUS      517 

the  pace  may  be,  the  limb  must  be  straight  in  order  that 
the  foot  may  be  placed  down  flat,  or,  as  in  the  faster  paces, 
heel  first.  This  straightening  of  the  knee  renders  the  bony 
column  of  the  leg  rigid  for  the  time  being ;  the  shock  of 
impact  is  therefore  greatest  at  that  part  of  the  column 
nearest  to  the  point  of  impact,  and  decreases  as  it  passes 
up  the  leg.  It  would  be  anticipating  our  subject  to  attempt 
to  deal  with  the  various  means  which  exist  in  the  foot  to 
render  this  shock  as  little  destructive  as  possible ;  we  can 
only  allude  to  the  weight  being  supported  on  the  laminae, 
to  the  presence  of  a  foot  articulation  which  is  yielding 
posteriorly,  the  existence  of  an  elastic  movement  of  the 
posterior  part  of  the  foot,  and  the  presence  of  an  elastic 
and  indiarubber-like  cushion,  the  foot-pad. 

There  are,  however,  two  distinct  strains  imposed  on  a 
limb — viz.,  the  shock  or  concussion  when  the  foot  comes  to 
the  ground,  and  the  strain  or  compression  occasioned  when 
it  is  leaving  the  ground ;  one  is  the  concussion  of  impact, 
the  other  the  compression  of  propulsion.  The  hind-leg 
differs  from  the  fore-limb  in  its  method  of  providing  for 
the  concussion  of  impact ;  here  we  find  that  the  limb 
instead  of  being  straight — as  the  fore-leg  is  from  the  elbow 
to  the  foot— is  bent,  and  it  is  bent  at  the  hock,  at  a  point 
which  we  may  take  to  be  midway  between  the  stifle  and 
the  ground.  The  shock  of  impact  comes,  therefore,  largely 
on  the  hock. 

The  fore-leg  in  providing  for  propulsion  rotates  over  the 
foot,  the  limb  still  being  straight  from  the  elbow  to  the 
ground,  and  the  shock  of  rotation  is  mainly  confined  to  the 
lower  end  of  the  bony  column.  In  the  hind-leg  propulsion 
is  obtained  not  only  by  the  foot  remaining  fixed  on  the 
ground,  but  also  at  the  same  time  by  a  straightening  or 
unbending  of  the  hock,  which  gradually  opens  until  the 
tibia  forms  with  the  metatarsal  bone  the  nearest  straight 
line  it  is  capable  of  making.  In  this  way  we  may  say  that 
the  hock  performs  twice  as  much  work  as  the  knee,  and 
such  a  statement  throws  some  possible  light  on  the 
frequency  with  which  this  joint  is  affected  with  disease. 


518     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  Anti-concussion  Mechanisms  existing  in  the  limb  are, 
roughly  speaking,  of  two  kinds — viz.,  (1)  those  for  receiving 
the  weight  of  the  body  on  the  leg  when  the  foot  comes  to 
the  ground,  without  the  part  suffering  from  the  concussion 
of  impact,  and  (2)  those  which  admit  of  propulsion  by  one 
fore-limb  without  the  parts  suffering  from  the  compression 
of  propulsion.  The  first  is  principally  provided  by  the 
yielding  joints  formed  in  the  pedal  and  fetlock  articulations, 
by  the  arrangement  of  the  foot,  and  b}'  the  tendinous  and 
ligamentous  material  at  the  back  of  the  limb ;  the  second 
is  furnished  by  the  column  of  bones  forming  the  limb  being 
broken  up  from  the  scapula  to  the  pedis,  and  progressively 
increasing  in  size  from  the  seat  of  the  largest  amount  of 
compression — viz.,  the  foot — to  the  least  amount  in  the 
shoulder.  Probably  the  coronet  and  pastern  represent  the 
weakest  part  of  the  fore-limb,  and  their  small  size  in 
comparison  with  the  weight  they  have  to  support  is 
evidence  of  this. 

To  ease  the  skeleton  from  concussion  the  muscles  and 
tendons  are  brought  into  play  and  rendered  taut ;  we  know, 
for  instance,  how  much  better  a  limb  is  prepared  to  stand 
a  sudden  shock  if  sufficient  warning  is  given  through  the 
sense  of  sight. 

The  tendons  and  muscles  of  the  limbs  help  to  take  the 
shock.  So  long  as  the  muscles  maintain  their  elasticity  the 
work  done  by  their  tendinous  attachments  is  comparatively 
slight ;  as  the  muscles  tire  the  strain  on  the  tendons 
increases,  and  in  consequence  they  may  give  way,  and  this 
will  occur  at  their  weakest  part.  In  this  tired  condition  of 
limb  the  skeleton  also  suffers,  the  bones  forming  the  column 
receive  more  shock  than  normal,  and  the  smallest  and 
shortest  bones  situated  nearest  to  the  seat  of  concussion, 
viz.,  the  ground,  may  even  fracture  under  the  strain,  and 
under  any  circumstances  run  a  grave  risk  of  becoming 
inflamed.  This  argument  is  based  on  clinical  observation ; 
we  do  not  believe  that  any  riding  horse  sprains  its  back 
tendons  or  suspensory  ligament  until  the  muscles  tire,  and 
are  no  longer  capable  of  exhibiting  that  perfect  elasticity 


THE  LOCOMOTOR  APPARATUS      519 

inherent  in  muscular  tissue.  We  do  not,  however,  say  that 
no  horse  suffers  in  its  pastern  bones  until  the  muscles  tire 
(for  example  the  cart  horse),  though  the  strain  on  them  is 
undoubtedly  greater  at  this  time  than  any  other.  The 
strain  on  the  pastern  bones  during  draught  depends  upon 
the  force  exerted,  viz.,  the  compression  of  propulsion,  and 
that  this  is  something  considerable  may  readily  be  seen  in 
any  heavy  draught  work. 

Fractures  of  the  pastern  also  teach  us  some  useful 
lessons ;  we  may  regard  them  for  our  purpose  as  experi- 
mental evidence  of  the  shock  inflicted  on  the  lower  bones 
of  the  limb.  This  shock  is  caused  when  the  foot  comes  to 
the  ground,  not  when  it  leaves  it,  and  it  may  occur  on  hard 
ground  or  on  sand ;  in  the  former  case  the  cause  of  the 
concussion  is  obvious,  in  the  latter  at  first  sight  it  is  not 
so  clear,  yet  when  we  remember  how  rapidly  horses  tire 
when  working  at  any  fast  pace  over  sand,  and,  owing  to 
the  nature  of  the  ground,  the  manner  in  which  they  must 
misjudge  the  application  of  that  muscular  bracing  which 
saves  the  skeleton  from  concussion,  it  is  not  difficult  to 
explain  the  well-known  fact  that  pasterns  frequently 
fracture  on  sandy  soil.  Direct  concussion  in  a  horse  which 
is  not  tired  and  is  not  working  on  sand  may  also  produce 
a  fracture  of  this  region.  Fractures  of  the  pastern  may 
occur  from  galloping  horses  on  the  wet  sand  of  a  seashore ; 
this  is  the  result  of  concussion  :  the  next  hardest  thing  to  a 
macadamized  road  is  a  wet  seashore. 

Our  only  object  in  dealing  with  a  subject  which  appears 
to  be  foreign  to  the  one  under  consideration  is  to  bring 
some  light  to  bear  on  the  strain  to  which  the  skeleton  is 
exposed.  This  strain  would  appear  to  be  greatest  on  the 
suffraginis  in  the  fore-limb,  for  fracture  of  this  bone  is 
incomparably  more  common  than  fracture  of  the  corona, 
though  this  might  be  accounted  for  by  the  density  of  the 
latter  and  the  absence  of  a  medullary  canal.  In  concluding 
these  remarks  on  fracture  of  the  pastern,  we  would  draw 
attention  to  the  fact  that  the  strain  imposed  on  the  bones 
in  all  cases  is  probably  nearly  identical  in  direction,  for 


520    A  MANUAL  OF  VETEKINAKY  PHYSIOLOGY 

there  is  a  remarkable  similarity  in  appearance  presented 
by  fractures  of  either  the  corona  or  suffraginis,  the  fractured 
portions  agreeing  in  shape  and  size,  in  some  cases  almost 
piece  for  piece. 

In  spite  of  what  we  have  said  about  direct  concussion 
affecting  the  pastern  bones,  we  do  not  think  that  this  is 
necessarily  the  only  factor  present  in  the  production  of 
ringbone.  The  compression  of  propulsion  must  take  a  part ; 
we  mean  by  this,  the  shock  imparted  to  the  pastern  bones 
while  the  foot  is  on  the  ground  and  the  body  is  passing 
over  it.  The  fore-leg  from  the  knee  to  the  foot  is  only 
intended  to  open  and  close  in  one  direction ;  we  can  readily 
make  the  foot  touch  the  point  of  the  elbow,  but  we  cannot 
make  it  touch  the  front  of  the  fetlock.  Now  if  we  study  the 
movement  the  limb  makes  from  the  time  the  foot  comes  in 
contact  with  the  ground,  we  observe  that  the  fetlock  at 
first  descends  and  then  ascends,  ;ind  having  reached  the 
desired  point  the  limb  passes  over  the  foot  which  remains 
fixed  on  the  ground,  and  at  this  moment  an  important 
movement  occurs  in  the  pastern,  viz.,  its  rotation  from 
rear  to  front.  While  the  fetlock  is  ascending  the  meta- 
carpal is  moving  on  the  suffraginis,  the  suffraginis  on  the 
corona,  and  the  latter  on  the  pedis ;  but  as  soon  as  the  limb 
becomes  vertical  (in  the  rotation  of  the  body  over  the  foot), 
the  movement  between  the  suffraginis  and  corona  becomes 
exceedingly  limited,  and  for  all  practical  purposes,  owing 
to  their  Immohility,  the  tiro  may  be  refiarded  as  one  hone  ; 
thus  the  remaining  rotation  of  the  body  occurs  between 
the  corona  and  pedis.  It  is  only  possible  to  understand 
this  by  following  it  out  on  the  dead  limb,  the  leg  being 
upright  and  the  foot  fixed. 

The  important  point  is  this — during  the  rotation  of  the 
body  over  the  foot  considerable  compression  and  strain 
must  be  experienced  in  the  pastern ;  this  strain  is  most 
severely  felt  at  the  articulation  between  the  suffraginis  and 
corona,  owing  to  the  fact  that  these  are  loeked  together 
during  the  main  extension  of  the  limb.  Further,  as  the 
upward  and  forward  propulsion  to  the  body  is  given  as  the 


THE  LOCOMOTOE  APPARATUS      521 

foot  is  leaving  the  ground,  much  of  the  shock  resulting 
from  it  must  be  expended  on  the  pastern  bones. 

Irregularities  in  the  ground  surface  are  a  severe  strain 
on  the  coronets,  especially  of  a  horse  out  of  condition. 
The  study  of  hoof  prints  will  readily  demonstrate  the  fact 
of  uneven  tread  ;  the  lateral  deviation  of  the  coronet  and 
pedal  joints  is  something  very  small,  yet  every  uneven 
tread,  be  it  caused  by  a  rut  or  a  pebble,  throws  a  strain  on 
these  parts,  and  there  can  be  no  doubt  that  many  cases  of 
ringbone  originate  in  strains  of  the  lateral  ligaments  of 
these  joints. 

In  the  Act  of  Standing  the  body  is  supported  on  four 
props ;  two  of  them  have  only  a  muscular  attachment  to 
the  trunk,  the  other  pair  are  united  by  a  ball  and  socket 
joint.  It  is  unnecessary  to  allude  by  name  to  the  muscles 
connecting  the  fore-leg  with  the  trunk,  excepting  the 
serratus  magnus  through  the  medium  of  which  the  body 
is  principally  slung  on  the  scapulae.  No  matter  what  the 
position  of  standing  may  be,  the  horse  never,  in  a  state  of 
health,  keeps  its  fore-feet  in  any  other  position  than  together ; 
one  fore-limb  advanced  in  front  of  the  other  is  abnormal 
excepting  when  grazing.  On  the  other  hand,  it  is  very 
rare  to  see  a  horse  standing  squarely  on  both  hind-legs,  he 
is  invariablj^  resting  the  limbs  alternately.  Some  years 
ago  we  drew  attention  to  this  as  being  an  explanation  of 
the  exemption  of  the  hind-limbs  from  navicular  disease  ; 
by  this  process  of  resting,  the  compression  of  the  navicular 
bone  (through  the  body  weight  above,  and  the  perforans 
tendon  below)  is  relieved.  The  horse  only  learns  to  rest 
the  fore-feet  when  too  late. 

In  Lying  Down  the  animal  brings  the  four  legs  together 
under  the  body,  and  bends  both  knees  and  hocks,  the  knees 
and  chest  touching  the  ground  before  the  hind-quarters. 
When  down  he  either  lies  extended  on  one  side  or  seated 
on  the  chest,  two  lateral  legs  being  under  the  body  and 
two  outside  it.  If  resting  on  the  chest  he  inclines  to  one 
side  or  the  other ;  he  cannot  like  a  ruminant  lie  plumb  on 
the   keel    of   the    sternum  owing   to  its   sharp  ridge.     If 


522    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

inclined  to  the  near  side,  the  near  fore-foot  is  placed  close 
to  the  breast-bone,  the  elbow  touching  the  ground,  the 
near  hind-foot  is  under  the  abdomen  and  he  lies  on  the 
outside  of  the  hock  and  shank  ;  the  oflf  fore-foot  lies  close 
to  the  off  elbow  but  as  a  rule  outside  it,  and  the  point  of 
the  off  hock  touches  the  ground.  A  horse  does  not  lie  long 
in  one  position  owing  probably  to  the  enormous  weight  of 
his  body.  It  will  be  observed  that  the  animal  lies  on  the 
point  of  the  elbow  which  is  underneath  the  body.  This  is 
the  cause  of  '  capped  elbow,'  and  not  that  usually  assigned, 
viz.,  resting  on  the  heel  of  the  shoe.  When  down  the 
animal  is  either  sitting  on  its  chest  or  lying  on  its  side,  but 
in  any  case  no  position  is  maintained  for  any  great  length 
of  time.  He  may  sleep  sitting  on  his  chest,  in  which  case 
he  rests  the  chin  on  the  ground  with  the  lower  lip 
frequently  everted  and  so  rests  on  the  incisor  teeth.  The 
eyes  are  never  completely  closed,  and  he  is  the  lightest 
sleeper  imaginable. 

Cattle  repose  on  the  breast  with  the  head  turned  round 
to  the  side. 

In  Rising  the  horse  can  only  get  up  by  extending  both 
fore-feet  in  front  of  the  body ;  the  hind-quarters  are  now 
pressed  upwards,  the  animal  securely  fixing  his  toes  in 
the  ground,  and  assisted  by  the  muscles  of  the  back,  the 
animal  is  immediately  on  his  feet,  the  fore-part  always 
rising  before  the  hind.  The  ruminant  rises  quite  dif- 
ferently, in  fact  the  reverse  of  the  horse,  the  hind-quarters 
being  the  first  to  ascend. 

Locomotion.  —  We  have  now  to  study  the  question  of 
locomotion  in  the  horse,  and  describe  how  the  legs  are 
moved  during  the  different  paces.  It  will  be  remembered 
that  our  knowledge  of  this  subject  chiefly  depends  upon 
graphic  records  and  instantaneous  photography,  the 
pioneers  in  the  field  being  Marey*  in  France,  Stanford, 
Stillman,  and  Muy bridge  in  America.!  We  have  selected 
typical  studies  in  order  to  elucidate  the  text,  but  it  must 

*  'Animal  Mechanics,'  International  Scientific  Series, 
t  '  The  Horse  in  Motion.' 


THE  LOCOMOTOE  APPARATUS 


523 


be  remembered  that  no  hard-and-fast  attitudes  can  be 
adopted.  The  sharp  quick  clever  horse  whose  muscular 
response  is  rapid  does  not  move  his  legs  quite  in  the  same 
way  as  the  slow  lethargic  indifferent  type  ;  and  similarly 
a  tired  horse  moves  differently  from  one  whose  muscles 
are  fresh  and  responsive. 

The  Walk  (Fig.  126)  is  the  slowest  pace,  the  movements 
are  somewhat  complex,  and  may  roughly  be  divided  into 


Fig.  126. — The  Walk  (Ellenberger). 

four  stages.  In  the  first  the  body  is  balanced  on  three 
legs,  in  the  second  stage  on  two  diagonal  legs,  in  the 
third  on  three  legs,  in  the  fourth  on  two  lateral  legs,  and 
the  next  movement  brings  it  back  to  the  first  stage,  only 
with  different  legs  employed.  Tracing  the  movements  in 
each  stage,  the  horse  advances  one  fore-leg — say,  the  off 
(Fig.  126,  i) — and  is  left  standing  on  the  near  fore,  near 
hind,  and  off  hind  ;  in  the  second  stage  the  near  hind  is 


524     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

picked  up,  and  the  animal  is  standing  on  the  near  fore 
and  off  hind,  viz.,  on  diagonal  legs  (Fig.  126,  ^)  ;  in  the 
third  stage  the  off  fore  has  come  to  the  ground,  and  the 
animal  is  balanced  on  both  fore  and  the  off  hind  leg 
(Fig.  12(),  3)  ;  in  the  fourth  stage  the  near  hind  is 
advanced  to  be  placed  over,  or  in  advance  of,  the  track 
of  the  near  .  fore ;  to  make  room  for  it  the  near  fore  is 
advanced,  and  the  horse  is  left  standing  on  two  lateral 
legs,  viz.,  off  fore  and  off  hind  (Fig.  126,  4).  The  next 
movement  brings  the  animal  into  the  first  position,  with 
the  near  fore  leading  instead  of  the  off  fore.  The  fore- 
leg remains  on  the  ground  for  a  longer  time  than  it 
takes  in  passing  through  the  air,  and  comprises  the  period 
during  which  the  body  is  passing  over  the  limbs.  The 
movement  in  the  air  both  of  fore  and  hind  legs  is  so 
extremely  rapid  as  almost  to  defy  detection.  The  snatching 
up  of  the  foot  from  the  ground  is  the  quickest  movement. 
Stillman  refers  it  to  the  spring  or  rebound  of  the  suspensory 
ligament,  but  it  is  doubtless  due  entirely  to  the  flexor 
muscles.  In  walking  on  level  ground  the  majority  of 
horses  rarely  extend  the  knee  any  great  distance  beyond  a 
vertical  line  dropped  from  the  point  of  the  shoulder.  A 
sudden  movement  of  the  extensors  now  straightens  the 
leg,  and  the  foot  is  placed  down  flat  or  heel  first.  If  the  leg 
is  not  fully  straightened  by  the  extensor  muscles,  the  foot 
comes  to  the  ground  toe  first,  with  the  knee  slightly  bent, 
and  a  stumble  follows.  In  heavy  draught  work  it  is  no 
uncommon  thing  to  see  the  toe  put  down  first,  but  here  the 
conditions  are  very  different.  It  appears  to  be  a  matter  of 
indifference  with  which  fore-leg  an  animal  starts  the  walk  ; 
under  some  conditions  he  may  indeed  make  the  first  step 
with  a  hind  leg,  in  which  case  the  next  to  move  is  the 
corresponding  fore-leg  in  order  to  make  way  for  the  hind 
foot. 

The  Trot  (Fig.  127)  is  a  very  simple  pace  to  analyse  ;  the 
body  is  supported  on  diagonal  legs  (Fig.  127,  1),  which  by 
their  propulsion  drive  it  off  the  ground,  during  which 
period  all  the  legs  are  in  the  air  (Fig.  127,  2) ;  when  the 


THE  LOCOMOTOR  APPARATUS 


525 


Fig.  127.— The  Trot. 
From  instantaneous  2>hoio(jra2)hs  by  0.  Anschiit~.     (Ellenherger.) 


526     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

body  comes  to  the  ground  again  the  next  pair  of  diagonal 
legs  receive  it  (Fig.  127,  3),  and  once  more  propel  it. 
There  are  thus  three  stages  to  the  trot ;  the  body  in  two  of 
them  is  supported  by  diagonal  legs,  and  in  one  of  them  it 
is  in  the  air. 

The  trot  appears  to  be  the  only  pace  in  which  instan- 
taneous photograi)hy  has  supported  the  conventional 
notions  of  this  movement.  We  can  see  the  trot,  first 
because  it  is  a  simple  pace,  and  secondly  because  the  body 
is  comparatively  long  in  the  air.  When  a  horse  falls  at 
the  trot,  he  does  so  either  through  not  flexing  his  knee 
sufficiently  before  bringing  the  leg  forward,  or  the  extension 
of  the  knee  is  not  perfect,  and  in  consequence  the  limb  is 
unfit  to  stand  weight.  The  knee  should  be  sufficiently  but 
not  unduly  bent  and  the  leg  brought  rapidly  forward,  the 
limb  then  sharply  extended,  well  braced,  and  the  foot 
placed  firmly  on  the  ground  heels  first. 

In  the  Amble  the  horse,  instead  of  using  diagonal  legs 
uses  the  lateral  limbs,  so  that  off  fore  and  off  hind  are  on 
the  ground  instead  of  off  fore  and  near  hind.  An  animal 
may  amble  both  at  the  walk  and  trot,  in  this  respect 
resembling  a  camel.  There  is  no  doubt  that  it  is  a  per- 
fectly natural  pace  for  some  horses  ;  others  are  taught  it, 
as  it  is  a  particularly  pleasant  one  for  the  rider  and  less 
fatiguing  for  the  horse. 

In  the  Canter  (Fig.  128)  the  body  is  pushed  upward  off 
the  ground  by  one  fore-leg — we  will  say  the  off  fore 
(Fig.  128,  1) — the  near  fore  and  both  hind  being  off  the 
ground ;  in  the  next  stage  all  the  legs  are  off  the  ground 
though  the  feet  are  no  great  distance  from  it  (Fig.  128,  3) ; 
in  the  third  stage  the  body  returns  to  the  ground,  alighting 
on  the  near  hind-leg,  which  is  not  placed  under  the  centre 
of  gravity  as  in  the  gallop,  but  behind  it,  the  animal  being 
balanced  on  one  limb  only  (Fig.  128,  3) ;  in  the  fourth 
stage  the  off  hind  and  near  fore  come  to  the  ground 
together,  so  that  the  body  is  now  balanced  on  three  legs — 
viz.,  near  fore  and  both  hind  (Fig.  128,  4) ;  in  the  fifth 
stage  the  off  fore  comes  to  the  ground,  but  as  it  does  so 


THE  LOCOMOTOR  APPARATUS      527 


Fig.  128.— The  Canter. 

From  imtantancous  phofo</n'jJis  hi/  0.  Anschutz.     {Ellcnher<jer. 


528     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  near  hind  rises ;  the  animal  is  still  left  on  three  legs — 
viz.,  both  fore  and  off  hind  (Fig.  128,  5) ;  in  the  sixth  stage 
the  near  fore  and  off  hind  leave  the  ground,  the  horse 
being  balanced  on  the  oft"  fore  only  (Fig.  128,  6') ;  the  next 
movement  is  a  repetition  of  the  first,  the  off  fore  ^Dressing 
the  body  upwards.  In  the  example  quoted  the  off  fore  is 
the  leading  leg,  and  it  will  be  seen  that  it  is  this  which 
gives  the  final  propulsion  to  the  body.  This  is  the  explana- 
tion of  why  the  leading  leg  tires  so  early.  Though  it  is  a 
matter  of  indifference  which  leg  a  horse  leads  off  with  in 
the  walk  and  trot,  this  is  not  the  case  in  the  canter  or 
gallop.  There  are  some  animals  which,  so  long  as  they  are 
leading  with  the  leg  of  their  own  choice,  are  pleasant  in 
their  paces,  but  if  forced  through  fatigue  or  other  cause  to 
lead  with  the  opposite  fore  leg,  their  movements  are  rough 
and  clumsy  and  wanting  in  co-ordination.  It  should  form 
part  of  the  training  of  every  horse  to  teach  him  to  change 
his  leading  leg  in  the  canter  or  gallop  with  facility ;  this 
education  would  prevent  many  cases  of  sprain. 

The  Gallop  is  a  very  difficult  pace  to  describe,  and  the 
analysis  I  give  of  it  here  is  from  one  of  Muybridge's 
numerous  instantaneous  photographs. 

The  gallop  (Fig.  129)  consists  of  seven  stages ;  for 
simplicity  we  will  elect  to  describe  it  from  the  time  the 
animal  is  in  the  air,  with  no  legs  on  the  ground,  but  all 
four  of  them  brought  well  under  the  body ;  this  is  the  first 
stage  (Fig.  129,  1) ;  in  the  second  stage  one  hind-leg,  say 
the  off,  comes  to  the  ground,  the  foot  being  placed  down 
close  under  the  centre  of  gravity  and  not  behind  it  as  in  the 
canter  (Fig.  129,  2)  ;  in  the  third  the  near  hind  comes  to 
the  ground,  the  horse  now  being  balanced  on  two  hind- 
legs,  both  fore  being  in  the  air  (Fig.  129,  3) ;  in  the  fourth 
stage  the  off  fore  comes  to  the  ground,  but  the  animal  is 
not  balanced  on  three  legs  as  in  the  canter,  for  at  the 
moment  the  oft'  fore  comes  to  the  ground  the  off  hind  is 
extended,  leaving  the  horse  on  diagonal  legs — viz.,  off  fore 
and  near  hind  (Fig.  129,  4) ;  in  the  fifth  stage  the  near 
hind  leaves  the  ground,  the  animal  being  balanced  on  the 


THE  LOCOMOTOE  APPAEATUS  529 


G  h  7 

Fig.  129.— The  Gallop. 

After  Stanford,  Muyhridgc,  and  Stillman.     ('  Thr,  Horse  in  Motion.') 


34 


530     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

off  fore-leg  (Fig.  129,  5) ;  in  the  sixth  stage  the  near  fore 
comes  to  the  ground  (Fig.  129,  Oa),  and  the  off  fore  leaves 
it  (Fig.  129,  6b) — the  body  is  still  supported  on  one  fore- 
leg; in  the  seventh  stage  the  body  passes  over  the  near 
fore-leg  (Fig.  129,  7),  and  by  a  contraction  of  its  muscles 
the  entire  weight  is  lifted  off  the  ground,  and  propelled 
forwards  and  upwards  (Fig.  129,  1).  The  simplest  descrip- 
tion of  the  gallop  is  that  the  horse  takes  a  stride  with 
the  hind-legs  which  then  leave  the  ground  ;  he  next  strides 
with  the  fore-legs,  and  at  the  end  of  this  propels  the 
body  for  several  yards  through  the  contraction  of  the 
muscles  of  the  fore-leg  on  which  he  was  last  bearing. 
During  the  true  gallop  he  never  has  more  than  two  legs 
on  the  ground  at  the  same  time,  and  they  are  always 
pairs,  excepting  in  position  4,  Fig.  129. 

The  points  of  importance  in  both  the  gallop  and  canter 
are  that  the  heel  of  the  foot  comes  to  the  ground  first,  that 
the  hind-legs  break  the  shock  of  the  falling  body,  and  that 
the  fore-legs  take  the  largest  share  in  propelling  the  weight. 
Two  of  these  facts  were  described  years  ago  by  Lupton,  but 
were  not  accepted.*  In  examining  the  track  of  a  galloping 
horse  it  is  remarkable  to  observe  what  a  very  straight  line 
the  hoof-marks  leave,  showing  that  each  foot  is  brought 
well  under  the  middle  line  of  the  body.  When  a  horse 
gallops,  no  matter  how  fast  the  pace,  the  fore-feet  never 
extend  beyond  a  vertical  dropped  from  the  muzzle. 

In  the  Jump  (Fig.  130)  the  horse  rises  to  it  by  the  pro- 
pulsion upwards  which  the  fore-legs  give  to  the  body 
(Fig.  130,  1),  the  knees  at  the  same  time  being  flexed  to 
enable  the  feet  to  clear  the  obstacle.  Both  hind-legs  being 
fixed  on  the  ground,  the  body  is  through  these  propelled 
forwards  (Fig.  130,  2).  In  alighting  the  animal  does  so 
through  the  medium  of  the  fore-legs,  either  together  or  one 
following  the  other  but  always  straight  (Fig.  130,  5).  In- 
stantaneous photography  disproves  the  theory  that  in  the 
jump  a  horse  naturally  alights  on  the  hind-legs,  though 
it  is  true  that  some  clumsy  horses  do. 
*  See  footnote,  p.  559. 


THE  LOCOMOTOR  APPARATUS 


531 


Fig.  130.— The   Jump. 
(A  7ischutz-Ellenbe7yrr. ) 


34—2 


532     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

In  Rearing  the  hind-legs  are  brought  well  under  the 
body,  the  head  and  neck  are  thrown  up,  and  the  propelling 
power  of  the  fore-legs  directs  the  body  upwards,  where  it  is 
sustained  by  the  muscles  of  the  back  and  loins.  So  long 
as  the  centre  of  gravity  falls  within  the  base  formed  by  the 
hind-feet,  the  body  is  in  a  position  of  stable  equilibrium ; 
but  if  it  passes  outside  this,  the  horse  comes  back  on  to 
the  point  of  both  hocks,  and  may  either  roll  over  on  its 
side  or  go  directly  backwards.  If  the  latter,  the  first  part 
of  the  body  to  strike  the  ground  is  the  occiput ;  in  this  way 
fracture  of  the  base  of  the  skull  may  occur. 

In  Kicking  with  both  hind-legs  the  head  is  depressed, 
and  a  powerful  contraction  of  the  muscles  of  the  quarter 
and  back  throws  the  croup  upwards,  and  at  the  same  time 
both  legs  are  violently  extended.  Kicking  may  be  practised 
either  with  one  hind-leg  backwards  or  one  hind-leg  for- 
wards. The  latter  is  very  dangerous ;  fortunately  only 
an  accomplished  horse  can  effect  it ;  it  is  known  as 
'  cow-kicking.'  Owing  to  the  pubo-femoral  ligament  a 
horse  can  only  kick  outwards  with  difficulty.  Striking 
with  the  fore-feet  is  not  common,  and  is  not  character- 
istic of  British  horses,  nor  are  '  cow-kicking '  or  '  buck- 
jumping.' 

In  Buck-jumping  the  animal  springs  bodily  off  the  ground, 
the  head  being  suddenly  depressed  between  the  fore-legs 
and  the  back  violently  arched. 

The  Normal  Daily  Work  of  Horses,  the  rate  at  which  they 
are  capable  of  performing  it,  and  the  power  they  exercise 
in  doing  so,  must  now  be  briefly  considered.  Rankine  has 
laid  down  that  mechanical  daily  work  is  the  product  of  three 
quantities  :  (1)  the  effort ;  (2)  the  rate ;  (3)  the  number  of 
units  of  time  per  day  during  which  the  work  is  continued. 
Our  only  difficulty  is  in  obtaining  the  value  of  the  effort, 
which  it  is  clear  must  depend  upon  the  nature  of  the  work, 
the  character  of  the  ground,  the  weight  carried  or  drawn, 
and  the  physical  fitness  of  the  animal.  The  normal  work 
of  horses  would  appear  to  be  3,000  foot-tons  per  diem ;  a 
hard  day's  work  is   equivalent  to  4,000  foot-tons,  and  a 


THE  LOCOMOTOR  APPARATUS      533 

severe  day's  work  is  5,000  foot-tons.  Redtenbacher*  places 
the  daily  work  of  a  horse  for  8  hours  at  6,700  foot- tons, 
and  Rankine's  tables  +  show  that  a  draught  horse  exercising 
a  force  of  traction  of  120  lbs.  for  8  hours  a  day,  performs 
6,200  foot-tons  of  work.  I  think  both  these  estimates  are 
without  doubt  too  high.  The  co-efficients  of  resistance 
employed  in  our  calculations  were  those  determined  for 
man  by  the  Rev.  Professor  Haughton ;  we  know  of  none 
specially  calculated  for  the  quadruped.  Assuming  the 
weight  of  the  animal,  plus  the  weight  carried  or  drawn,  to 
be  equal  to  1,000  lbs.,  then  3,000  foot-tons  of  work  will  be 
obtained  by  the  following  work  : 

Walking   at   3  miles  per  hour  for  8"7  hours. 

M     4  „  „    5-3      „ 

))  )»     5  )i  )»    3'7       „ 

Trotting    .,8  ,,  ,,    1'5      „ 

Cantering  „  11  ,,  ,,1         „ 

This  table  is  only  given  as  a  means  of  conveying  to  the 
mind  the  value  of  3,000  foot-tons  of  work,  though  trotting 

12  miles  or  walking  18t  miles  are  commonly  done  in  practice 
as  a  day's  work. 

The  Velocity  of  the  gallop  has  been  variously  stated,  but 
it  is  certain  that  no  horse  has  galloped  1  mile  in  1  minute 
as  is  reported  of  Flying  Childers.  A  horse  named  Salvator 
in  1890,  carrying?  stones  12 lbs.,  was  galloped  on  a  straight 
course  against  time,  and  did  a  mile  in  1  minute  35 1^  seconds. 
The  most  severe  galloping  ever  recorded  was  performed  by 
Quibbler  in  1786,  who  galloped  23  miles  round  the  flat  at 
Newmarket  in  57  minutes  10  seconds.  The  fastest  pace  at 
which  trotting  has  been  performed  is  1  mile  in  2  minutes 
S^  seconds.  The  celebrated  American  trotting-horse  Tom 
Thumb  trotted  100  miles  in  10  hours  7  minutes,  including 
a  stoppage  of  37  minutes ;  an  English  mare  did  the  same 
distance  in   10  hours  14  minutes,  including  a  stoppage  of 

13  minutes,   while    Sir   E.   Astley's   Phenomenon   trotted 

*  Quoted  by  M'Kendriek. 

t  '  Encyclopsedia  Britannica,'  art.  '  Animal  Mechanics.' 


534     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

17  miles  in  53  minutes.  All  the  old  performances  here 
quoted  are  from  Youatt's  work  on  '  The  Horse.' 

Turning  now  to  what  may  be  expected  of  ordinary 
horses,  it  may  be  noted  that  the  average  walk  of  a  cavalry 
horse  is  3"75  miles  per  hour  ;  the  average  trot  is  7'5  miles 
per  hour,  or  a  mile  in  8  minutes,  and  a  fast  trot  is 
8h  miles  per  hour.  A  gallop  is  from  12  to  14  miles  per 
hour.  The  stride  of  horses  at  various  paces  was  measured 
in  a  very  ingenious  manner  by  Stillman  and  Muybridge. 
They  give  the  stride  at  the  walk  as  5  feet  6  inches ;  at  the 
trot  between  7  feet  and  8  feet ;  at  the  canter  about  10  to 
12  feet ;  and  the  gallop  between  16  feet  and  20  feet — they 
even  speak  of  a  stride  of  25  feet.  An  American  pacer  has 
been  known  to  cover  21  feet  in  a  stride. 

The  question  of  the  Weight  which  a  horse  can  carry  is 
one  affecting  the  vital  interests  of  the  cavalry  service  ; 
there  is  a  great  difference  between  the  actual  or  total  weight 
a  horse  can  carry  and  the  effective  weight  he  can  carry. 

The  question  of  weight  is  greatly  influenced  by  the  pace 
at  which  it  has  to  be  carried,  and  under  any  circumstances 
is  largely  governed  by  the  weight  of  the  animal's  own 
body.  We  have  shown  that  horses  should  not  be  asked 
to  carry  more  than  one-fifth  of  their  body-weight,  and 
this  conclusion  will  doubtless  apply  to  all  riding  horses.* 
One-fifth  of  the  body- weight  of  a  cavalry  horse  is  roughly 
14|  stones ;  instead  of  this  they  carry  about  20  stones. 

The  physiological  features  of  Draught  can  only  be  glanced 
at.  The  subject  of  draught  is  a  complex  one,  and  our  in- 
formation is  still  very  incomplete.  Quadrupeds  appear  to 
be  designed  for  the  purpose  of  draught,  a  horizontal  spine 
is  not  intended  for  carrying  weight ;  such  can  only  be 
satisfactorily  met  by  an  upright  column,  as  in  man,  who 
from  his  conformation  is  essentially  devised  for  carrying  a 
burden ;  the  horse,  on  the  other  hand,  is  constructed  for 
hauling  or  draught.     Brunei,  in  his  article  on  '  Draught,' -f- 

*  '  The  Effective  Weight  Horses  can  Carry,'  Jozirnal  of  Comparative 
Pathology  and  Tlierapeutics,  vol.  xi.,  No.  4. 
t  '  Book  of  the  Horse,'  Youatt. 


THE  LOCOMOTOR  APPARATUS      535 

points  out  that  the  reason  why  a  horse  is  more  suited  for 
draught  than  for  carrying  weight,  is  that  he  can  throw  his 
weight  considerably  in  front  of  his  centre  of  gravity,  the 
feet  forming  the  fulcrum,  and  '  allowing  the  weight  of  the 
body  in  its  tendency  to  descend  to  act  against  the  resistance 
applied  horizontally  and  drag  it  forward ;  as  the  resistance 
yields  the  feet  are  carried  forward  and  the  action  continued.' 
Such  is  the  theory  of  draught.  The  nature  of  the  vehicle, 
the  condition  of  the  roads,  the  angle  the  trace  forms  with 
the  horizontal,  the  presence  or  absence  of  springs,  four 
wheels  or  two,  high  or  low  front  wheels,  and  the  width  of 
the  track,  are  features  which  singly  or  combined  greatly 
complicate  the  question. 

The  force  exerted  in  draught  depends  upon  the  load  and 
the  pace ;  in  the  light  or  mail  stage-coach,  where  10  and 
11  miles  an  hour  were  attained,  the  strain  or  force  of  trac- 
tion employed  by  each  horse  was  only  40  lbs. ;  in  the  heavy 
coach  it  was  62i  lbs,  for  each  horse.  The  higher  the 
velocity  the  less  the  force  of  traction  which  can  be  em- 
ployed, and  the  shorter  the  duration  of  labour.  For  slow 
draught  work  at  2h  to  3  miles  per  hour,  and  for  8  hours  a 
day  (which  appears  to  be  the  most  suitable  pace  and  dura- 
tion of  labour),  a  force  of  traction  of  from  100  lbs.  to 
125  lbs.,  or  150  lbs.,  is  quoted  by  Brunei  as  being  the  most 
suitable.  But  a  force  of  traction  of  120  lbs.  for  8  hours  a 
day  is  too  much  to  expect.  Watt  found  that  a  horse  could 
raise  a  weight  of  150  lbs.  passed  over  a  pulley,  220  feet  per 
minute.  This,  as  applied  to  engines,  is  termed  '  horse 
power,'  and  is  equal  to  33,000  lbs.  lifted  1  foot  high  per 
minute,  33,000  foot-pounds  per  minute.  This  standard  of 
comparison  cannot  be  generally  applied  to  horse  labour,  as 
it  is  far  too  high.  An  animal  could  only  perform  this 
amount  for  Sh  hours  per  diem,  whereas  its  most  useful 
work  is  performed  in  8  hours. 

The  actual  dead  pull  which  a  horse  can  exert  depends 
upon  his  body-weight ;  no  animal  tested  by  me  against  a 
dynamometer  has  pulled  his  own  weight,  nor  should  we 
expect  it.     From  65  to  78  per  cent,  of  the  body-weight  was 


536     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

found  by  us  to  represent  the  maximum  muscular  effort  of 
the  horse.*  The  animals  tested  were  grouped  according  to 
the  spirit  they  put  into  their  work  : 

Group  '  excellent '  pulled  78'5  per  cent,  of  their  body-weight. 
M      'good'  „      77-6 

„       'fair'  „      70-6 

'bad'  „      65-6  „  -  „  „ 

Pathological. 

The  question  of  lameness  in  horses  must  always  occupy  a  prominent 
position  in  veterinary  practice.  It  is  intimately  bound  up  with  the 
locomotor  apparatus,  and  it  is  not  possible  to  attempt  any  useful 
summary  of  the  troubles  met  with.  The  anatomy  and  physiology  of 
the  locomotor  system  should  be  thoroughly  understood  by  the  student 
if  he  is  to  become  a  good  practitioner. 

*  '  The  Maximum  Muscular  Effort  of  the  Horse,'  Journal  of 
Physiology,  vol.  xix.,  1896. 


CHAPTEE  XVII 

THE  FOOT 

The  foot  is  largely  a  modified  form  of  skin,  the  vascular 
tissues  represent  the  corium,  while  the  horn  represents  the 
epidermis.  It  is  no  uncommon  thing  to  have  a  horn-like 
tissue  produced  by  the  skin,  as,  for  example,  in  the  human 
nail,  in  the  hand  of  the  labourer,  and  in  the  chestnut  and 
ergot  found  on  the  limbs  of  the  horse.  In  spite  of  its 
origin  from  the  skin,  the  foot  is  a  specialised  structure 
presenting  not  only  a  surface  for  wear  and  tear,  but 
mechanisms  for  supporting  the  weight,  and  others  devoted 
to  warding  off  from  both  the  foot  and  limb  the  concussion 
and  jar  to  which  such  a  structure  is  necessarily  exposed. 
If  it  were  not  for  the  mechanisms  just  alluded  to,  and 
were  the  foot  a  structure  simply  devoted  to  offering  a 
surface  of  sufficient  density  for  the  horse  to  stand  upon,  it 
would  present  little  of  special  interest. 

The  foot  may  primarily  be  divided  into  two  parts,  the 
insensitive  or  horn  foot  and  the  sensitive  or  vascular  foot. 
The  horn  is  produced  from  the  vascular  foot,  but  the  latter 
does  not  exist  solely  for  the  production  of  horn ;  it  is 
provided  with  a  fibrous  pad,  elastic  tissues,  a  peculiar 
arrangement  of  joint,  and  a  remarkable  corium,  the  collec- 
tive function  of  which  is  devoted  to  saving  the  parts  from 
destruction  during  the  battering  process  to  which  the  foot 
is  exposed,  and  further  to  support  the  weight.  These  two 
feet  the  sensitive  and  insensitive  are  closely  united ;  in  their 
general  configuration  one  is  an  exact  counterpart  of  the 
other,  and  one  fits  into  the  other  much  as  a  finger  fits  into 

537 


538     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

a  glove.  It  would  be  out  of  place  here  to  give  anything 
like  a  detailed  account  of  the  anatomy  of  the  foot,  but 
there  are  certain  structural  features  so  intimately  associated 
with  the  physiology  of  the  organ  that  it  is  impossible  to 
separate  them. 

Bones  of  the  Foot. — The  core  of  the  foot  consists  of  bone 
around  which  all  the  other  structures  are  moulded.  The 
bone  is  not  one  solid  piece,  as  we  might  imagine  would  be 
necessary  in  such  a  position,  but  on  the  other  hand  consists 
of  three  pieces.  One  of  these  is  the  pedal  bone  which  in 
shape  resembles  a  miniature  foot,  and  the  substance  of 
which  is  porous  to  such  an  extent  as  to  resemble  pumice- 
stone  in  appearance.  A  second  bone,  the  navicular,  is  very 
small,  of  peculiar  shape,  dense  in  structure,  rests  slightly 
on  the  pedal  bone,  and  is  mainly  held  in  position  by 
ligamentous  tissue.  The  third  bone  only  belongs  partly 
to  the  foot  and  partly  to  the  limb.  One  would  suppose  that 
the  pedal  bone  should  occupy  the  whole  of  the  interior 
of  the  hoof,  as  high  as  the  coronary  edge  and  as  far  back 
as  the  heels,  but  this  is  not  so.  It  only  occupies  a  com- 
paratively small  portion  of  the  internal  foot  (Fig.  131),  and 
that  portion  is  mainly  situated  towards  the  anterior  and 
lateral  parts ;  the  posterior  part  of  the  foot  contains  very 
little  pedal  bone,  but  the  deficiency  is  made  up  by  the 
introduction  of  two  large  plates  of  cartilage  attached  to 
the  bone,  and  over  which  the  structures  are  reflected  and 
moulded  as  on  the  bone  itself.  This  singular  deficiency  of 
bone,  in  a  part  where  one  might  be  led  to  regard  its 
existence  in  large  amounts  as  a  necessity,  and  the  presence 
of  large  cartilaginous  plates  to  take  its  place,  is  due  to  the 
various  movements  which  the  foot  has  to  perform,  and 
which  could  not  be  carried  out  if  the  bone  of  the  foot  were 
relatively  proportioned  to  the  structure  within  which  it  fits. 

The  Foot-joint. — Three  bones  form  the  foot-joint.  The 
question  naturally  arises  why  the  joint  is  not  composed  of 
two  bones  instead  of  three,  and  what  advantage  is  gained  by 
the  introduction  of  a  small  dense  bone  such  as  the  navicular 
into  the  articulation  ?      The  articulation  furnished  by  the 


THE  FOOT  539 

pedis  is  much  smaller  than  that  furnished  by  the  corona, 
but  by  the  introduction  of  the  navicular,  the  pedis  plus 
navicular  surface  is  nearly  but  not  quite  equal  to  the  corona 
surface ;  one  use,  therefore,  of  the  navicular  bone  is  to 
increase  the  articular  surface  of  the  pedis.  But  it  is  con- 
ceivable that  this  small  articular  surface  of  the  pedis  might 
have  been  increased  in  some  other  way  than  by  the  introduc- 
tion of  a  distinct  bone  and  other  complicated  apparatus,  and 
one  is  forced  to  recognise  that  the  value  of  the  navicular 
articulation  does  not  depend  entirely  on  the  fact  that  it 
increases  the  size  of  the  joint,  but  that  it  supplies  what  else- 
where we  have  spoken  of  as  a  yielding  articulation.  The 
value  of  this  yielding  articulation  appears  to  be  in  the  saving 
of  direct  concussion ;  the  weight  through  the  corona  comes 
in  the  first  instance  mainly  on  the  navicular,  which  under 
its  influence  yields  slightly  in  a  downward  direction ;  from 
the  navicular  the  weight  is  transferred  to  the  pedis  itself, 
which,  as  we  shall  later  have  to  point  out  more  particularly, 
also  yields  slightly  under  its  influence,  and  in  this  way  it  is 
undoubted  that  direct  concussion  to  the  joint  is  prevented. 

The  Navicular  Bone  and  Bursa. — It  is  quite  certain  that 
the  navicular  would  be  of  very  little  use  for  the  above 
purpose,  if  it  depended  on  being  kept  in  position  solely  by 
the  delicate  ligaments  which  have  origin  from  it.  The 
chief  support  to  the  navicular  bone  is  the  broad  expansion 
of  the  perforans  tendon  which  passes  beneath  it ;  between 
the  tendon  and  the  bone  the  most  intimate  fitting  occurs, 
and  a  synovial  apparatus  exists  here  to  save  friction.  It 
is  probable  that  the  perforans  tendon  and  the  inferior  face 
of  the  navicular  are  more  closely  adapted  to  each  other 
than  any  articulations  in  the  body,  excepting  those  found 
in  the  knee  and  hock  joints.  Briefly,  then,  the  small 
dense  navicular  bone  is  enabled  to  form  a  yielding  articula- 
tion in  the  foot,  owing  to  the  manner  in  which  it  is 
supported  in  position  by  the  powerful  perforans  tendon. 
It  might  be  argued  on  purely  theoretical  grounds  that 
a  small  bone  thus  placed  in  the  foot  would  be  very  liable 
to  damage,  and  such  we  know  practically  to  be  the  case. 


540    A  MANUAL  OF  VETEETNABY  PHYSIOLOGY 

It  is  not  our  intention  here  to  touch  on  the  subject  of 
navicular  disease,  excepting  in  so  far  as  it  helps  to  eluci- 
date the  physiology  of  the  part,  but  it  is  permissible  to 
regard  the  lesions  of  navicular  disease  in  the  light  of  a 
physiological  experiment,  and  we  learn  from  them  how 
intimately  the  freedom  and  elasticity  of  a  horse's  action 
depend  upon  the  navicular  bone,  and  the  stilty,  pottering, 
shuffling  gait  conferred  on  the  animal  when  the  navicular 
bone   is   no   longer    capable   of    properly   performing    its 


Fig.  131. — Longitudinal  Section  of  the  Foot. 

1,  The  corona  ;  2,  the  pedis ;  3,  the  navicular  ;  4,  the  horn  wall ;  5,  the 
horn  sole ;  6,  6,  the  foot-pad ;  7,  7,  the  plantar  cushion ;  8,  the 
perforans  tendon  passing  under  the  navicular  bone,  to  be  inserted 
in  pedis ;  9,  the  wall-secreting  substance ;  10,  the  extensor  pedis 
tendon  ;  11,  junction  of  wall  and  sole,  the  '  white  line.' 

functions.  The  very  close  support  afforded  to  this  bone  by 
the  perforans  tendon  may  possibly  be  a  cause  of  disease, 
for  the  conclusion  has  been  forced  on  us  that  under  the 
influence  of  the  weight  of  the  animal,  and  the  counteracting 
influence  of  the  perforans  tendon,  the  navicular  bone  must 
be  exposed  to  considerable  compression  (see  Fig.  131).  This 
view  is  mentioned  here  not  so  much  as  a  pathological  as  a 
physiological  factor. 

We  cannot  recognise  in  the  navicular  bone  any  pulley 
function  in  connection  with  the  perforans  tendon,  such  as 


THE  FOOT  541 

has  been  usually  described,  that  is  if  by  the  use  of  the  term 
pulley  it  is  sought  to  convey  the  impression  that  some 
mechanical  advantage  is  obtained.  It  is  true  that  by 
passing  beneath  the  navicular  bone  the  direction  of  the 
pull  of  the  tendon  is  slightly  altered,  but  no  mechanical 
advantage  is  thereby  derived.  The  perforans  tendon  at  its 
insertion  spreads  out  fan-shaped,  and  is  attached  over  a 
considerable  semilunar  surface  of  the  pedal  bone ;  so 
extensive  is  this  attachment  that  it  is  erroneous  to  believe 
the  tendon  plays  over  the  navicular  bone.  It  is  true  that 
movement  does  occur  between  the  tendon  and  the  bone, 
but  the  tendon  is  passive,  while  the  yielding  of  the 
navicular  bone  under  the  influence  of  the  body-weight  is 
the  active  agent.  It  is  curious  to  observe  the  direction  in 
which  the  largest  amount  of  friction  occurs  between  these 
two  surfaces,  Reasoning  from  the  position  of  the  parts 
one  would  think  the  greatest  amount  of  wear  must  occur 
at  the  moment  the  foot  comes  to  the  ground,  but  if  the 
eroded  tendon  of  navicular  disease  be  examined,  it  will  be 
observed  that  the  fibres  are  all  stripped  upwards  and  rarely 
or  never  downwards.  This  would  point  to  the  greatest 
friction  occurring,  not  when  the  bone  yields  under  the 
weight,  but  when  it  returns  to  its  place  as  the  body  passes 
over  the  foot ;  but  it  may  be  that  the  fact  is  capable  of 
a  different  explanation.  The  frequency  with  which  the 
central  ridge  of  the  navicular  bone  is  affected  with  disease, 
would  point  to  this  part  as  being  the  position  of  the 
largest  amount  of  pressure. 

Lateral  Cartilages. — Attached  to  the  heel  of  each  pedal 
bone  is  a  large  curved  plate  of  cartilage,  in  parts  fibrous, 
in  others  hyaline  in  nature.  So  extensive  is  this  plate  that 
it  reaches  high  above  the  margin  of  the  hoof — viz.,  outside 
the  foot  in  an  upvv'ard  direction  as  far  forward  as  the 
coronet  and  as  far  back  as  the  heel  (Fig.  141,  p.  568). 
There  is  no  other  structure  in  the  body  with  which  it  can 
be  compared :  a  bone  possessed  of  two  large  cartilaginous 
wings  is  a  something  peculiar  to  the  foot.  The  use  of 
these   cartilages   is   intimately   connected   with    the   main 


542     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

principles  of  the  physiology  of  the  foot,  to  be  dealt  with 
later. 

Plantar  Cushion.  —  Placed  between  the  two  plates  of 
cartilage  is  a  large  somewhat  pyramidal-shaped  body 
known  as  the  plantar  cushion  (Fig.  131,  7,  7).  In  appear- 
ance it  resembles  a  fibro-fatty  mass,  composed  of  inter- 
lacing fibres  and  fat,  pale  yellow  in  colour,  almost  destitute 
of  bloodvessels,  firm  to  the  touch,  yet  yielding  in  its  nature. 
It  occupies  the  posterior  part  of  the  foot,  rising  above  the 
hoof  into  the  hollow  of  the  heel,  whilst  its  inferior  face  is 
V-shaped,  and  a  complete  counterpart  of  the  horn  cushion 
or  food-pad  which  covers  it. 

The  Corium  of  the  foot  completely  covers  the  structures 
just  described,  viz.,  the  whole  of  the  pedal  bone,  a  large 
surface  of  the  lateral  cartilages,  and  the  plantar  cushion. 
This  tissue  has  received  various  names — viz.,  from  its 
colour  the  vascular  foot,  from  its  appearance  the  fleshy, 
from  its  character  the  velvety  foot,  whilst  from  one  of  its 
functions  it  has  been  termed  the  horn-secreting  foot. 

The  Vascular  Wall  or  laminal  tissue  (Fig.  132-2)  is  com- 
posed of  a  number  of  leaves  lying  side  by  side,  which  run 
from  the  coronet  downwards  and  forwards  to  the  edge  of 
the  wall.  In  number  there  are  about  500  or  600,  and  they 
invest  the  entire  surface  of  the  pedal  bone  and  the  greater 
part  of  the  lateral  cartilages,  their  extreme  vascularity 
giving  the  appearance  of  a  thin  layer  of  muscle.  The 
leaves  at  the  toe  are  longer  than  those  at  the  heel,  where 
they  are  short  and  turned  in  under  the  foot,  running  for- 
wards beneath  it  to  form  the  sensitive  bars. 

If  a  single  leaf,  say  at  the  toe,  be  removed  and  examined, 
it  is  found  to  commence  immediately  under  the  thick 
cornice-like  structure  known  as  the  coronary  substance, 
and  to  be  most  firmly  attached  to  the  pedal  bone ;  in  fact, 
so  intimate  is  the  attachment  that  it  is  almost  impossible 
to  remove  this  tissue  cleanly  from  the  bone.  The  edge 
of  the  leaf  is  not  regular  but  denticulated,  and  when  viewed 
from  its  face  it  is  observed  that  it  is  narrower  near  the 
coronet  than  at  its  inferior  part,  at  which  latter  place  it 


THE  FOOT  543 

terminates  in  five  or  six  papillae.  The  leaf  is  extremely 
vascular,  in  fact  quite  scarlet  in  colour,  the  effect  over  the 
whole  mass  of  leaves  being  very  striking  in  appearance. 
If  the  tissue  be  examined  microscopically  it  is  found  that 
part  of  its  substance  is  devoted  to  leaf  formation,  whilst 
the  remainder  is  a  sub-laminal  tissue,  the  function  of  which 
is  to  secure  the  laminas  firmly  to  the  wall  of  the  pedal 
bone.      This    sub-laminal    tissue    has   been   described   by 


V 

K 

M  -- 

■■ 

'■'^' 

1 

k  vV 

1^ 

-     2. 

s- 

iHi 

Fig.  1o2. — The  Vascular  Wall,  the  Hoof  being  removed. 

1,  The  wall-secreting  substance  with  its  papilLe ;  2,  the  sensitive 
laminae  ;  3,  where  the  upper  end  of  the  sensitive  laminte  run  into 
and  fuse  with  the  coronary  substance  ;  4,  a  line  between  1  and 
the  skin,  wliich  secretes  the  periople ;  5,  the  heels  of  the  plantar 
cushion. 

Moeller  as  consisting  of  two  layers  ;  the  one  nearest  the 
bone  is  designated  the  stratum  pcriostalc,  and  acts  as  the 
periosteum  of  the  bone  (Fig.  133,  e).  Outside  this  is  a 
layer  of  fibrous  connective  tissue  and  elastic  fibres,  arranged 
in  bundles,  crossing  and  forming  networks,  and  containing 
few  cellular  elements,  though  more  than  were  found  in  the 
periosteal  stratum  ;  this  layer  is  extremely  vascular  and 
has  been  designated  the  stratum  vasculoswn.  External  to 
this  layer  are  the  lamina3  formed  of  elastic  and  connective 


544    A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

tissue  fibres  as  in  the  previous  layer,  only  the  network  is 
much  finer.  The  laminae  contain  numerous  bloodvessels 
and  nerves. 

If   a   horizontal   section   of  the   laminae   be   made   and 


Fig.  133. — Horizontal  Section  of  the  Horn  and  Vascular  Wall 
OF  THE  Horse's  Foot.     Low  Magnification, 

a,  b,  c,  The  outer,  middle  and  inner  portions  of  the  wall,  showing  the 
canal  system  with  the  tubular  and  intertubular  horn  ;  d,  the 
horn  laminie  bearing  on  their  side  the  lamellae,  shown  black ; 
there  are  sometimes  a  few  short  lamina;  to  be  seen,  one  is  shown 
in  the  figure ;  e,  the  sub-laminal  tissue,  from  which  the  sensitive 
laminae  may  be  seen  dovetailed  between  the  horn  laminte,  and 
from  the  sides  of  which  grow  the  sensitive  lamellse. 

examined  microscopically,  it  can  easily  be  seen  that  each 
lamina  has  growing  from  its  free  edge  a  number  of  delicate 
processes  which  are  miniature  laminae,  or  as  they  have 
been  termed  secondary  laminae  or  lamellae  (Fig.  133,  (0  5  ^^ 


THE  FOOT  545 

number  they  are  from  100  to  120  on  each  leaf,  depending 
upon  the  size  of  the  primary  lamina.  The  appearance  thus 
presented  (Fig.  138,  d)  is  very  characteristic,  and  has  been 
likened  by  Chauveau  to  a  feather,  the  barb  being  the 
lamina  and  the  barbules  the  secondary  laminae. 

The  Wall-secreting  or  Coronary  Substance  is  a  thick,  half- 
round,  cornice-shaped  welt  of  material  situated  above  the 
laminae  (Fig.  132,  1) ;  it  has  received  several  names,  the 
most  rational  being  that  based  on  its  function  as  the 
wall-secreting  substance.  Externally  this  body  is  covered 
by  a  membrane  possessing  long  papillae  which  are  highly 
vascular,  and  readily  seen  by  immersing  the  foot  in 
water,  while  the  body  itself  on  section  is  fibro-fatty  in 
appearance.  It  extends  all  round  the  coronet  from  heel  to 
heel,  and  here  joins  the  plantar  cushion.  On  its  superior 
margin  is  a  narrow  groove  (Fig.  132,  4),  which  is  the 
dividing  line  between  skin  and  hoof,  and  from  which  the 
periople  is  secreted.  On  the  lower  margin  the  substance 
fuses  with  fibres  from  the  sensitive  lamina?.  The  entire 
coronary  substance  fits  into  a  half-round  groove  in  the  wall, 
and  the  papillae  on  its  surface  are  lodged  in  canals  formed 
in  the  horn.  Beneath  the  coronary  welt  is  a  well-developed 
subcutis,  which  unites  it  to  the  tissues  covering  the  corona 
and  to  the  lateral  cartilages.  The  vascular  papillated  mem- 
brane covering  the  coronary  substance  is  irregularly  pig- 
mented, corresponding  to  the  colour  of  the  horn  wall. 

The  Vascular  Sole  is  scarlet  in  colour,  and  covered  by 
long  papillae  which  are  lodged  in  the  horn  sole.  In  each 
papilla  an  artery  and  one  or  more  veins  may  be  found. 

The  corium  covering  the  plantar  cushion  is  similarly 
arranged,  the  papillae  being  lodged  in  the  foot-pad  or  horn 
frog. 

The  Blood  Supply  to  the  foot  is  exceedingly  rich. 
We  have  alluded  to  the  scarlet  appearance  presented  by 
the  laminae,  the  vascular  sole,  and  the  tissue  covering  the 
plantar  cushion ;  but  besides  these  the  coronary  cushion, 
pedal  bone,  etc.,  are  richly  supplied  with  blood.  The 
pumicestone-like  appearance  presented  by  the  latter  is  for 

85 


546  A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  purpose  of  affording  passage  to  the  mnumerable  vessels 
which  are  passing  from  the  interior  of  the  bone  in  an  out- 
ward direction  to  reach  the  vascular  tissues  ;  in  fact,  no 
description  or  drawing  can  adequately  convey  an  idea  of 
the  appearance  presented  by  this  vascular  body.  The 
veins  are  large  and  numerous  (Fig.  134)  and  are  not  pro- 
vided with  valves  ;  some  pass  through  the  substance  of  the 
lateral  cartilage,  and  a  large  plexus  exists  both  outside  and 
inside  the  cartilage.  The  relation  of  these  vessels  to  the 
lateral  cartilages  and  the  absence  of  valves  are  points 
which  will  occupy  our  attention  again  when  we  deal  with 
the  use  of  the  various  parts  of  the  foot. 


Fig.   184. — The  Venous  System  of  the  Horse's  Foot  (Storch). 

The  insensitive' foot  or  Hoof  is  moulded  over  the  sensitive 
structures  in  such  a  way  as  to  cover  them  completely,  and 
form  in  horn  a  perfect  counterpart  of  the  sensitive  foot. 
The  hoof  is  composed  of  a  wall  with  its  inflections  the  bars, 
a  sole,  and  a  foot-pad  or  frog ;  each  of  these  parts  must  be 
considered  separately. 

The  Wall  is  that  part  of  the  hoof  which  can  be  seen 
when  the  foot  is  on  the  ground ;  its  division  into  toe, 
quarters,  and  heels  is  for  convenience  of  description,  as  no 
natural  division  exists.  On  the  exterior  of  the  wall  is  a 
layer  of  horn  known  as  the  periople,  which  is  more 
apparent  near  the  coronet  where  it  is  white,  soft,  and  thick 


THE  FOOT 


547 


(Fig.  135),  than  lower  down  where  it  is  extremely  thin 
and  more  of  the  nature  of  a  varnish,  while  at  the  toe  it  is 
practically  absent.  This  layer  is  formed  from  the  upper 
edge  of  the  coronary  substance.  In  a  foot  which  has 
been  poulticed,  the  periople  at  the  coronet  stands  out  as 
a  white  band  running  from  heel  to  heel ;  this  appearance 
is  due  to  the  absorption  of  water  by  the  layer  of  soft  cells 
of  which  it  is  composed  at  the  coronet.  The  use  of  the 
periople  appears  to  be  to  cement  over  the  junction  of  the 
skin  and  hoof,  and  by  the  covering  it  affords  the  wall  to 


Fig.  135. — Thk  External  Foot  or  Hoof. 

The  fibrous  appearance  of  the  wall  maj'  be  seen,  also  the  ^jeriq/)Ze 
marked  x  ;  the  hair  of  the  edge  of  the  coronet  is  cHpped  away  to 
show  this  band  of  white  horn,  which  for  the  purpose  of  the  photo- 
graph was  swollen  by  immersion  in  water. 

assist  in  preventing  evaporation  from  its  surface.  The 
colour  of  the  wall  is  black,  or  black  and  so-called  white, 
really  buff;  a  black  horn  is  produced  by  a  pigmented 
coronary  substance,  a  buff  horn  has  no  pigment.  Non- 
pigmented  horn  is  weak  and  brittle,  and  grows  slowly. 
Such  feet  are  always  a  source  of  trouble. 

The  wall  is  thickest  and  longest  at  the  toe,  thinnest  and 
shortest  at  the  heel.  A  gradual  decrease  in  thickness  occurs 
from  front  to  rear,  but  if  a  section  of  the  wall  be  made  in 
the  direction  of  its  fibres,  it  will  be  found  that  whatever 

35—2 


548     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  thickness  may  be  at  that  j)articular  part,  this  thickness 
is  maintained  from  the  coronet  to  the  ground  surface.  The 
greater  thickness  of  the  wall  at  the  toe  and  quarters  as 
compared  with  the  heels,  is  connected  with  the  wear  and 
tear  of  the  hoof,  and  the  movements  which  the  latter 
undergoes  under  the  influence  of  the  body  weight.  If  the 
wall  were  as  thick  at  the  heels  as  at  the  toe  it  would  have 
been  a  rigid  box  ;  we  shall  have  to  show  that  it  is  a  yielding 
box,  and  that  the  yielding  which  occurs  corresponds  to  the 
thin  wall  of  the  heels.  The  reason  why  the  wall  is  thick 
at  the  toe  is  that  this  is  the  region  of  the  greatest  friction. 
The  wall  at  the  heels  is  suddenly  inflected,  running  under 
the  foot  in  a  forward  direction  for  a  short  distance,  and 
forming  an  acute  angle  with  the  wall.  This  inflected 
portion  of  the  wall  is  called  the  Bars,  and  in  the  gap  formed 
by  the  inflection  is  lodged  the  foot-pad.  Thus  the  wall  is 
an  incomplete  circle  of  horn,  the  circle  being  broken  at  the 
posterior  part  of  the  foot,  and  the  piece  of  wall  which  might 
have  completed  the  circle  is  sharply  bent  on  itself  and 
caused  to  run  in  practically  the  opposite  direction.  When 
we  consider  this  arrangement  it  is  easy  to  see  the  advantages 
gained  by  it ;  the  foot  is  not  a  rigid  body  but  a  yielding  one. 
It  would  be  difficult  to  understand  how  any  lateral  move- 
ment could  take  place  had  the  wall  been  a  complete  circle. 
The  value  of  the  inflected  portion  of  the  wall  is  rendered 
evident  when  we  bear  in  mind  the  lateral  movement  of 
the  foot.  From  their  position  the  bars  afford  additional 
strength ;  they  knit  the  structures  together  at  the  heel 
in  a  remarkable  way,  and  prevent  any  rupture  between 
the  wall  and  foot-pad  during  the  lateral  movements  of  the 
foot,  such  as  would  undoubtedly  have  occurred  had  the  wall 
and  foot-pad  been  directly  united. 

The  hind  feet  differ  from  the  fore  feet  in  shape,  being 
more  upright  and  narrower. 

On  examining  the  inside  of  the  hoof-wall  a  very  complex 
arrangement  presents  itself.  At  the  upper  edge,  corre- 
sponding to  the  coronet,  is  a  deep  semicircular  groove, 
deepest  at  the  toe  and  narrowest  at  the  heels,  in  which  is 


THE  FOOT  549 

lodged  the  thick  welt  of  tissue  previously  described  as  the 
wall-secreting  substance.  Covering  the  entire  surface  of  this 
groove  are  innumerable  pin-point  holes,  into  which,  as  may 
easily  be  seen,  the  papillne  which  project  from  the  '  substance ' 
are  lodged.  The  thickness  of  the  wall  at  any  one  place 
corresponds  to  this  coronary  substance,  and  from  it  the 
entire  horn  wall  is  secreted.  The  most  perfect  contact 
exists  between  the  wall- secreting  substance  and  the  groove 
in  which  it  is  lodged,  and  this  contact  is  further  assisted 
by  the  vascular  papilla  which  run  for  a  short  distance  into 
the  depths  of  the  horn  wall. 

Horn  Laminge. — On  the  inside  of  the  wall  of  the  hoof 
a  number  of  leaves  are  found  arranged  side  by  side,  running 
all  round  the  foot  from  heel  to  heel,  and  composed  of  delicate 
plates  of  horn.  It  is  easy  to  see  that  they  correspond 
in  size,  direction,  and  length,  with  the  vascular  or  sensitive 
laminae  previously  described,  and  like  them  they  possess 
secondary  horn  laminae  or  lamella.  These  insensitive  and 
sensitive  laminee  are  arranged  towards  each  other  in  a 
peculiar  way,  by  which  an  enormous  amount  of  strength  is 
obtained,  viz.,  by  the  process  of  dovetailing.  Each  insensitive 
lamina  fits  in  between  two  sensitive  laminae,  and  so  power- 
ful is  this  union,  that  in  endeavouring  to  separate  them  the 
vascular  lamina  will  often  tear  from  the  pedal  bone  rather 
than  rupture  the  dovetail.  In  this  way  the  most  intimate 
and  perfect  union  between  the  vascular  and  horn  wall  is 
brought  about,  and  in  addition  other  advantages  are 
obtained  which  will  be  dealt  with  shortly.  The  horn 
laminae  as  their  name  implies  are  composed  of  horn,  but 
the  secondary  laminae  which  invest  them  are  composed  of 
cells  which  are  a  something  between  horn  and  epithelium, 
viz.,  the  cells  have  not  undergone  a  true  horn  conversion 
but  remain  protoplasmic  in  nature  ;  this  is  recognised  by 
the  fact  that  they  readily  stain  with  carmine  whereas  horn 
does  not.  If  our  description  has  been  clear,  it  will  be 
observed  that  though  the  sensitive  and  insensitive  laminae 
dovetail  yet  they  are  never  in  actual  contact,  for  between 
them  are  the  lamellae,  both  sensitive  and  insensitive,  and  it 


550    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

is  actually  through  these  structures  that  the  intimate  union 
is  maintained  (Fig.  133,  d). 

It  will  be  remembered  in  speaking  of  the  vascular  laminae 
that  we  described  some  as  being  found  beneath  the  foot ;  in 
the  same  way  horn  lamina  corresponding  in  position  and 
number  to  these  are  also  found  under  the  foot,  and  are 
situated  at  that  part  which  has  been  described  as  the  bars. 
Clearly,  therefore,  the  bars,  though  situated  under  the  foot 
at  its  posterior  part,  are  a  part  of  the  wall,  inasmuch  as 
they  possess  all  the  essential  anatomical  elements  of  the 
wall  proper. 

The  Sole  of  every  normal  foot  is  concave,  that  of  the  hind 
feet  being  more  concave  than  of  the  fore.  This  concavity 
agrees  with  the  concavity  of  the  solar  surface  of  the  pedal 
bone,  which  in  itself  is  ample  evidence  that  the  general 
surface  of  the  sole  is  not  intended  to  bear  weight.  Soles 
vary  in  thickness,  some  being  very  rigid  and  firm,  others 
very  thin  and  yielding  ;  the  sole  cannot  be  too  thick.  The 
one  shown  in  Fig.  131  is  an  excellent  specimen  of  a  good  sole. 
The  growth  of  the  sole  is  peculiar ;  in  exactly  the  same  way  as 
noticed  in  the  wall,  the  papillae  from  the  vascular  sole  fit 
into  pin-point  holes  in  the  horn  sole,  and  horn  is  developed 
around  them.  But  here  the  resemblance  ends  ;  while  the 
horn  of  the  wall  is  capable  of  growing  to  almost  any  lengthy 
until  in  fact  it  curls  like  a  ram's  horn,  the  horn  of  the  sole 
can  only  grow  a  very  short  distance  before  the  fibres  break 
off,  and  scales  or  flakes  of  horn  are  the  result ;  these  either 
fall  out  or  are  pulled  out.  In  other  words,  the  foot  deter- 
mines for  itself  how  thick  the  sole  shall  be,  and  without 
any  assistance  the  fibres  break  off  when  the  proper  thick- 
ness has  been  attained,  and  allow  the  part  to  drop  out. 
This  shelling  out  of  the  sole  is  advantageous  in  the 
shod  foot,  inasmuch  as  the  part  not  being  exposed  to 
friction  cannot  wear  away.  In  parts  of  the  foot  such  as 
the  wall,  which  in  the  unshod  foot  are  exposed  to  friction, 
no  breaking  off  of  horn  fibres  is  required,  as  the  wear  and 
tear  maintain  the  part  at  its  proper  length  and  thickness. 
The  union  between  the  vascular  and  horn  sole  is  brought 


THE  FOOT  551 

about  by  the  papillafe  on  the  surface  of  the  former.  The 
extraordinary  length  and  number  of  these  can  only  be 
appreciated  by  examining  the  sensitive  sole  under  water. 

The  sole  and  wall  are  united,  the  place  of  union  being 
marked  by  a  white  line  which  extends  around  the  complete 
circumference  of  the  hoof  (Fig.  131).  That  part  of  the  sole 
situated  just  within  the  white  line  is  capable  of  bearing 
weight,  inasmuch  as  it  is  not  immediately  under  the  vascular 
sole.  The  arrangement  of  horn  at  the  junction  of  the  wall 
and  sole  is  peculiar.  It  will  be  remembered  that  the  horn 
laminiB  have  on  them  secondary  laminae ;  these  secondary 
laminte  exist  wherever  the  sensitive  horn  laminte  digitate. 
But  in  the  horn  of  the  line  of  junction  of  the  wall  and  sole 
there  are  obviously  no  sensitive  laminae,  and  though  the 
horn  lamincTB  are  there  and  can  be  distinctly  seen  with  the 
naked  eye,  there  are  no  traces  of  secondary  laminiE  on 
them ;  these  have  been  left  behind  in  the  sensitive  foot  as 
the  wall  grows  down.  The  horn  formed  between  the 
junction  of  the  wall  and  sole  is  softer  than  that  of  any 
other  part  of  these  two  structures ;  this  softness  allows  of 
a  slight  yielding  of  the  sole  in  an  up  and  down  direction, 
and  this  we  shall  find  actually  occurs. 

The  Foot-pad*  or  'frog,'  as  it  is  vulgarly  known,  is  a 
pyramidal-shaped  piece  of  horn,  accurately  moulded  over 
the  plantar  cushion,  and  filling  up  the  space  left  by  the 
inflection  of  the  wall  at  the  posterior  part  of  the  foot.  In 
the  foot-pad  we  meet  for  the  first  time  with  a  peculiar  soft 
elastic  horn,  possessing  something  of  the  characters  and 
appearance  of  indiarubber  ;  nothing  in  its  microscopical 
appearance  accounts  for  this  physical  difference  in  the  horn 
of  the  pad  as  compared  with  that  of  the  wall.  Chemistry, 
however,  comes  to  our  assistance,  and  shows  that  the  horn 

*  The  term  '  Foot-pad  "  is  introduced  not  only  to  define  the  function 
of  the  part,  but  in  order  to  eUminate  the  senseless  language  of  the 
stable  from  scientific  discussion.  We  should  have  preferred  the  term 
'  cushion '  to  pad,  but  this  would  have  created  confusion  with  the 
plantar  cushion.  Foot  'buffer,'  from  some  points  of  view  would  be  a 
better  term,  but  there  are  also  objections  to  this. 


552     A  MANUAL  OF  VETEKINARY  PHYSIOLOGY 

of  the  pad  contains  much  more  moisture  than  that  of  any 
other  part  of  the  foot,  and  it  is  the  moisture  which  confers 
on  it  its  pecuHar  soft  pliable  condition.  The  foot-pad  grows 
from  the  vascular  membrane  covering  the  plantar  cushion, 
in  the  same  way  as  we  have  already  seen  it  in  the  wall  and 
sole.  At  the  heels  of  the  foot,  where  the  wall  is  inflected, 
the  soft  horn  of  the  pad  not  only  fills  up  the  gap  between 
them,  but  plasters  over  the  inflected  edge  of  the  wall  for 
some  little  distance,  so  that  an  inspection  of  the  heel  gives 
the  impression  that  the  horn  found  at  the  posterior  part  of 
the  hoof  is  a  continuation  of  the  wall.     The  overgrowth  of 


Fig.  136. — Horizontal  Section  of  the  Horn  of  the  Wall, 
Highly  Magnified. 

a,  Horn  tube,  a  canal  containing  cellular  elements  ;  b,  the  tubular  horn, 
that  is,  the  horn  secreted  from  the  papilke,  forming  an  oval  or 
circular  nest  of  cells  around  the  canal  ;  c,  the  intertubular  horn. 


the  foot-pad  is  provided  against  by  a  method  which  is  a 
combination  of  that  found  in  the  wall  and  sole,  viz.,  it  is 
cast  off  after  growing  to  a  certain  thickness,  while  the  part 
next  the  ground  is  worn  away  by  friction ;  in  consequence, 
owing  to  its  rubber-like  nature,  rags  of  horn  along  the 
edges  of  the  foot-pad  are  a  common  and  natural  condition. 
The  Structure  of  Horn.^ — ^The  horn  of  the  foot  consists  of 
epithelial  cells  which  have  undergone  compression  and 
keratinisation,  by  which  latter  process  they  become  hard 
and  tough.  It  is  possible  to  have  horn  in  the  foot  which 
is  not  keratinised,  and  the  two  are  very  readily  dis- 
tinguished by  the  process  of  staining.     The  double  stain 


THE  FOOT  553 

picro-carmine  has  a  selective  affinity  for  each  kind  of 
horny  tissue ;  the  carmine  picks  out  the  protoplasmic 
and  non-corneous  cells  and  stains  them  red,  whilst  the 
picric  acid  stains  all  tissue  which  has  undergone  the 
process  of  keratinisation  of  a  yellow  colour.  By  means  of 
this  stain  we  possess  a  very  easy  means  of  determining  the 
character  of  the  horn  under  examination. 

The  ultimate  horn-cell  is  a  very  thin,  spindle-shaped, 
oblong,  or  irregular  body,  containing  granular  matter,  a 
nucleus,  and  frequently  pigment  (Fig.  136).  In  all  cases 
the  cells  are  united  at  their  edges  and  sides  by  a  cement 
substance.  By  acting  upon  horn  with  caustic  alkalies  the 
cells  are  in  the  first  instance  rendered  clear,  they  then 
gradually  dissolve,  are  converted  into  a  gelatinous  mass, 
and  finally  they  disappear.  There  is  no  necessity  to  use  a 
caustic  alkali  to  destroy  horn,  any  alkali  has  the  effect  of 
eroding  it.  Bearing  in  mind  the  highly  alkaline  nature  of 
the  horse's  urine,  the  practical  application  of  this  fact  in 
the  care  and  management  of  the  feet  is  very  obvious. 

If  a  portion  of  horn  be  examined  microscopically,  it  is 
found  that  the  compressed  epithelial  structure  is  tunnelled 
in  such  a  way  as  to  form  canals  or  tubes,  or,  at  any  rate,  to 
form  a  structure  which  is  tube-like  in  nature.  These  tubes 
exist  wherever  the  growing  surface  is  invested  with  papillae 
or  projections,  so  that  where  the  i^apillie  are  numerous  the 
tubes  are  numerous,  where  they  are  absent  the  tubes  are 
absent.  The  only  horny  structure  not  secreted  from  a 
papillated  surface  is  the  horn  laminae,  and  here  conse- 
quently we  find  no  horn  tubes,  but  everywhere  else  the 
horn  is  found  to  possess  a  more  or  less  tubular  structure. 
The  method  of  tube  formation  in  horn  is  very  simple ;  the 
papillae  growing  from  the  various  secreting  surfaces  are 
lodged  in  depressions  in  the  horn,  in  this  way  a  canal  is 
formed  for  the  reception  of  the  papilla.  As  the  horn  grows 
down  from  the  surface  which  secretes  it,  the  canal  lodging 
the  papilla  gradually  slides  oft",  but  throughout  the  length 
of  the  horn  a  tubular  appearance  indicates  where  the 
papilla  was  at  one  time  lodged,  and  the  cells  of  these  tubes 


554     A  MANUAL  OF  VETERINAKY  PHYSIOLOGY 

from  their  reaction  with  carmine  prove  themselves  to  be 
different  to  true  horny  structure. 

The  horn  which  is  secreted  in  the  foot  is  therefore 
formed  (1)  from  papillae  found  on  the  secreting  surface, 
and  (2)  from  the  spaces  between  the  papilUe.  The  papillae 
form  tubular  horn,  the  spaces  between  them  form  inter- 
tubular  horn  (Fig.  136,  b  and  c).  The  tube  or  canal  in  horn 
is  the  outcome  of  the  existence  of  papilhe ;  the  horn  is 
arranged  in  an  oval  or  circular  manner  around  the  canal 
(Figs.  133  and  136),  the  cells  composing  it  being  so  placed 
that  their  edges  are  towards  the  papillae.  There  is,  however, 
a  layer  of  cells  which  actually  forms  the  wall  of  the  canal, 
and  these  are  arranged  with  their  sides  next  it,  or,  to  put  it 
another  way,  they  stand  on  their  edges.  The  horn  formed 
by  the  papilla  is  consequently  arranged  concentrically,  and 
this  gives  a  laminated  appearance  around  the  canal,  which 
is  best  seen  in  the  external  and  middle  layers  of  the  wall 
(Fig.  136,  a,  b).  In  the  deep  layer  of  the  wall  the  papillae 
produce  a  much  greater  secretion,  and  here  the  circular  or 
oval  masses  of  cells  investing  the  canal  are  more  prominent, 
and  further,  unlike  those  in  the  anterior  and  middle  parts 
of  the  wall,  they  need  no  reagent  to  identify  their  cellular 
nature  (Fig.  136,  b).  If  a  section  of  wall  be  stained  with 
picro-carmine  only  the  canal-contents  of  the  external  and 
middle  wall  stain  with  carmine  ;  all  the  remaining  substance 
takes  up  the  picric  acid.  In  the  deep  wall  this  is  different ; 
here  the  whole  of  the  cellular  material  secreted  by  the 
papillae  is  stained  red,  showing  that  these  cells  are  proto- 
plasmic rather  than  horny,  and  partly  accounts  for  the 
fact  that  this  deep  horn  is  always  softer  than  the  middle  or 
external  horn  of  the  wall.  The  horn  formed  between  the 
papillae  surrounds  and  knits  together  that  formed  by  the 
papillae.  If  a  section  of  horn  be  examined  without  under- 
going any  special  preparation,  it  is  quite  impossible  to  see 
the  cells  of  which  it  is  composed  ;  Fig.  133  gives  a  good  idea 
of  this,  and  represents  the  fibrous  appearance  presented  by 
a  horizontal  section  of  the  wall.  To  see  the  cells  the  pre- 
paration has  to  be  treated  with  a  solution  of   potash  or 


THE  FOOT 


555 


other  reagent,  when  the  appearance  presented  in  Fig.  136 
is  obtained. 

At  the  junction  of  the  wall  and  the  sole  the  horn  of  the 
laminae  is  firmly  interdigitated  with  the  soft  horn  of  the 
margin  of  the  sole.  This  can  be  perfectly  seen  micro- 
scopically, and  further  it  may  be  demonstrated  that  the 
portion  of  the  sole  thus  thrust  between  the  horn  laminae 
is  perforated  in  five  or  six  places  for  the  reception  of  the 
papillae  which  grow  from  the  inferior  extremity  of  the 
sensitive  laminae. 

If  a  vertical  section  of  horn  be  made,  we  can  study  the 
canals  now  divided  in  their  length.     It  is  readily  seen  that 


Fig.    137. — Microscopical    Structure    of    Horn  :     Longitudinal 
Section  of  the  Wall.     Low  Magnification  (after  Lungwitz). 

Note  the  dififerent  size  of  canals ;  those  on  the  right  are  nearest  the 
laminiE ;  those  on  the  left  are  towards  the  outside  wall ;  they  are 
smaller  and  more  numerous  than  those  deeper  seated  :  ^  is  a 
portion  of  a  horn  lamina. 

though  spoken  of  as  canals  or  tubes  they  are  really  not 
empty,  but  throughout  their  entire  length  contain  cells 
which  are  protoplasmic  in  nature.  These,  owing  to  the 
manner  in  which  they  reflect  light,  give  to  the  part  a 
beaded  appearance.  The  cells  contained  within  the  canal 
are  secreted  by  the  apex  of  the  paj^illa ;  they  do  not 
fill  up  the  entire  lumen  of  the  canal.  The  use  of  the 
canal  system  in  horn  is  for  the  purpose  of  irrigation ;  the 
horn  must  be  supplied  with  moisture,  the  bulk  of  this  is 
obtained  through  these  imperfect  canals,  the  soft  proto- 
plasmic canal  wall  readily  admitting  of  transudation.  It 
is  not  intended  to  represent  that  anything  like  a  fluid  is 


556     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

circulating  along  the  tubes,  but  moisture  certainly  does  find 
its  way  down,  and  is  readily  imbibed  by  the  surrounding 
cells.  Besides  this  arrangement  for  maintaining  the 
moisture  in  horn,  there  is  no  doubt  that  in  the  intertubular 
horn  moisture  passes  from  the  secreting  surface  from  cell 
to  cell,  and  in  this  way  is  transmitted  throughout  the 
length  of  the  foot.  Constant  evaporation  is  taking  place 
from  the  foot,  and  the  loss  is  made  good  in  the  manner 
indicated. 

If  the  invisible  moisture  which  is  always  escaping  from 
the  foot  be  hindered  in  its  evaporation,  the  horn  becomes 
sodden,  crumbles  away,  and  closely  resembles  a  grey  cheese. 
This  experiment  can  readily  be  performed  on  the  sole  and 
foot-pad,  by  accurately  moulding  to  their  surface  a  sheet 
of  gutta-percha  and  leaving  it  there.  The  practical  lesson 
is  obvious ;  no  impervious  material  should  be  applied  to 
the  foot  as  a  protection,  or  if  used  it  should  be  venti- 
lated. 

Use  of  the  Moisture  in  Horn. — The  amount  of  moisture 
contained  in  horn  is  something  considerable,  and  the  rate 
at  which  it  evaporates  is  quite  extraordinary.  If  parings 
of  the  foot-pad  be  enclosed  in  a  bottle,  in  a  short  time  the 
interior  will  become  bedewed  with  moisture.  The  use  of 
moisture  in  horn  is  to  keep  the  foot  elastic  and  so  prevent 
it  from  becoming  brittle.  The  agency  which  is  at  work  to 
prevent  the  too  rapid  evaporation  of  moisture  from  the 
wall  is  the  periople,  or  thin  varnish-like  layer  which  covers 
the  wall,  and  in  addition  there  is  the  natural  hardness  of  the 
external  fibres  of  the  wall ;  the  latter  is  sufficient  to  retain 
the  fibres  in  their  elastic  condition  by  preventing  evapora- 
tion. In  the  case  of  the  sole,  the  layers  of  exfoliated 
material  which  accumulate  as  the  result  of  the  breaking 
off  of  the  horn  fibres  prevent  undue  evaporation.  Horn 
containing  but  little  moisture  is  in  an  abnormal  condition, 
it  is  rigid  and  brittle  ;  nails  driven  into  the  part  cause  it  to 
crack,  and  that  elasticity  on  which  so  largely  depends  the 
natural  shape  and  usefulness  of  the  foot  becomes  impaired, 
or  even  destroyed.  A  museum  specimen  of  a  foot  will 
very  clearly  illustrate  our  meaning ;  in  its  dried  condition 


THE  FOOT  557 

it  is  so  brittle  that  if  dropped  it  will  occasionally  fracture 
like  a  piece  of  glass  ;  but  if  this  foot  be  placed  in  water  for 
a  few  days,  it  comes  out  as  fresh  and  elastic  as  though  it 
had  just  been  removed  from  the  body  instead  of  being 
probably  twenty  years  old.  All  that  the  horn  has  done  is 
to  imbibe  water,  and  the  previously  brittle  substance  now 
becomes  yielding  and  elastic.  The  entire  physiology  of 
the  horse's  foot  is  centred  around  this  question  of  the 
moisture  contained  in  horn.  One  of  our  main  objects  in 
shoeing  should  be  to  protect  the  wall  from  unnecessary 
interference ;  the  removal  of  the  varnish  layer  formed  by 
the  periople,  and  the  cutting  across  of  some  thousands  of 
horn-fibres  by  the  unnecessary  use  of  the  rasp,  lead  to 
destruction  owing  to  the  evaporation  of  water. 

The  necessity  for  elasticity  in  the  foot  is  evident,  when 
we  consider  the  concussion  to  which  the  part  is  exposed 
during  work,  which  would  inevitably  lead  to  its  destruction 
by  fracture  or  otherwise  unless  some  such  provision  as 
this  were  present.  Clinically  we  are  perfectly  acquainted 
with  the  fractures  which  occur  in  the  wall  of  the  hoof  from 
violence. 

In  one  part  of  the  foot  undoubted  evidence  of  sweat 
glands  exists ;  these  are  found  in  a  particular  part  of  the 
plantar  cushion  near  to  and  on  the  sides  of  the  cleft.  The 
glands  are  very  large,  coiled,  and  a  spiral  duct  passes 
through  the  horn  of  the  foot-pad  and  opens  on  the  surface. 
Fig.  138  is  after  Franck,  who  carefully  described  these 
structures,  though  the  original  discovery  was  made  by 
Ercolani.* 

Chemistry  of  Horn. — An  analysis  of  the  horn  of  the  foot 
has  given  us  the  following  results  :t 

Wall.  Sole.  Foot-pad. 

Water  -  -     24-735  37-065  42-54 

Organic  matter  -     74-740  62-600  57-27 

Salts  -  -         -525  -335  -19 


100-000  100-000  100-00 

*  Veterinary  Journal,  vol.  i.,  No.  1. 

t  '  Chemistry'   of   the    Hoof   of   the    Horse,'    Veterinary   Jonrnal, 
>'o\.  XXV.,  1887. 


558     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  pad  contains  the  largest  amount  of  moisture,  and 
the  wall  the  least.  The  salts  are  small  in  amount,  and 
consist  principally  of  those  of  sodium,  magnesium,  iron  and 
silica,  in  the  form  of  chlorides,  sulphates,  and  phosphates. 

Hoof  consists  of  a  horny  material  or  keratin,  a  substance 
which  replaces  the  protoplasm  originally  existing  in  the 
cells.     Keratin  is  a  proteid-like  body  found  in  hair,  nails, 


Fig.  138. — The  Sweat  Glands  of  the  Plantar  Cdshion  (Franck). 

d,  d,  The  glands,  the  corkscrew-like  ducts  of  which  {e,  e,  b)  pass  out 
through  the  horn  of  the  foot-pad,  opening  at //on  to  the  surface 
of  the  foot.  At  c  is  the  deep-seated  portions  of  the  horn  of  the 
foot-pad,  where  it  grows  from  the  papillae  of  the  corium  of  the 
plantar  cushion  ;  g,  g  are  horn  tubes  seen  in  longitudinal  section. 

and  even,  in  a  modified  form,  in  the  nervous  system ;  it 
consists  of  Carbon  51-41,  Hydrogen  6-96,  Nitrogen  17-46, 
Oxygen  19-49,  and  Sulphur  4-23  per  cent.  The  sulphur 
is  loosely  combined,  and  it  is  this  in  combination  with 
hydrogen  which  causes  horn  undergoing  decay  or  disease 
to  have  such  an  offensive  odour,  sulphuretted  hydrogen 
being  formed.     Keratin  is  a  very  insoluble  substance,  but  is 


THE  FOOT  559 

dissolved  by  strong  and  boiling  acids  and  by  alkalies.  With 
sulphuric  acid  it  yields  leucine,  tyrosine,  and  volatile  sub- 
stances ;  the  latter  conferring  the  peculiar  odour  on  burnt 
horn,  feather,  nail,  etc. 

Provisions  for  Elasticity  and  Toug^hness. — From  what  we 
have  previously  said,  it  can  be  seen  that  it  is  the  wall  of 
the  foot  which  supports  the  horse's  weight.  On  examining 
the  wall  it  is  found  to  be  thickest  at  the  toe,  thinner  at 
the  quarters  and  thinnest  at  the  heels;  it  is  thickest  at 
the  toe  owing  to  the  wear  and  tear  of  the  foot  at  this  part. 
As  the  pad  and  posterior  part  of  the  foot  are  the  first  to 
make  contact  with  the  ground  (at  any  rate  in  all  fast 
paces),*  so  the  toe  is  the  last  part  to  leave  it.  The  final 
propulsion  being  given  to  the  body  by  the  toe,  as  we  have 
seen  in  studying  locomotion,  we  can  readily  understand 
how  necessary  it  is  for  this  part  to  be  thick  and  strong. 
The  object  of  the  wall  becoming  thin  towards  the  posterior 
part  of  the  foot,  is  to  allow  of  the  elastic  movement  which 
we  have  yet  to  describe. 

Two  physical  conditions  have,  therefore,  to  be  provided 
for  in  the  wall — viz.,  elasticity  of  the  posterior  part,  and 
toughness  of  the  anterior  portion.  The  first  is  provided  by 
the  wall  being  thinner  at  the  heels  than  elsewhere  ;  but 
besides  being  thinner,  the  wall  of  the  heel  contains  more 
moisture  than  the  wall  of  the  toe,  and  this  moisture 
ensures  its  elasticity.  The  younger  the  horn,  viz.,  the 
nearer  to  the  coronet  at  which  it  is  examined,  the  more 
moisture  it  contains  ;  the  further  away  from  the  coronet 
the  less  moisture,  the  tougher  and  more  resisting  the  horn. 

The  wall  grows  evenly  from  the  coronet  all  the  way 
round  ;  if  it  grows  half  an  inch  in  the  month  at  the  toe, 

*  Fifty  years  ago  Mr.  J.  Irvine  Lupton  communicated  a  paper  on 
'  Physiological  Eedections  on  the  Position  assumed  by  the  Fore  Foot 
of  the  Horse  in  the  Varied  Movements  of  the  Limbs,'  Veterinarian 
vol.  xxi.,  1858.  In  this  communication  he  states  that  the  heel  comes 
to  the  ground  before  the  toe  ;  further,  he  clearly  describes  the  use  of 
the  foot-pad,  the  expansion  of  the  foot,  and  the  final  propulsion  given 
by  the  toe.  Mr.  Lupton's  advanced  views  did  not  meet  with  approval 
at  the  time  ;  to  day  they  are  accepted  facts. 


5(J0     A  MANIAL  OF  VETERINARY  PHYSIOLOGY 

it  grows  the  same  length  at  the  quarters,  and  the  same  at 
the  heels.  The  anterior  part  of  the  wall  is  longer  than  the 
posterior,  therefore  the  anterior  is  tougher  than  the  pos- 
terior, for  the  reason  that  the  horn  is  much  older  at  the 
extremity  of  the  toe  than  it  is  at  the  heel,  and  being  further 
away  from  the  coronet,  it  contains  less  moisture.  The 
wall  at  the  heel  is  some  months  younger  than  that  at  the 
toe  ;  it  is  thinner,  and  contains  more  moisture,  therefore 
it  is  more  elastic  but  not  so  tough. 

The  age  of  the  wall  is  an  important  factor  in  the  wear 
of  the  foot.  If  it  takes  from  nine  to  twelve  months  for  the 
wall  to  grow  from  the  coronet  to  the  toe,  the  piece  of  wall 


i'  c   cC  C 

Fig.  139. — Di.\gram  Illustrating  the  Age  of  the  Wall. 

a,  h^  c,  cl,  e,  f,  are  circles  drawn  round  the  hoof  parallel  to  the  coronet ; 
in  this  way  it  is  ascertained  that  the  age  of  the  wall  at  a  is  the 
same  as  the  heel  at  a\  the  age  of  the  wall  at  d  corresponds  with 
the  age  of  the  quarter  at  d'.  Every  portion  of  the  ground  surface 
of  the  wall  is  of  a  different  age,  being  oldest  and  hardest  at  /',  and 
youngest  and  most  elastic  at  a'. 

at/,  Fig.  139,  is,  say,  twelve  months  old,  whilst  that  at  a  is 
less  than  six  months  old.  The  horn  of  the  quarter  is  older 
than  the  horn  of  the  heel,  and  the  horn  of  the  toe  older 
than  that  of  the  quarter.  This  excellent  provision  admits 
in  the  unshod  foot  of  considerable  friction  occurring  at  the 
toe  without  producing  undue  wear,  for  the  part  is  hard 
and  tough,  while  the  younger  and  moister  horn  at  the 
posterior  part  of  the  foot  allows  of  expansion.  In  this  way 
varying  degrees  of  toughness  and  elasticity  are  provided  in 
the  wall. 

The  toe  of  the  wall  appears  to  grow  faster  than  either 


THE  FOOT  561 

the  quarters  or  the  heels,  but  this  is  more  imaginary  than 
real ;  it  is  the  tendency  of  the  foot  to  grow  forward  as  well 
as  downward  which  produces  the  illusion.  That  the  foot 
does  grow  forward  may  readily  be  determined  by  experi- 
ment, for  if  we  cut  or  saw  a  groove  in  the  wall  at  the 
coronet,  say  an  inch  or  so  from  the  heels,  the  groove  will 
in  course  of  time  be  carried  some  considerable  distance 
towards  the  toe  ;  the  exact  amount  can  be  determined  by 
observing  the  obliquity  of  the  horn  fibres. 

Kow  the  Weight  is  carried  by  the  Foot. — It  is  universally 
recognised  that  the  weight  of  the  body  is  supported  by  the 
union  of  the  insensitive  with  the  sensitive  laminae.  That 
the  enormous  weight  of  the  horse's  body  should  be  carried 
upon — or,  rather,  slung  upon — thin  delicate  strips  of 
sensitive  material  on  the  one  hand,  and  correspondingly 
delicate  strips  of  horn  on  the  other,  is  perhaps  the  most 
remarkable  feature  in  the  physiology  of  the  foot.  We  know 
how  firm  this  union  is,  the  extreme  difficulty  in  a  state  of 
health  in  separating  the  two  surfaces,  even  by  mechanical 
means,  while  the  structurally  delicate  nature  of  the  parts 
yielding  this  firm  connection  we  have  previously  considered. 

In  one  foot  the  weight  is  carried  on  600  or  more  primary 
laminae,  and  72,000  or  more  secondary  laminae.  Those 
laminae  situated  at  the  anterior  part  of  the  foot  are  exposed 
to  more  strain  than  those  posteriorly  placed,  for  the  reason 
that  they  are  longer,  and  they  have  no  plantar  cushion  and 
foot-pad  to  assist  them  as  the  shorter  posteriorly-placed 
lamina  have.  Moreover,  during  progression,  the  final 
propulsion  of  the  toe  comes  entirely  on  them.  The  short 
posteriorly-placed  laminae  have  their  strength  increased 
by  the  direction  in  which  the  weight  of  the  body  comes 
upon  them.  Instead  of  bearing  the  weight  on  the  length 
of  the  laminae,  as  at  the  toe,  they  carry  it  on  the  side,  in 
such  a  manner  that  where  we  have,  say,  one  lamina  at 
the  toe,  there  are  twenty  at  the  quarter.  It  is  not  possible 
to  describe  this  condition  clearly,  but  Fig.  140  will  help  to 
explain  it. 

It  will  be  remembered  that    the   laminae   are   attached 

36 


562    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Fig.  140. — Diagrams  to  Illustrate  the  Direction  taken  by  the 
Lamin.e  at  Different  Parts  of  the  Foot,  as  seen  in  Trans- 
verse Section. 

In  the  upper  figure  the  section  is  made  through  the  toe  of  the  foot : 
a,  being  part  of  the  pedal  bone ;  b,  the  horn  wall ;  c,  the  laminae, 
the  latter  are  practically  straight,  the  weight  being  imposed  from 
top  to  bottom  in  their  length.  The  middle  figure  is  a  vertical 
section  just  behind  the  point  of  the  foot-pad :  the  laminae,  c,  give  the 
appearance  of  being  placed  above  one  another.  The  lower  figure 
is  a  vertical  section  through  the  posterior  part  of  the  foot  :  a,  being 
the  lateral  cartilage,  and  c,  the  laminae.  It  will  be  observed  that 
the  laminae,  as  in  the  previous  figure,  are  placed  one  above  the 
other  ;  this  arrangement  gives  strength,  and  is  a  compensation  for 
shortness. 


THE  FOOT  563 

at  the  anterior  and  part  of  the  lateral  face  of  the  foot  to 
hone,  but  for  the  remaining  lateral  face  and  posterior  part 
of  the  foot  they  are  attached  to  stout  cartilage ;  *  if  a  line 
be  drawn  through  the  foot  separating  the  osseous  attach- 
ment of  the  laminte  from  the  cartilaginous  attachment 
(see  Fig.  141),  it  will  be  found  that,  roughly  speaking,  one- 
half  is  cartilaginous  and  one-half  osseous  ;  the  cartilaginous 
portion  is  situated  just  where  elasticity  is  required,  viz., 
the  posterior  face  of  the  wall.  One  function  of  the  lateral 
cartilages  of  the  foot  is  to  afford  a  movable  wall-attachment 
to  the  sensitive  laminae,  and  enable  them  to  be  carried 
outwards  during  expansion.  A  knowledge  of  the  relation 
of  the  posterior  laminae  to  the  lateral  cartilage  explains  the 
cause  of  lameness  in  '  side-bone,'  viz.,  the  squeezing  of  the 
sensitive  laminte  between  the  wall  on  the  one  hand,  and  the 
ossifying  cartilage  on  the  other. 

The  folding  up  of  the  horn  and  sensitive  leaves  in  the 
foot,  in  the  manner  above  described,  has  another  function 
besides  that  of  merely  supporting  the  weight  and  rendering 
the  union  firm.  The  first  thing  which  strikes  one  in  con- 
nection with  the  foot  is  its  remarkably  small  size  in 
proportion  to  the  size  of  the  body.  Comparing  the  horse's 
foot,  so  far  as  size  is  concerned,  with  the  human  foot,  the 
advantage  in  the  majority  of  cases  lies  on  the  side  of  the 
biped.  The  most  interesting  fact  which  physiology  has  to 
demonstrate  is  that,  though  the  foot  presents  a  small 
circumference,  in  reality  it  encloses  a  vast  area  due  to  the 
anatomical  arrangement  of  the  laminte.  It  is  clear  that  by 
folding  up  this  amount  of  material  the  surface  of  the  foot  is 
considerably  increased.  In  other  words,  by  this  arrange- 
ment the  foot  has  been  kept  within  small  proportions  with- 
out affecting  its  strength.  A  book,  say  of  600  pages,  may, 
by  placing  one  leaf  on  the  other,  be  made  to  occupy  a  bulk 
represented  by  a  few  inches  ;  but  if  each  page  be  laid  out 
separately  on  the  ground,  and  made  to  touch  the  others, 
the  surface  covered  will  be  considerable.     This  is  exactly 

*  Some  of  the  laminae  are  attached  to  the  tendon  of  the  extensor 
pedis  and  the  lateral  ligaments  of  the  foot-joint. 

36—2 


564     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

what  occurs  in  the  foot,  the  insensitive  and  sensitive  leaves 
by  their  singular  arrangement  increase  the  surface  of  the 
foot,  and  yet  keep  it  within  reasonable  limits.  Bracy 
Clarke,  who  first  had  a  calculation  made  as  to  the  increased 
surface  afforded  by  this  arrangement,  came  to  the  conclu- 
sion that  it  was  equal  to  li  square  feet;  but  Moeller*  has 
estimated  that  it  is  equivalent  to  8  square  feet,  whilst 
Gader's  estimate  t  is  lOf  square  feet.  For  safety  we  adopt 
Moeller' s  number.  The  bearing  surface  afforded  by  each 
foot  is  equivalent  to  8  square  feet,  giving  a  total  area  of 
32  square  feet ;  but  it  is  evident  that  as  feet  vary  greatly  in 
size,  this  surface  must  be  greater  or  less  depending  on  the 
size  of  the  foot. 

The  physiological  function  of  the  leaves  of  the  foot  is 
demonstrated  by  pathological  observation.  Inflammation 
of  the  laminas  occurs  either  through  overwork,  or  through 
an  animal  standing  too  long  in  one  position  ;  in  either  case 
the  parts  get  strained,  and  resent  it.  The  practical  value 
of  exercising  horses  which  from  any  reason  have  to  stand 
for  a  length  of  time  is  well  known  ;  the  exercise  overcomes 
the  tendency  of  the  lamina  to  congestion  from  continual 
strain,  and  the  feet  not  only  become  cool,  but  the  animal 
may  continue  standing  for  a  considerable  time  if  exercised 
daily.  The  treatment  of  laminitis  by  exercise,  first  taught 
by  Broad,  of  Bath,  possesses  a  sound  physiological  basis. 
If  any  doubt  exists  as  to  the  function  of  the  laminae  in 
supporting  the  weight  of  the  horse's  body,  we  have  only 
to  look  at  the  processes  which  occur  in  them  as  the  result 
of  disease.  Laminitis  is  often  attended  by  separation  of 
the  horn  and  sensitive  laminse,  when  the  horse's  weight 
being  no  longer  properly  supported,  the  pedal  bone  under 
its  influence  is  actually  forced  through  the  sole  of  the  foot. 

The  Origin  of  the  Horn  Laminae, — No  one  doubts  that  the 
wall  grows  from  the  coronet,  but  great  controversy  has 
taken   place  over  the  origin   of   the   horn    laminae,  some 

*  Veterinary  Journal,  vol.  v. 

t  Quoted  by  Goubaux  and  Barriere,  '  Exterior  of  the  Horse ' 
(translation). 


THE  FOOT  565 

saying  they  grow  like  the  wall  from  a  part  of  the  coronary 
cushion,  and  others  affirming  that  they  obtain  their  origin 
from  the  sensitive  laminae.  If  we  were  to  judge  solely  by 
the  result  of  pathological  processes,  we  should  be  inclined  to 
say  the  sensitive  secreted  the  horn  laminss;  but  Moeller* 
points  out  that  the  sensitive  and  insensitive  laminae  are 
never  in  actual  contact,  and  that  between  them  are  placed 
the  secondary  lamina  both  vascular  and  horny.  Therefore 
he  argues,  and  with  great  weight,  that  the  vascular  cannot 
secrete  the  horn  laminae,  but  that  the  secondary  vascular 
secrete  the  secondary  horn  laminae.  If  a  portion  of  wall 
be  removed  experimentally  and  the  vascular  laminae  be 
exposed,  in  the  course  of  a  short  time  the  part  becomes 
covered  with  a  layer  of  horn,  and  this  has  been  used  as  a 
strong  argument  in  favour  of  the  secretion  of  horn  from 
sensitive  laminas ;  but  the  horn  which  is  thus  secreted  is 
derived  from  the  secondary  vascular  laminae,  and  no  one 
contends  that  these  secrete  the  primary  horn  laminae.  The 
following  explanation  appears  to  us  to  be  the  correct  one : 
The  horn  laminae  are  secreted  from  the  lower  edge  of 
the  coronary  substance,  here  white  protoplasmic  cells  are 
poured  out  between  the  papillae  :  these  cells  are  carried  down 
with  the  wall,  being  pressed  into  and  moulded  between  the 
sensitive  leaves,  thus  becoming  horn  laminae,  the  exact 
counterpart  in  shape  of  the  mould  in  which  they  are  made. 
All  this  occurs  in  the  region  marked  3,  Fig.  132.  As  the 
wall  grows  down  the  horn-leaves  are  carried  with  it,  so 
that  there  is  a  perpetual  movement  occurring  between  the 
slowly  travelling  insensitive  and  the  fixed  vascular  laminae. 
The  rate  of  this  movement  is  probably  about  '0125  inch 
in  twenty-four  hours,  on  the  assumption  that  the  wall 
grows  f  of  an  inch  in  the  month.  During  the  time  the 
horny  are  gliding  through  the  sensitive  leaves  the  vascular 
lamellae  furnish  them  with  horny  lamellae ;  and,  as  we 
have  previously  seen,  when  the  wall  reaches  the  sole  the 
horny  lamellae  are  left  behind,  while  the  laminfe  emerge 
with  the  wall  destitute  of  these  structures. 

*  Op.  cit. 


566     A  MANUAL  OF  VETERINAKY  PHYSIOLOGY 

The  Use  of  the  Bars.^ — The  inflected  portion  of  the  wall, 
known  as  the  '  Bars,'  runs,  as  we  have  previously  mentioned, 
forwards  under  the  foot  instead  of  completing  the  circle  of 
the  wall.  The  object  of  turning  aside  from  completing  the 
ring  the  wall  originally  looked  like  forming,  is  to  make 
room  for  the  elastic  posterior  foot,  viz.,  the  plantar  cushion 
and  foot-pad ;  and  the  explanation  why  the  wall  is  turned 
in  instead  of  ending  abruptly,  is  to  afford  a  solid  bearing 
to  the  posterior  part  of  the  foot,  to  give  additional  strength, 
and  to  secure  a  more  intimate  union  with  the  sole.  The 
bars  being  part  of  the  wall  are  intended  to  bear  weight ; 
in  the  foot  of  the  wild  horse  and  zebra,  they  present  the 
most  extraordinary  development  as  the  result  of  weight- 
bearing. 

The  Use  of  the  Sole  is  quite  clear,  it  is  to  afford  protection 
to  the  sensitive  parts  above.  Its  normally  concave  shape 
(Figs.  131  and  140)  proves  that  it  is  not  intended  to  bear  on 
the  ground  over  its  general  surface,  and  the  acute  lameness 
which  results  from  a  stone  in  the  foot  gives  further  proof, 
if  any  were  required,  of  its  indifferent  weight-supporting 
properties  ;  that  margin,  however,  in  contact  with  the  wall 
can  bear  weight.  Under  the  influence  of  the  body-weight 
the  sole  becomes  slightly  flatter,  especially  that  portion  of 
it  situated  posteriorly,  the  horns  of  the  crescent.  When  we 
come  to  study  the  expansion  of  the  foot  the  object  of  this 
flattening  will  be  more  apparent.  The  sole  growls  from  the 
sensitive  sole,  as  previously  described. 

The  Use  of  the  Foot-pad. — This  is  one  of  the  chief  anti- 
concussion  mechanisms  in  the  foot ;  it  is  there  to  break 
the  jar,  and  it  does  so  by  receiving,  in  conjunction  with 
the  posterior  wall,  the  impact  of  the  foot  on  coming  to 
the  ground  ;  this  is  imparted  to  the  plantar  cushion,  and 
through  the  lateral  cartilages  to  the  wall  of  the  foot,  which 
bulges,  or,  as  it  is  termed,  expands.  In  breaking  the  jar 
(not  only  to  the  foot  but  to  the  whole  limb),  it  is  assisted 
by  its  elastic  rubber-like  nature.  The  foot-pad  needs  for 
its  perfectly  healthy  condition  contact  with  the  ground  ;  it  is 
strange  that  in  this  respect  two  structures  situated  side  by 


THE  FOOT  567 

side,  viz.,  the  sole  and  pad,  should  be  so  opposed  in 
function.  We  know  practically  that  if  the  latter  be  kept 
off  the  ground  the  part  atrophies,  the  heels  contract,  the 
foot  is  rendered  smaller,  and  the  pad  becomes  diseased. 
This  wasted  condition  of  the  pad  and  narrow  foot  may  be 
restored  by  pressure,  but  that  pressure  must  be  ground 
pressure.  It  is  possible  by  means  of  a  bar-shoe  to  throw 
considerable  pressure  on  the  pad  and  heels,  but  the  foot 
still  contracts :  it  is  only  when  the  pad  is  bearing  on  the 
ground  that  it  continues  in  a  healthy  condition,  and  retains 
its  normal  size.  Foot-pad  pressure  is,  therefore,  one  of 
the  rules  in  shoeing  if  the  part  is  to  be  able  to  exercise 
its  natural  functions. 

The  Lateral  Cartilages. — "We  have  dealt  with  certain 
functions  of  the  lateral  cartilages,  but  it  will  not  be  amiss 
to  summarise  our  knowledge  of  their  use. 

1.  They  form  an  elastic  wall  to  the  sensitive  foot,  and 
afford  attachment  to  the  vascular  laminae. 

2.  As  the  foot  increases  in  w^idth  (expansion),  the  car- 
tilages carry  outwards  the  sensitive  laminse  which  are 
attached  to  them,  and  so  prevent  any  disturbance  of  the 
union  of  the  insensitive  and  sensitive  laminae. 

3.  Large  venous  trunks  pass  through  and  close  to  the 
cartilages  of  the  foot,  and  the  movements  of  the  cartilages 
assist  the  venous  circulation. 

4.  The  object  of  having  lateral  cartilages  in  the  foot  is 
to  admit  of  expansion  under  the  influence  of  the  bod}-- 
weight.  This  increase  in  the  width  of  the  foot  is  brought 
about  by  pressure  on  the  pad,  which  widens  and  presses  on 
the  bars  at  H,  Fig.  141,  and  at  the  same  time  tends  to 
flatten  the  plantar  cushion,  both  of  which  factors  force 
the  cartilages  slightly  outwards.  When  the  posterior  wall 
retracts  the  cartilages  are  carried  back  to  their  originaf 
position.  Should  this  elastic  cartilage  under  pathological 
conditions  become  converted  into  bone,  its  functions  are 
destroyed,  and  lameness  may  occur.  By  a  simple  operation 
relief  from  this  lameness  in  a  large  proportion  of  cases 
may  be  secured.     It   is  possible  to  demonstrate   that   by 


568     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


surgical  interference  the  hoof  can  be  made  permanently 
wider,  and  thereby  rendered  capable  of  accommodating 
lateral  cartilages  which  have  undergone  an  increase  in  size 
as  the  result  of  ossification.  This  operation  is  based  on 
physiological  principles.* 

Anti-Concussion  Mechanism. — Practically  the  whole  physi- 
ology of  the  foot  is  a  consideration  of  the  factors  whereby 

G 


Fig.  141. — Portion  of  the  "Wall  removed,  to  show  the  Position 
OF  THE  Rigid  and  Elastic  Sensitive  Foot. 

A,  Wall  of  the  foot ;  b,  the  lateral  cartilage  ;  g,  a  line  which  represents 
the  coronet ;  c,  the  pedal  bone — the  line  of  union  between  the 
pedal  bone  and  lateral  cartilage  is  well  seen  ;  f,  is  a  portion  of  the 
corona  ;  d,  a  portion  of  the  sole  exposed  by  the  removal  of  the  wall ; 
e,  the  heel  of  the  wall  left  at  its  plantar  surface  to  show  the 
arrangement  of  the  bar,  h,  which  passes  behind  and  within  the 
lateral  cartilage  b.  The  figure,  which  is  accurately  drawn  from  a 
photograph,  is  intended  to  show  what  an  extensive  surface  the 
lateral  cartilage  presents,  and  the  variety  of  surfaces  to  which  the 
sensitive  laminae  are  attached ;  they  cover  b,  c,  and  f,  the  latter  in 
the  living  animal  being  the  position  of  the  extensor  pedis  tendon 
and  lateral  ligament  of  the  foot,  to  which  the  laminae  are  attached. 
Further,  the  figure  sliows  the  division  of  the  internal  foot  into  an 
elastic  and  a  rigid  portion. 

the  parts  are  saved  from  concussion,  in  spite  of  wear  and 
tear,  batter  and  jar.  The  weight  carried  on  each  fore  foot 
when  the  horse  is  standing  is  rather  more  than  one  quarter 

*  '  A  New  Operation  for  the  Cure  of  Lameness  arising  from  Side- 
Pones,'  Veterinary  Journal,  vol.  xxv,,  1887. 


THE  FOOT 


569 


the  weight  of  the  body ;  during  locomotion  the  amount 
varies  from  half  the  weight  in  the  trot,  to  the  entire 
weight  in  certain  stages  of  the  canter  and  gallop.  The 
mechanisms  which  exist  in  the  foot  to  save  concussion  are 
not  only  intended  for  the  protection  of  the  foot  but  also  to 
save  the  limb,  and  they  may  be  tabulated  as  follows  : 

1.  The  yielding  articulation  in  the  pedal  joint. 

2.  The  increase  in  the  width  of  the  foot  when  the  heels 
come  to  the  ground,  known  as  expansion. 

3.  The  elastic  foot-pad. 

4.  The  slight  descent  of  the  pedal  bone  and  with  it  of  the 
sole. 


Fig.  142. — Diagram   to   Illustrate   the  Expansion  of   the  Foot 

(lungwitz). 

The  unbroken  outline  illustrates  the  shape  of  the  foot  at  rest ;  the 
dotted  outline  shows  the  portion  of  the  foot  which  has  yielded 
laterally  under  the  influence  of  the  body-weighi. 


Expansion. — We  have  here  retained  a  word  warranted  by 
custom  though  perhaps  not  free  from  objection.  By  its 
use  is  indicated  the  fact  that  the  wall  of  the  foot  opposite 
to  the  heels  becomes  wider  when  the  weight  comes  on  this 
part  (Fig.  142).  The  increase  in  the  width  of  the  foot  is  not 
due  to  a  something  being  added  to  it,  but  to  an  alteration 
in  the  shape  of  certain  of  its  structures ;  if,  therefore,  the 
foot  becomes  wider  it  does  so  at  the  expense  of  other  parts 
9,ltering  their  shape.     As  a  matter  of  fact  an  increase  in 


570     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  width  of  the  foot  is  not  the  only  change  which  occurs  ; 
it  can  be  shown  that  the  heels  at  the  coronary  edge  sink 
closer  to  the  ground,  while  the  coronary  edge  of  the  wall 
in  line  with  the  toe  of  the  foot  retracts,  or  travels  back- 
wards and  downwards.  Fig.  143,  A. 

In  all  fast  paces,  when  the  foot  comes  to  the  ground, 
the   posterior  wall  and  foot-pad  first  receive  the  weight. 


Fig.  143. — Diagrams   to   show   the   Area  over   which  the  Wall 

EXPANDS,    AND    TO    ILLUSTRATE    THE    ReTREAT    OF     THE    ANTERIOR 

Coronary  Edge  of  the  Hoof,  and  the  sinking  of  the  Heels 
(lungwitz). 

A.  The  unbroken  outline  shows  the  shape  of  the  foot  with  no  weight 

on  it  ;  the  dotted  outline  illustrates  the  retreat  of  the  coronary 
edge  in  front  and  sinking  of  the  heels. 

B.  In  this  figure  the  hoof  is  looked  at  from  above  ;  the  unbroken  out- 

line is  the  coronary  edge  from  heel  to  heel.     The  dotted  line  shows 
the  change  in  shape  it  undergoes  under  the  influence  of  the  weight 
of  the  body. 
In  A  the  shaded  part  of  the  wall  is  to  illustrate   the    area   which 
expands. 


Under  the  influence  of  the  body-weight  the  foot-pad  is 
compressed  and  becomes  wider,  the  plantar  cushion  with 
which  it  is  closely  in  contact  is  also  compressed  and 
becomes  wider.  The  effect  of  this  increase  in  width  is  that 
the  foot-pad  presses  on  the  bars,  while  the  plantar  cushion 
presses  on  the  cartilages,  both  of  which  yielding  laterally 
force  apart  the  wall  at   the  heels.     When  the  weight   is 


THE  FOOT  571 

taken  off  the  foot  the  heels  return  to  their  original  position, 
and  the  foot  becomes  narrower.  The  increase  in  width 
which  the  foot  undergoes  is  something  very  small ;  this  is 
probabl}'  the  reason  why  for  years  it  has  never  been 
accepted  as  a  fact,  and  that  in  this  countr}-  in  particular 
few  were  found  who  gave  the  theory  any  support.*  The 
employment  of  delicate  apparatus  such  as  that  used  by 
Lungwitzf  and  others,  and  experimenting  upon  feet  which 
have  not  been  mutilated  in  shoeing,  have  placed  the 
question  beyond  all  doubt. 

The  area  over  which  the  w^all  expands  can  be  seen  in 
Fig.  143,  A  ;  the  shaded  portion  of  the  heel  represents  the 
part  which  yields  laterally.  At  times  expansion  is  obtained 
at  the  coronet  and  little  or  none  on  the  ground  surface,  but 
as  a  rule  the  amount  obtained  at  the  coronet  can  also  be 
obtained  near  the  ground.  As  to  the  amount  of  expansion 
no  definite  statement  can  be  made,  it  is  small  but  is 
influenced  by  the  shape  of  the  foot ;  horses  with  low  heels 
and  full  well-developed  foot-pads  register  a  larger  amount 
than  where  the  heels  are  high  and  rigid.  The  measurements 
obtained  by  us  with  very  delicate  apparatus  were  smaller 
than  those  obtained  in  Germany  by  Lungwitz.  On  an 
average  we  obtained,  by  simply  lifting  up  one  fore-foot,  and 
so  causing  the  horse  to  throw  double  weight  on  the  other  limb, 
an  expansion  of  -yu  of  an  inch  for  half  the  foot,  or  ttV  of  an 
inch  total  increase  in  width.  Doubtless  during  locomotion 
a  greater  expansion  than  this  occurs.  The  question  may 
be  asked  what  advantage  can  be  gained  by  such  a  small 
increase  in  the  width  of  the  foot  ?  Small  as  the  increase 
is,  it  still  makes  all  the  difference  between  a  yielding  and  an 
unyielding  block  of  horn  being  brought  to  the  ground  ;  it 
'  gives  '  instead  of  offering  resistance,  and  this  '  give  '  is 
sufficient  to  prevent  the  hoof  from  being  fractured,  while 
the  pad  which  has  largely'  caused  the  expansion  has  acted 
as  a  buffer  and  assisted  to  destroy  concussion. 

*  See  footnote,  p.  559. 

t   TJie    Journal   of    Comparative   Pathology    and    Therapeutics, 
vol.  iv.,  3. 


572     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

There  is  no  point  in  the  physiology  of  the  foot  which  has 
given  rise  to  greater  discussion  than  the  question  of  expan- 
sion, but  we  submit  that  its  existence  is  not  only  proved,  but 
that  it  is  provided  for  by  the  anatomical  construction  of  the 
part  and  the  elastic  nature  of  horn. 

The  retraction  of  the  coronary  edge  of  the  foot  in  front, 
and  the  sinking  behind  (see  Fig.  143),  are  accompanied  by 
a  tense  condition  of  the  coronary  substance  which  Lungwitz 
describes  as  an  elastic  ring.  Macdonald*  in  this  country 
regards  it  as  a  hydraulic  ligament  which  supports  the  pedal- 
joint  under  the  strain  to  which  it  is  exposed.  The  view  we 
hold  is  that  the  tense  state  of  the  coronary  substance  is  due 
to  the  alteration  in  the  shape  of  the  coronary  edge  of  the 
foot,  and  that  the  value  or  existence  of  any  hydraulic 
support  in  connection  with  the  joint  has  yet  to  be  demon- 
strated. 

In  addition  to  the  changes  in  the  coronary  edge  of  the 
foot  during  the  period  of  expansion,  another  condition  is 
present,  viz.,  a  compression  of  the  wall  under  the  influence 
of  the  body-weight,  which  produces  a  diminution  in  its 
height.  This  can  be  roughly  demonstrated  in  the  following 
manner :  If  a  portion  of  the  wall,  say  between  the  heel 
and  quarter,  be  cut  away  so  as  just  to  clear  the  shoe  when 
the  latter  is  fitted,  it  will  be  found  on  placing  weight  on 
the  limb  by  lifting  up  the  opposite  fore-foot,  that  the  wall 
has  now  descended  sufficiently  to  touch  the  shoe.  The 
only  explanation  which  can  be  afforded  is  that  given  above, 
viz.,  the  wall  has  undergone  sufficient  compression  to 
allow  the  part  which  was  originally  clear  of  the  shoe  to 
come  in  contact  with  it,  and  to  produce  this  it  must  have 
diminished  in  height. 

The  Descent  of  the  Pedal  Bone  is  the  last  factor  employed 
in  saving  concussion,  and  the  existence  of  this  has  been  as 
strenuously  denied  as  the  expansion  of  the  wall ;  there  is, 
however,  no  difficulty  in  demonstrating  it,  and  we  can  readily 
see  the  value  of  this  function.  Concussion  to  the  sensitive 
foot  is  prevented  by  a  slight  up-and-down  play  between  the 

*  Veterinary  Record,  No.  145,  1892. 


THE  FOOT  573 

sub-laminal  tissue  and  the  pedal  bone ;  as  the  weight 
comes  on  to  the  foot  the  pedal  bone  descends  slightly,  to  rise 
again  when  the  weight  is  taken  off  it.  As  the  pedal  bone 
descends,  the  horn  sole  on  which  it  is  resting  also  slightly 
descends  and  comes  nearer  to  the  ground ;  this  is  one 
reason  why  the  sole  is  concave  instead  of  flat.  The  descent 
of  the  internal  foot  saves  concussion,  in  the  same  way  that 
it  is  easier  to  catch  a  cricket-ball  with  a  retreating  move- 
ment of  the  hand  than  by  rigid  opposition  ;  further  it 
facilitates  the  circulation.  The  descent  of  the  pedal  bone 
is  a  most  important  physiological  factor  and  one  of  the 
safeguards  of  the  sensitive  foot. 

Vascular  Mechanism. — Lying  as  the  foot  does  furthest 
from  the  heart,  we  are  led  to  inquire  why  it  is  that  the 
blood  is  able  to  circulate  through  it  so  thoroughly,  and 
whether  other  means  are  at  hand  for  assisting  the  force  of 
the  heart  in  facilitating  the  circulation.  Such  means  do 
exist.  Though  the  contraction  of  the  left  ventricle  is  sufficient 
under  ordinary  circumstances  to  bring  the  blood  back  to  the 
right  side  of  the  heart  from  any  part  of  the  body  (as  we 
have  pointed  out  in  dealing  with  the  circulation),  it  is 
doubtful  whether  it  would  be  wholly  sufficient  to  empty  the 
foot  of  blood  or  keep  the  considerable  plexus  of  veins  full. 
This  plexus  is  shown  in  Fig.  134,  p.  546,  which  is  a 
reproduction  from  a  photograph  of  a  corrosion  injection.* 
The  figure  conveys  very  accurately  an  idea  of  the  remarkable 
venous  arrangement  of  the  foot. 

The  venous  circulation  is  assisted  by  two  movements  in 
the  foot,  viz.,  the  expansion  and  recoil  of  the  outer  foot, 
and  the  descent  and  elevation  of  the  inner  foot.  There  is 
no  difficulty  in  seeing  the  movement  imparted  to  a  column 
of  fluid  circulating  in  these  parts,  for  if  a  plantar  vein  be 
divided  and  the  horse  made  to  walk,  every  time  the  foot 
comes  to  the  ground  the  blood  spurts  out  from  the  vein  as 
if  from  an  artery ;  when  the  foot  is  taken  ofi"  the  ground 

*  The  figure  appeared  in  an  article  by  Dr.  C.  Storch,  of  Vienna,  on 
'  The  Venous  System  of  the  Horse's  Foot,'  Oesterreichischen  Monat- 
schriftfilr  Thierheilkunde,  1893. 


574     A  MANUAL  OF  VETERINAKY  PHYSIOLOGY 

the  stream  of  blood  becomes  greatly  reduced.  A  perfect 
pumping  action  is  in  this  way  produced.  The  mechanism 
can  also  be  demonstrated  on  the  dead  limb,  by  placing  a 
manometer  tube  filled  with  water  in  each  plantar  vein,  and 
then  pressing  downward  on  the  limb,  thus  roughly  imitating 
the  weight  on  the  leg.  With  every  compression  of  the 
foot  the  water  rises  in  the  manometer  tube,  and  falls 
during  the  period  of  no  pressure,  a  period  corresponding  in 
the  living  animal  to  the  foot  being  off  the  ground. 

We  must  accept  it,  therefore,  as  a  proved  fact  that  the 
venous  circulation  is  largely  facilitated  by  the  expansion 
and  contraction  of  the  posterior  part  of  the  foot ;  during 
expansion  the  blood  is  being  driven  upwards,  and  during 
recoil  the  veins  aspirate  the  blood  into  their  interior. 
Indeed,  so  perfect  are  these  mechanisms  that,  as  previously 
pointed  out,  there  are  no  valves  in  the  veins  of  the  foot,  and 
none  are  found  nearer  than  the  middle  of  the  pastern.  To 
assist  the  circulation,  the  large  venous  trunks  at  the 
postero-lateral  part  of  the  foot  are  in  close  connection  with 
the  lateral  cartilages,  and  some  of  the  vessels  even  pass 
through  their  substance. 

In  conclusion  we  may  with  advantage  summarise  what 
has  been  said  about  the  anti-concussion  mechanisms : 

When  the  weight  comes  on  to  the  foot,  it  is  received  by  a 
yielding  foot-articulation,  an  elastic  wall,  bars  and  pad,  and 
through  these  by  the  plantar  cushion.  The  elastic  posterior 
wall  is  pressed  outwards  by  the  compressed  indiarubber- 
like  pad  and  plantar  cushion,  and  it  expands  slightly  from 
the  ground  surface  to  the  coronet.  At  the  moment  of 
expansion,  the  bulbs  of  the  heel  of  the  foot  at  the  coronary 
edge  sink  under  the  body-weight  and  come  nearer  the 
ground,  and  as  a  result  of  this  the  anterior  coronary  edge 
retracts.  The  pedal  bone  descends  slightly  through  its 
connection  with  the  sensitive  laminte,  and  presses  the  sole 
down  with  it,  while  the  wall  of  the  foot  diminishes  in 
height  under  the  compression  to  which  it  is  exposed.  Under 
these  conditions  the  blood-pressure  in  the  veins  of  the  foot 
rises,  and  the  vessels  are  emptied.      When  the  weight  is 


THE  FOOT  575 

removed  from  the  foot  the  bloodvessels  fill,  the  pad  and 
posterior  walls  recoil,  the  bulbs  of  the  heel  rise,  and  the 
foot  becomes  narrower  from  side  to  side  ;  at  the  same  time 
the  anterior  edge  of  the  coronet  goes  forward,  and  the 
pedal  bone  and  sole  ascend. 

Such  are  the  physiological  features  of  the  foot  which 
facilitate  the  circulation  and  help  to  counteract  concussion. 
Foot-lameness  is  only  too  frequent,  but  if  it  were  not  for 
the  mechanisms  we  have  described,  it  would  not  be  possible 
for  horses  to  work  for  a  single  day. 

Physiological  Shoeing. — It  is  impossible  to  conclude  this 
chapter  on  the  foot  without  some  mention  of  what  might  be 
termed  physiological  shoeing. 

We  all  recognise  the  evils  of  shoeing  as  strongly  as  we 
realise  its  necessity.  By  bearing  in  mind  the  functions 
of  the  various  parts  of  the  foot,  we  can  certainly  reduce 
these  evils  to  comparatively  narrow  limits.  The  following 
rules  form  the  basis  of  physiological  shoeing : 

1.  The  reduction  of  the  wall  to  its  proper  proportions, 
such  as  would  have  occurred  through  friction  had  no  shoe 
been  worn. 

2.  Fitting  the  shoe  accurately  to  the  outline  of  the  foot, 
and  not  rasping  away  the  exterior  of  the  crust  to  fit  the 
shoe,  since  this  not  only  renders  the  horn  brittle,  but  is  so 
much  loss  of  bearing  surface. 

3.  The  exterior  of  the  wall  should  be  left  intact.  The 
practice  of  rasping  the  wall  for  the  sake  of  appearance 
destroys  the  horn,  and  allows  of  such  considerable  evapora- 
tion from  the  surface  of  the  foot  that  the  part  becomes  brittle. 

4.  The  sole  should  not  be  touched  with  the  knife  ;  it 
cannot  be  too  thick,  as  it  is  there  for  the  purpose  of  pro- 
tection. 

5.  The  bars  should  not  be  cut  away,  they  are  part  of 
the  wall,  and  intended  to  carry  weight.  The  shoe  should 
rest  on  them. 

6.  The  foot-pad  should  not  be  cut,  but  left  to  attain  its 
full  growth.  No  foot-pad  can  perform  its  functions  unless 
on  a  level  with  the  ground  surface  of  the  shoe. 


576     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

7.  The  pattern  of  shoe  is  immaterial  so  long  as  it  has  a 
true  and  level  bearing,  and  rests  well  and  firmly  on  the 
wall  and  bars.  We  believe,  the  simpler  the  shoe  the  better, 
viz.,  one  flat  on  both  ground  and  foot  surface.  It  should 
be  secured  with  no  more  nails  than  are  absolutely  neces- 
sary, as  the  nails  destroy  the  horn  ;  further,  the  nails  should 
not  be  driven  higher  than  needful,  for  high  nailing  is 
ruinous  to  feet. 

Such,  briefly,  are  the  conditions  which  fulfil  physiological 
shoeing. 

Pathological. 

No  useful  summary  of  foot  trouble  can  be  given.  Practically  every 
structure  in  it  is  liable  to  disorder.  It  is  the  most  common  seat  of  in- 
curable lameness,  and  has  always  been  so  since  the  horse  was  subjected. 
'  No  foot,  no  horse '  is  as  old  as  the  days  of  Xenophon.  This  horse- 
master  tells  us  how  to  keep  the  horn  of  the  feet  of  cavalry  horses  hard 
— a  very  necessary  matter  at  a  time  when  shoes  were  unknown.  It  is 
a  remarkable  fact  that  the  horn  of  unshod  feet  is  infinitely  harder  than 
that  of  horses  wearing  shoes.  The  wall  may  be  so  hard  as  even  to 
resist  a  nail  being  driven  in. 


CHAPTEE  XVIII 

GENERATION  AND  DEVELOPMENT 

The  Sexual  Season  of  animals  is  a  subject  which  in  recent 
years  has  received  exact  expression  at  the  hands  of 
Heape,*  whose  communication,  quoted  below,  we  have 
followed  in  connection  with  this  question.  Heape  divides 
female  mammals  into  two  classes,  Moncestrous,  or  those 
which  have  one  oestrous  cycle,  and  Polycestrous,  or  those 
having  a  series  of  cestrous  cycles.  The  first  phase  of 
generative  activity  at  the  beginning  of  a  sexual  season  is 
known  as  Proa'strum,  or  the  Procestrons  period ;  it  corre- 
sponds to  the  period  '  coming  on  heat,'  or  '  coming  in 
season.'  The  period  lasts  a  variable  time  in  different 
animals,  and  is  succeeded  by  the  period  oj  desire  or 
(Jtlstrus  ;  it  is  only  during  this  period  that  sexual  inter- 
course is  permitted,  or  that  fruitful  coition  is  possible.  If 
conception  does  not  occur  or  is  prevented,  restrus  is 
followed  by  Meto:stra)n,  or  the  )net(rstroiis  period,  during 
which  sexual  activity  passes  away,  and  is  succeeded  by 
a  period  of  complete  rest  or  freedom  from  sexual  excite- 
ment known  as  Ana'strum.  The  anct'strous  period  may  last 
two,  three,  eleven,  or  more  months,  depending  on  the  species. 
The  sexual  cycle  is  not  always  as  above  described ;  there 
are  animals  in  which  metcestrum  is  not  followed  by  a 
period  of  complete  rest,  but  by  a  short  quiescence  known 
as  Dio'strnm,  which  lasts  a  certain  number  of  days,  and 
is  then  followed  by  a  new  prooestrum,  oestrus,  metcestrum, 
and  dioestrum. 

*  'The    Sexual   Season   of   Mammals,'    etc.,   by  W.  Heape,  M.A., 
Quarterly  Journal  of  Microscopical  Science,  vol.  xliv.,  p.  1,  1901. 

577  87 


578    A  MANUAL  OF  VETEKINARY  PHYSIOLOGY 

Among  moncestrous  mammals  is  the  wolf,  which  in  the 
wild  state  has  only  one  sexual  season  in  the  year ;  another 
is  the  dog,  though  in  this  case  the  sexual  season  may  recur 
during  the  year,  but  the  periods  in  each  case  are  quite 
distinct,  and  followed  by  complete  rest,  which  is  the 
essentially  distinguishing  feature.  Among  polyoestrous 
mammals  are  the  mare,  cow,  sheep,  pig,  and  all  of  these 
during  a  portion  of  the  year  exhibit  a  series  of  dia3strous 
cycles  (in  the  absence  of  pregnancy),  followed  by  anoestrum 
until  the  next  year. 

The  number  of  annual  sexual  cycles  which  any  given 
species  passes  through  is  vastly  influenced  by  domestica- 
tion. Probably  in  all  primitive  species  one  sexual  season 
yearly  was  the  rule.  Domestication  alters  this.  The 
abundant  food  supply  renders  the  struggle  for  existence 
no  longer  acute,  the  dread  of  being  preyed  upon  by  the 
enemies  peculiar  to  each  8})ecies  is  removed,  and  one  of  the 
responses  to  these  altered  conditions  is  a  greater  desire  to 
multiply,  for  the  reason  that  the  energies  previously  ex- 
pended in  the  struggle  for  life  are  turned  into  a  fresh 
channel.  The  cat  in  a  wild  state  has  one  sexual  period  a 
year,  the  domestic  variety  has  three  or  four.  The  wild  dog 
and  wolf  breed  once  annually,  in  captivity  twice  annually. 
The  lioness  in  a  wild  state  has  probably  but  a  single 
breeding  season,  in  captivity  the  oestrous  period  may  be 
three  or  four  times  a  year.  Bears  in  a  wild  state  have  a 
single  breeding  season,  in  captivity  more  than  one.  The 
wild  otter  has  a  single  season,  but  in  a  state  of  captivity 
she  comes  '  in  season  '  every  month  (Marshall  and  Jolly). 
So  far,  in  fact,  as  evidence  is  available,  a  single  sexual 
season  for  animals  in  a  state  of  freedom  appears  to  be  the 
natural  condition,  jxAyo^strion  being  an  acquired  character. 
The  frequency  of  oestrus  under  domestication  is  essentially 
influenced  by  food,  temperature,  and  environment. 

The  complete  cestrous  cycle  in  the  dog*  under  domesti- 

*  '  Contribution  to  the  Physiology  of  Mammalian  Eeproduction.' 
Part  I. :  '  The  CEstrous  Cycle  in  the  Dog.'  By  F.  H.  A.  Marshall  and 
W.  A.  Jolly.     Phil  Trans.,  B.  vol.  198.     1905. 


GENERATION  AND  DEVELOPMENT  579 

cation  is  six  months.  Every  six  months,  in  spring  and 
autumn,  the  majority  of  dogs  come  '  on  heat,'  though  there 
are  many  exceptions  to  this  rule,  some  of  the  smaller  breed 
of  dogs  having  a  three  and  four  heat  period  in  the  year. 
The  })eriod  of  procestrum  lasts  from  seven  to  ten  days  and 
oestrus  lasts  another  week. 

In  the  mare  the  complete  testrous  cycle  with  its  dicestrous 
intervals  may  last  for  months,  in  the  majority  of  mares 
from  February  to  June  or  July,  and  unless  rendered 
pregnant  the  dicBstrous  periods  last  twenty-one  days,  and 
are  followed  by  prottstrum,  oestrus,  etc.,  as  previously 
described,  though  the  time  duration  of  these  is  irregular, 
generally  brief,  and  always  uncertain.  For  instance,  the 
exact  period  at  which  the  mare  is  ripe  to  receive  the  male 
may  only  be  a  matter  of  a  few  hours,  whereas  she  may 
be  several  days  in  a  highly  unsettled  sexual  condition. 
The  mare  is  in  a  condition  of  cestrus  on  the  seventh  to 
tenth  day  after  foaling,  with  thoroughbred  mares  it  may  be 
the  sixth  ;  at  this  period,  though  still  nursing,  she  desires 
intercourse,  and  in  this  respect  differs  from  the  nursing 
cow  and  sow.  If  she  does  not  conceive  the  period  of 
dioestrum  is  twenty-one  days,  and  followed  by  oestrus, 
the  returning  heat  usually  lasting  longer  by  two  or  three 
days  than  the  original  heat. 

The  cote  under  domestication  will  breed  at  any  time  of 
the  year  (Goodall).  She  ordinarily  takes  the  bull  six  weeks 
or  two  months  after  calving,  but  it  is  unusual  for  her  to 
accept  the  bull  while  suckling  her  calf.  If  the  latter  be' 
removed  or  weaned  she  shows  signs  of  oestrus  six  or  seven 
days  later,  the  duration  of  which  may  be  twelve  hours. 
The  period  of  dioestrum  is  twenty- one  days,  at  the  end  of 
which  time  both  cows  and  heifers  exhibit  oestrus ;  this 
cycle  continues  until  they  are  settled  in  calf. 

With  slieep"^  oestrus  may  only  last  one  or  two  days,  or 
it  may  pass  away  very  quickly,  the  dioestrum  which  follows 
lasting  from  thirteen  to  eighteen  days.     The  number  of 

■*  '  The  Qistrous  Cycle  and  Function  of  the  Corpus  Luteum  m  the 
Sheep,'  by  F.  H.  A.  Marshall,  B.A.     Phil.  Trans.,  B.  vol.  196.     1903. 

37—2 


580     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

recurrent  periods  in  any  one  cycle  in  the  sheep  have  been 
observed  to  depend  upon  breed  ;  two,  three,  or  four  re- 
current periods  have  been  noted.  There  are  some  breeds 
of  sheep  which  may  produce  two  sets  of  lambs  in  one  year. 
The  period  of  oestrus  may  be  induced  almost  at  any  time 
in  the  late  summer  and  autumn  by  the  introduction  of  the 
ram  to  the  ewes  (Goodall). 

The  soiv  takes  the  l)oar  about  a  week  after  she  has 
weaned  her  litter,  or  about  eight  weeks  after  farrowing. 
The  period  of  oestrus  lasts  about  two  days,  the  dioestrous 
period  twenty-one  days. 

The  only  known  animal  which  in  a  wild  state  exhibits  a 
continuous  series  of  ditestrous  cycles  is  the  monkey,  but 
even  in  this  case  there  is  a  limited  season  when  concep- 
tion is  possible  (Heape). 

The  oestrous  period  may  appear  in  the  dog  after  a  portion 
of  the  spinal  cord  has  been  excised,  proving  that  it  is  a 
process  quite  independent  of  any  reflex  act,  that  it  may 
exist  in  the  absence  of  any  knowledge  on  the  part  of  the 
animal,  and  that  its  production  is  under  no  central  control. 
Furthermore,  such  an  animal  may  become  pregnant  and 
be  delivered  in  the  ordinary  way,  though  quite  unconscious 
of  the  process. 

The  external  signs  of  pro<xstriun  in  all  animals  is  a 
swelling  of  the  vulva  more  or  less  pronounced,  with  a 
slight  flow  of  mucus  which  may  be  blood-stained.  There 
is  excitement,  the  mare  may  refuse  to  work,  squeals  and 
•  kicks  when  approached,  elevates  and  protrudes  the  clitoris, 
micturates  frequently,  the  material  being  very  mucoid. 
The  cow  bellows,  is  excited,  and  mounts  its  fellows.  Sheep 
become  restless  and  follow  the  ram.  The  dog  is  playful, 
excited,  and  desires  the  attention  and  companionship  of 
the  males  of  her  own  species.  In  all  animals  it  is  only 
during  the  actual  period  of  cestrus  or  desire  that  copulation 
is  permitted,  and  in  all  polycestrous  domestic  animals  this 
period  is  variable  in  extent. 

Changes  in  the  Uterus  during  Sexual  Excitement. — During 
prooestrum  there   is  an   increase   in   the   uterine   stroma. 


GENERATION  AND  DEVELOPMENT  581 

injection  of  the  mucous  membrane  in  consequence  of  a 
dilatation  of  the  capillaries,  and  usually  a  breaking  down 
of  the  walls  of  the  latter,  leading  to  extravasation  of  blood 
into  the  stroma,  or  even  into  the  cavity  of  the  uterus.  The 
glands  of  the  uterus  swell  and  pour  out  a  slight  secretion. 
In  some  animals  such  as  the  monkey  the  epithelial  lining 
of  the  uterus  is  destroyed  during  this  period,  but  with 
ungulates  desquamation  of  the  uterus  is  probably  very  rare, 
while  in  carnivores  it  occurs  more  or  less  in  every  case. 
The  pigmentation  found  in  the  mucous  membrane  of  the 
uterus  after  cestrus  is  due  to  the  extravasation  of  blood ; 
this  blood  is  also  the  source  of  the  blood-stained  discharge, 
and  on  a  more  extensive  scale  is  the  cause  of  the  menstrual 
flow  in  monkeys  and  women,  in  both  of  which  there  is  in 
addition  blood  collected  in  lacunse  in  the  wall  of  the  uterus 
and  destruction  of  the  epithelium.  Gradually  in  all  animals 
the  uterus  recovers  its  normal  appearance,  procestrum 
passes  away,  and  is  followed  by  oestrus.  Bearing  in  mind 
the  rapidity  with  which  cestrus  may  follow  procestrum  in 
such  animals  as  the  mare,  cow,  and  sheep,  it  is  evident  that 
the  whole  of  the  above  process  cannot  always  be  fully  gone 
through  ;  but  in  the  dog,  whose  cycle  is  far  more  regular, 
the  uterus  undergoes  the  changes  described. 

By  systematically  preventing  animals  from  breeding  the 
sexual  season  may  be  interfered  with  to  the  extent  of 
complete  cessation  (Heape).  Certainly  the  mare  used  late 
in  life  for  breeding  purposes  often  proves  barren.  Yet 
there  are  mares  which,  though  deprived  of  the  services  of 
the  male,  never  lose  their  desire,  and  may  for  the  greater 
part  of  their  life  be  a  source  of  danger  from  sexual 
excitement.  The  cause  of  cestrus  is  an  internal  secretion 
produced  by  the  stroma  of  the  ovary.  (See  Corpus  Luteum, 
p.  591.) 

When  male  animals  suffer  from  a  periodic  sexual  excite- 
ment it  is  known  as  rutting.  This  term  should  he  confined 
entirely  to  a  male  sexual  season,  such  as  is  experienced 
by  the  camel,  stag,  elephant,  and  ostrich.  In  the  rutting 
stag  the  neck  becomes  enormously  swollen  (Leeney),  the 


582     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

elephant  experiences  a  discharge  from  the  temporal  gland, 
and  the  ostrich  becomes  red  in  the  legs.  All  these  are  at 
this  time  dangerous  to  approach,  and  are  frequently  violent 
and  aggressive. 

Effect  of  Removal  of  Testicles  and  Ovaries. — The  influence 
of  the  removal  of  the  ovaries  and  testicles  on  general  meta- 
bolism is  a  subject  which  has  been  referred  to  in  dealing 
with  internal  secretions  (p.  265),  and  attention  was  there 
drawn  to  the  fact  that  both  in  cats  and  dogs  the  complete 
removal  of  the  ovaries,  and,  it  may  be  added,  of  both  horns 
of  the  uterus,  may  not  in  every  case  prevent  an  animal 
exhibiting  oestrus.  Such,  of  course,  are  exceptional  cases, 
for  ovariotomy  usually  suppresses  the  function.  If  an 
animal,  for  instance,  be  operated  upon  before  puberty, 
viz.,  before  an  cestrual  period  has  had  time  to  appear,  such 
an  one  will  not  subsequently  experience  any  sexual  excite- 
ment. If  the  operation  be  performed  during  the  first  preg- 
nancy, the  '  heat '  period  does  not  occur.  If  operated  upon 
after  one  or  more  oestrual  periods,  and  not  being  at  the 
time  pregnant,  there  may  be  a  few  returning  heat  periods 
and  free  sexual  intercourse. 

If  castration  of  the  stag  be  practised  the  antlers  fall  off 
from  the  seventh  to  the  ninth  day  after  operation  ;  fourteen 
days  is  said  to  be  the  longest  time  they  remain.  This  is 
evidence  of  an  internal  secretion  of  the  testicle  (p.  265) 
which  influences  the  growth  and  shedding  of  the  antlers, 
while  the  chain  of  evidence  is  completed  by  the  fact  that  cas- 
tration on  one  side  only  affects  the  growth  of  the  antler  on 
that  side.  If  the  epididymis  be  left  after  complete  castra- 
tion, its  presence  modifies  the  growth  of  the  next  pair  of 
antlers.* 

Similarly  the  growth  of  parts  in  other  male  animals  is 
affected  by  castration.  Cats  operated  upon  while  very  young 
have  heads  which  are  indistinguishable  from  the  female ; 
the  tissues  of  the  jowl,  which  give  the  head  of  the  male 
cat  such  a  massive  appearance,  are  lost  after  castration, 

*  I  am  indebted  to  Mr.  H.  Leeney,  M.R.C.V.S.,  Hove,  for  these 
facts  and  much  other  information  on  the  subject. 


GENERATION  AND  DEVELOPMENT  583 

and  this  may  occur  even  when  the  operation  is  performed 
late  in  life.  Female  cats  operated  upon  while  young  acquire 
a  head  of  the  male  type,  and  even  if  the  operation  be  per- 
formed when  approaching  middle  life,  there  is  a  disposition 
to  broadening  of  the  skull  (Leeney). 

The  alteration  in  the  shape  of  the  male  and  female  skull 
observed  in  the  cat  when  castration  or  ovariotomy  is  prac- 
tised in  early  life,  supports  the  view  advocated  by  Heape 
that  no  being  is  wholly  male  or  wholly  female,  but  a 
portion  of  each  sex  with  one  predominant.  Cocks  con- 
verted into  capons  when  young  do  not  develop  such  full 
male  plumage,  and  the  combs  and  wattles  are  more  like 
those  of  the  hen.  Pullets  from  which  the  '  clutch '  has 
been  taken  grow  fat,  and  sometimes  put  on  male  plumage. 
Hen  pheasants  injured  by  shot  in  the  ovarium  have  fre- 
quently been  found  with  male  plumage,  and  disease  of  the 
ovary  in  hens  or  pheasants  may  lead  to  their  crowing 
(Leeney). 

The  Testicles  are  solid  organs  with  an  external  covering 
of  serous  membrane,  and  possessing  a  tunica  allmginea, 
and  a  stroma  or  framework  of  fibrous  tissue.  The  spaces 
of  the  meshwork  are  occupied  by  the  seminiferous  tubules. 
These  tubules  are  highly  convoluted  in  the  parts  imme- 
diately concerned  in  the  formation  of  spermatozoa,  and 
commence  usually  by  blind  extremities.  If  the  changes  in 
the  cells  found  on  the  basement  membrane  of  the  tubules  be 
followed,  it  is  found  that  the  cells  of  the  lining  epithelium 
divide  into  two  daughter  cells,  one  remaining  attached  to 
the  basement  membrane,  the  su>itentacular  cell,  the  other 
becomes  a  spermatogen.  The  spermatogen-cells  divide  and 
subdivide  to  form  other  cells  that  are  recognised  as  sperma- 
toblasts. These  spermatoblasts  elongate  and  pass  into 
spermatozoa,  collecting  into  sheaves  as  they  do  so,  and 
becoming  attached  to  the  sustentacular  cells  that  are 
placed  on  the  basement  membrane.  These  sustentacular 
cells  minister  to  the  needs  of  the  developing  sperms  until 
they  are  fully  matured.  The  latter  are  then  set  free,  and 
pass  into  the  lumen  of  the  tubule.     The  sjreimatozca  are 


584  A  MANUAL  OF  YETEKINAEY  PHYSIOLOGY 

produced  in  enormous  numbers;  it  is  estimated  in  man 
that  each  cubic  centimetre  of  seminal  fluid  contains  from 
sixty  to  seventy  milhons  of  cells. 

A  mature  spermatozoon  under  favourable  conditions  is 
active,  moving  about  rapidly  in  the  seminal  fluid  by  means 
of  its  long  vibratile  tail.  It  is  formed  of  a  head,  a  middle 
piece  or  body,  and  a  tail.  The  head  corresponds  to  the 
nucleus,  and  is  constantly  present,  the  middle  piece  and 
tail  are  developed  to  a  varying  degree  in  different  animals. 
In  the  horse  the  length  of  the  head,  which  is  bluntly  pear- 
shaped,  is  about  5  ^u.,*  the  tail  is  eight  or  nine  times  as  long 
as  the  head.  It  is  supposed  that  the  sperm-cell  extrudes 
polar  bodies  as  does  the  ovum  (p.  589),  but  they  have 
not  been  recognised.  The  head  of  the  sperm  may  be  con- 
sidered as  the  male  pronucleus. 

The  Spermatic  Fluid  is  alkaline  or  neutral  in  reaction,  of 
viscid  consistence,  contains  proteids,  nuclein,  lecithin, 
cholesterin,  fat,  leucine,  tyrosine,  kreatine,  inosite,  sulphur, 
alkaline  earths,  chloride  of  sodium,  and  phosphates. 

The  essential  element  is  the  sjjermatozoa,  without  which 
the  fluid  is  not  fertile.  Spermatozoa  exhibit  spontaneous 
movement,  the  long  tail  moving  from  side  to  side,  by 
which  means  the  organism  is  propelled  when  placed  in  the 
body  of  the  female.  The  vitality  of  spermatozoa  under 
suitable  conditions  is  considerable,  and  when  placed  in 
the  body  of  the  female  they  have  been  found  very  active 
many  days  after  copulation.  Colin  found  them  active  in 
the  vesiculffi  seminales  eight  days  after  castration.  The 
spermatozoa  are  readily  killed  by  ordinary  or  acidulated 
water,  glycerin,  etc. 

It  is  not  known  in  what  way  the  secretions  of  the  pros- 
tate and  seminal  vesicles  influence  the  main  secretions  with 
which  they  are  ejected,  but  it  is  supposed  they  maintain 
the  motility  of  the  spermatozoa.  The  prostatic  fluid  pre- 
cedes the  spermatic  in  ejaculation,  and  in  stallions  and 
bulls,  when  excessive  daily  demands  are  made,  the  fluid 
ejaculated  is  largely  prostatic  and  infertile.  The  testicular 
products  of  hybrids,  such  as  the  mule,  are  infertile. 
*  j«  =  a  micron  ;  xooo  niillimetre  =  v,- J^y  inch  (nearly). 


GENERATION  AND  DEVELOPMENT  585 

The  Period  of  Puberty,  or  that  time  in  the  animal's  life 
when  it  is  capable  of  procreation,  has  been  put  at  1^  years 
for  the  horse,  8  to  12  months  for  bovines,  and  6  to  8  months 
for  the  sheep,  pig,  and  dog.  There  is,  however,  a  great 
difference  between  capability  and  fitness  for  procreation. 
Breeding  from  immature  animals  is  one  explanation  of  a 
great  deal  of  the  worthless  material  in  the  shape  of  horses 
which  may  be  seen  in  all  countries. 

The  advent  of  maturity  is  marked  by  certain  changes  in 
form,  particularly  in  horses.  They  lose  their  awkwardness, 
the  outline  of  the  frame  becomes  more  consolidated  and  in 
greater  unison.  In  the  male  the  neck  becomes  thick  and 
curved,  the  voice  deepens,  and  the  whole  appearance 
denotes  life  and  vigour.  In  both  the  stallion  and  bull  the 
temper  is  usually  irritable  and  uncertain,  and  often  ex- 
tremely vicious.  The  age  at  which  procreation  ceases  is  not 
known.  Fleming  says  that  mares  have  been  known  to  pro- 
duce foals  at  28,  32,  and  38  years  of  age,  and  it  is  certain 
that  some  good  stallions  have  been  advanced  in  years. 

The  Act  of  Erection  is  a  vascular  phenomenon  produced 
by  an  engorgement  of  the  erectile  tissue  of  the  penis  with 
blood.  This  engorgement  is  brought  about  by  stimulation 
of  the  nervi  erigentcs,  which  arise  from  the  sacral  portion 
of  the  cord.  These  nerves  furnish  dilator  fibres  to  the 
vessels  of  the  penis,  and  under  their  influence  the  cavernous 
spaces  of  the  erectile  tissue  become  gorged  with  blood 
under  pressure.  The  nervi  erigentes  act  reflexly  through 
an  erection  centre  in  the  cord,  while  the  erection  centre 
is  under  the  influence  of  higher  centres  in  the  brain. 
Erection  and  ejaculation  in  the  dog  may  be  produced  by 
stimulation  of  a  definite  area  of  the  cortex  of  the  cerebrum, 
and  they  may  also  be  produced  after  section  of  the  spinal 
cord  in  the  lumbar  region.  The  sensory  nerves  in  the 
penis,  by  which  erotic  sensations  are  carried,  are  the  pudic  ; 
if  the  pudic  nerve  be  cut  erection  is  impossible  ;  if  the 
central  cut  end  Ije  stimulated  it  leads  to  ejaculation. 

The  first  portion  of  the  penis  which  receives  the  excess  of 
blood  in  erection  is  the  corpus  cavernosum  ;  the  spongiosum 


580     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

and  glans  are  not  fully  erect  in  the  stallion  until  the  penis 
is  introduced  into  the  vagina  ;  at  the  moment  of  ejacula- 
tion in  this  animal  the  glans  swells  enormously,  apparently 
to  cover  or  grasp  the  os  uteri.  Though  the  organ  in  the 
horse  assumes  such  considerable  proportions,  in  the  bull 
this  is  not  marked.  The  peculiar  penis  in  this  animal 
comes  to  a  narrow  point  without  any  of  the  swelling  observ- 
able in  the  stallion.  In  the  act  of  erection,  the  S-shaped 
curve  of  the  penis  is  removed,  and  the  organ  elongates ;  at 
the  same  time  the  retractor  muscles  of  the  sheath  draw 
back  the  prepuce  and  the  organ  is  exposed.  In  the  ram, 
also,  the  penis  is  narrow  and  pointed,  and  the  vermiform 
appendix  at  its  extremity  appears  essential  for  successful 
impregnation,  for  if  it  be  removed,  it  is  said  the  animal 
proves  sterile.  In  the  dog  the  increase  in  the  size  of  the 
penis  is  mainly  at  its  posterior  part,  and  the  bulbous 
swellings  there  observable  are  the  portions  grasped  by  the 
spasm  of  the  sphincter  cunni  of  the  female,  rendering  with- 
drawal impossible  until  complete  relaxation  occurs.  The 
bone  in  the  penis  of  the  dog  facilitates  its  introduction  into 
the  vagina. 

Sexual  Intercourse. — Copulation  is  not  necessary  in  all 
animals,  nor  indeed  in  any.  What  is  required  is  merely 
an  interchange  of  elements  from  the  nucleus  of  two  different 
cells.  To  this  last  statement  a  slight  exception  might  be 
taken,  because  there  is  a  condition,  partlienogenesis,  where 
the  access  of  a  second  element  is  not  required,  but  this 
method  of  development  is  unknown  in  the  higher  animals. 
The  act  of  intercourse  is  of  short  duration  in  the  majority 
of  animals,  excepting  the  dog,  pig,  and  camel.  Colin 
places  it  at  ten  to  twelve  seconds  for  a  vigorous  stallion ; 
it  is  exceedingly  rapid,  almost  instantaneous,  in  the  bull 
and  ram,  probably  from  the  peculiar  shape  of  their  intro- 
mittent  organ.  The  spermatic  fluid  is  forced  into  the 
vagina,  or  even  directly  into  the  uterus.  The  peculiar 
termination  of  the  urethra  of  the  horse,  and  the  bulbous 
enlargement  of  the  glans  during  the  final  act  of  coition, 
point   to   the  organ   grasping   the  os   at   the   moment  of 


GENEEATTON  AND  DEVELOPMENT  587 

ejaculation,  while  the  projecting  portion  of  the  urethra  is 
inserted  into  it,  by  which  means  some  of  the  fluid  is 
undoubtedly  directly  injected  into  the  uterus ;  the  pointed 
penis  of  the  bull  and  ram  makes  it  certain  that  such  is 
also  the  case  in  these  animals.  An  examination  of  the 
uterus  of  the  sheep  and  dog  a  few  minutes  after  coition 
has  revealed  the  presence  within  it  of  spermatozoa.  There 
is  ample  evidence  that  the  spermatozoa  may  remain  alive 
for  several  days  within  the  uterus.  At  the  moment  of 
intercourse  the  uterus  becomes  erect,  and  the  introduction 
of  the  male  element  into  it  is  further  assisted  by  the 
aspiration  following  its  subsidence. 

The  actual  mechanism  of  ejaculation  is  produced  by  a 
contraction  of  the  vesiculte  seminales,  the  prostate  gland, 
and  probably  of  the  vasa  deferentia,  through  the  reflex 
action  of  the  ejaculation  centre  in  the  lumbar  and  sacral 
portions  of  the  cord.  By  this  means  the  seminal  fluid  is 
forced  out  of  the  vesiculae  into  the  urethra,  and  by  means 
of  the  muscles  of  the  perinreum  is  forcibly  ejected  from  the 
urethra.  In  animals  possessing  no  vesiculfe,  such  as  the 
dog,  ejaculation  takes  place  direct  from  the  testicle  and 
vas  deferens. 

The  Ovaries. — They  are  solid  organs,  and  about  half  the 
size,  or  a  little  less  than  half  the  size,  of  the  testicle  of  the 
male.  An  exception  to  this  must  be  made  in  the  case  of 
the  sheep ;  here  the  ovary  is  very  small  compared  to  the 
testicle  of  the  ram,  this  animal  for  its  size  having  probably 
the  largest  of  testicles,  certainly  among  the  domestic  males. 
The  ovaries  of  the  mare,  cow,  and  sheep,  are  somewhat 
ovoid  with  a  slight  depression  termed  the  hilum ;  the 
ovaries  of  the  pig  and  dog  are  lobulated  and  resemble  a 
bunch  of  grapes ;  the  ovary  of  the  cat  and  rabbit  is  more 
or  less  lenticular.  The  substance  of  the  ovary  is  divided 
into  cortex  and  medulla  ;  the  cortex  being  that  portion 
containing  the  developing  eggs  or  ova,  the  medulla  being 
the  solid,  connective  tissue,  vascular  core.  Covering  the 
ovary  is  a  modified  endothelium,  the  (jerminal  epitheUum. 
This    is    of   the    columnar  type    (a    modification    of    the 


588    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

endothelium  of  the  body  cavity),  and  is  found  over 
the  whole  ovary  except  where  the  ligament  of  the  ovary 
passes  to  the  uterine  horn,  and  where  the  broad  ligament 
of  the  uterus  is  attached  to  the  ovary  itself.  The  epithelium 
is  called  germinal  because  from  it  the  eggs  are  developed 
during  intra-uterine  life ;  it  is  probable  that  no  new  ova 
are  formed  after  birth.  During  development  the  germinal 
epithelium  grows  into  the  body  of  the  ovary  as  a  long 
cylinder  of  cells.  These  cells  eventually  are  cut  off  from 
any  connection  with  the  epithelium  covering  the  ovary, 
and  one  cell,  it  may  be  two,  takes  on  the  appearance  and 
characters  of  an  ovum.  The  other  cells  that  have  accom- 
panied and  been  constricted  off  with  the  ovum  take  on  the 
duties  of  the  membrana  granulosa,  which  is  merely  a 
cellular  sphere  containing  the  ovum.  The  earliest  ova  are 
found  in  the  cortex  as  large  cells  enclosed  in  the  simple 
one-layered  membrana  granulosa. 

The  changes  that  occur  from  this  primitive  condition 
until  the  ovum  is  mature  are  chiefly  indicated  in  the  wall 
of  the  structure  containing  the  egg,  the  so-called  Graafian 
follicle.  A  connective-tissue  capsule,  the  tunica  fibrosa, 
originates  around  the  follicle,  and  finally  a  cavity  appears 
owing  to  a  splitting  of  the  membrana,  a  cavity  containing 
a  fluid  under  pressure,  the  liquor  follicuU.  The  ovum  con- 
tinues to  grow  slowly  until  it  reaches  about  yl-"  ('IS  to 
•2  mm.)  in  diameter,  and  is  found  in  an  upheaval  of  the 
cells  of  the  membrana  granulosa  known  as  the  discus  or 
cumulus  proligerus. 

The  Graafian  follicle  of  the  adult  animal  consisting  of 
the  above  -  mentioned  parts,  and  containing  the  ovum, 
extends  throughout  the  thickness  of  the  cortex  of  the  ovary, 
and  daily  becoming  larger,  it  appears  eventually  as  a 
vesicle  on  the  surface.  The  formation  of  the  liquor 
folliculi  under  pressure,  and  its  tendency  to  move  in  the 
direction  of  the  least  resistance,  will  influence  the  point  of 
rupture,  which  is  said  generally  to  occur  at  the  hilum  or 
thereabouts. 

"When  rupture  of  the  Graafian  follicle  occurs  the  ovum  is 


GENERATION  AND  DEVELOPMENT    589 

flushed  out,  and  at  the  same  moment,  according  to  Henson, 
the  fimbriated  extremity  of  the  Fallopian  tube  becoming 
erect  grasps  the  ovary,  and  thus  the  escaping  ovum  is 
received  into  its  '  duct.'  Probably  the  converging  furrows 
found  on  the  plicated  extremity  of  the  Fallopian  tube  may 
assist  in  du'ecting  the  ovum  to  the  ostiiDii  ahdominale.  If 
by  chance  the  ovum  be  not  caught  and  carried  away  to  the 
uterus  as  described,  it  may  fall  into  the  peritoneal  cavity 
and  perish,  or  if  it  has  been  already  fertilized  abdominal 
fcetation  may  occur,  the  peritoneum  acting  as  a  matrix  in 
which  the  embryo  may  develop. 

The  Ovum. — With  the  exception  of  those  produced  by  the 
pyototlieria  (duck-mole  and  spiny  ant-eater),  the  mammalian 
ova  are  extremely  small.  They  vary  in  size  from  15  o  to 
T^  of  an  inch,  and  although  not  to  be  compared  to  those 
of  birds,  reptiles,  or  amphibians,  yet  they  are  undoubtedly 
the  largest  cells  found  in  the  mammalian  body.  The 
greater  size  of  the  eggs  of  birds,  reptiles  and  amphibians 
is  due  to  the  quantity  of  deutoplasm  or  yolk  contained 
therein.  In  mammals  this  is  small  in  amount,  owing  to 
the  speedy  union  of  the  developing  ovum  to  the  uterine 
wall,  which  effects  an  intimate  connection  with  an  abundant 
food  supply. 

The  ovum  is  a  typical  cell,  it  is  spherical  and  more  or 
less  translucent.  It  has  a  thick  cuticle  or  zona  radiata, 
within  which  lies  the  protoplasm  and  vitellus  or  yolk, 
confined  in  a  special  membrane  the  vitelline  membrane. 
Within  the  vitellus  or  yolk  is  the  germinal  vesicle  or  the 
nucleus  of  the  cell,  and  within  this  the  germinal  spot  or 
nucleolus.  This  is  the  structure  of  the  ovum  prior  to  its 
extrusion  from  the  Graafian  follicle  ;  but  either  just  before 
or  immediately  after  escape  from  this,  and  prior  to  impreg- 
nation, changes  occur.  These  changes  more  especially 
involve  the  nucleus  or  germinal  vesicle,  and  are  known  as 
the  formation  of  the  polar  bodies. 

The  extrusion  of  the  Polar  Bodies  has  been  studied  by 
Van  Beneden  in  the  ova  of  the  ascaris  megalocephala  of  the 
horse,  and  what  is  true  of  this  is  believed  to  be  true  for  the 


590    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

mammalian  cell.  The  first  stage  is  that  of  indirect  division 
of  the  nucleus,  and  its  movement  towards  the  periphery 
of  the  cell.  The  nucleolus  probably  divides  in  a  similar 
manner,  but  its  fate  is  not  known.  The  nucleus  having 
divided,  one  half  is  extruded  into  a  space  beneath  the  zona 
radiata ;  thus  the  Jirst  polar  body  is  got  rid  of.  The  half  of 
the  nucleus  still  remaining  in  the  ovum  divides,  and  for  a 
second  time  a  nucleus  is  extruded,  forming  the  second  polar 
body. 

There  have  been  many  explanations  offered  of  the  sig- 
nificance of  the  polar  bodies.  Two,  however,  are  important 
and  worthy  of  mention.  Minot  and  Balfour  believed  that 
they  were  intended  to  prevent  parthenogenesis,  or  the 
possibility  of  a  new  creature  developing  from  an  ovum  that 
had  never  received  a  male  element.  Weismann  believes 
that  by  the  loss  of  certain  elements  by  means  of  the  polar 
bodies,  the  ovum  is  rendered  receptive  for  characters  of  the 
male  ;  in  other  words,  it  has  a  bearing  upon  the  hereditary 
properties  of  ovum  and  sperm,  the  polar  bodies  carrying 
away  superfluous  histological  and  genetic  properties. 

As  a  result  of  the  rupture  of  the  Graafian  follicle,  a  rent 
is  made  in  the  ovary.  This  wound  j&lls  with  blood  from 
the  opened  vessels,  and  for  some  time  afterwards  appears 
as  a  pigmented  spot.  If  pregnancy  has  not  supervened,  it 
undergoes  a  retrogressive  metamorphosis  and  soon  dis- 
appears. If,  however,  the  ovum  is  fecundated,  the  corpus 
lateam,  as  this  pigmented  spot  is  termed,  continues  to  grow, 
and  may  be  observed  in  the  ovary  even  near  term. 

The  Corpus  Luteum  of  the  pregnant  animal  is  very  much 
larger  than  that  of  the  non-pregnant,  and  it  appears  to  be 
conclusively  settled  that  the  existence  of  this  yellow  tissue 
in  the  ovary  is  not  for  the  mere  purpose  of  filling  up  a 
cavity  in  its  structure,  but  that  the  yellow  body  is  a 
ductless  gland  which  becomes  an  active  secreting  agent, 
producing  a  secretion  by  which  the  ovum  is  anchored  to 
the  wall  of  the  uterus,  and  its  nourishment  and  develop- 
ment assisted.  This  ductless  gland  is  functional  until 
about  the  middle  of  pregnancy,  when  it  is  no  longer  a 


GENERATION  AND  DEVELOPMENT  591 

necessary  factor  in  the  nourishment  of  the  embryo  and 
consequently  degenerates.  That  the  corpus  luteum  takes 
httle  or  no  share  in  the  production  of  seasonal  sexual 
excitement  appears  quite  clear ;  this  is  the  function  of  the 
stroma  of  the  ovary  which  pours  an  internal  secretion  into 
the  blood,  and  so  brings  about  menstruation  and  astrus. 

Ovulation  is  the  process  of  egg  extrusion.  In  some 
animals,  as  the  rabbit  and  ferret,  it  can  only  occur  as  the 
result  of  coition,  the  presence  of  spermatozoa  in  the  uterus 
being  essential  to  the  act.  In  others,  and  they  represent 
the  majority,  such  as  the  mare,  donkey,  cow,  sheep,  pig, 
and  dog,  ovulation  occurs  during  oestrus,  but  the  act  of 
copulation  is  not  necessary  to  extrusion,  and  in  such 
animals  artificial  insemination*  is  therefore  possible.  The 
period  of  oestrus  is  not  necessarily  identical  with  the 
period  of  ovulation,  cestrus  may  occur  without  ovulation, 
and  ovulation  may  occur  without  cestrus.  Ewart  says  the 
mare  may  mature  and  discharge  one  or  more  eggs  after  she 
has  become  impregnated.  Ovulation  occurs  at  the  moment 
the  Graafian  vesicle  ruptures  and  the  ovum  is  ejected. 
The  number  of  ova  which  may  be  extruded  during  one 
sexual  period  is  not  known  with  any  degree  of  certainty ;  in 
the  case  of  the  cat  and  dog  there  is  evidence  of  several 
being  ejected,  for  each  foetus  represents  a  separate  egg. 
The  number  of  eggs  laid  is  always  greatly  in  excess  of  the 
number  impregnated,  and  the  mare  which  probably  only 
produces  one  egg  at  a  time,  and  with  whom  twin  births  are 
very  rare,  is  believed  by  Ewart  to  shed  about  ten  or  twenty 
ova  annually.  Whether  an  equal  number  is  discharged  by 
each  ovary  is  unknown.  Probably  one  ovum  for  the  mare, 
cow,  ass,  deer,  elephant,  or  monkey  at  each  testrous  period 
is  the  rule,  though  two  may  be  discharged.  The  sheep 
probably  discharges  one  to  four,  the  dog,  wolf,  and  cat  five 
to  six,  the  pig  ten  or  even  fifteen. 

Determination  of  Sex. — Heape  t  maintains  that  there  is  no 

*  First  practised  on  the  dog  by  Spallanzani,  1784. 

t  '  Notes  on  the  Proportion  of  Sexes  in  Dogs,'  W.  Heape,  M.A., 
F.E.S.,  rroceedimjH  of  the  Canihrulge  Philosojihical  Societi/,  vol.  xiv., 
pt.  ii.,  1907. 


592     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

such  thing  as  a  pure  male  or  female  animal,  but  that 
all  contain  a  dominant  and  a  recessive  sex,  excepting 
hermaphrodites,  in  which  both  sexes  are  equally  repre- 
sented. The  assumption  of  male  characteristics  in  old 
females,  and  of  female  characteristics  in  old  males  of  the 
human  species,  is  noted  by  Heape.  We  have  referred  on 
p.  582  to  the  remarkable  effect  of  castration  and  ovariec- 
tomy on  the  skull  of  young  cats,  castration  producing  a 
female  skull,  ovariectomy  a  skull  of  the  male  type.  Capon- 
ning  also  induces  female  plumage.  Injuries  of  the  ovarium 
in  birds  are  associated  with  crowing  and  male  plumage,  all 
of  which  is  evidence  that  the  recessive  sex  asserts  itself 
when  the  dominant  sex  becomes  impaired,  and  supports 
the  view  held  by  Heape  that  there  is  no  such  thing  as  a 
pure  male  or  female  animal.  If  this  be  true,  it  naturally 
follows  that  a  male  ovum  is  fertilized  by  a  female  sperma- 
tozoan  and  a  female  ovum  by  a  male  spermatozoan  (Heape). 
Everything,  in  fact,  points  to  ova  and  spermatozoa  being 
sexual — that  is  to  say,  there  are  male  and  female  ova, 
male  and  female  spermatazoa.  Microscopic  differences  in 
the  structure  of  spermatozoa  have  also  been  observed  which 
have  led  to  their  classification  into  two  groups,  which  are, 
in  all  probability,  male  and  female. 

The  bearing  of  Heape's  work  on  the  determination  of 
sex  is  of  great  importance.  He  maintains  that  the  sex  of 
the  offspring  is  determined  at  the  time  of  fertilization,  and 
that  no  influence  exerted  subsequently  can  alter  it.  This 
is  opposed  to  generally  accepted  doctrines,  but  results  from 
an  acceptance  of  his  hypothesis  that  an  ovum  in  which  one 
sex  is  dominant  must  be  fertilized  by  a  spermatozoan  in 
which  the  opposite  sex  is  dominant ;  whether  the  sex  be 
determined  by  the  ovum  or  spermatozoan  depends  upon 
which  is  the  more  powerful  of  the  two. 

Heape's  study  of  the  ovary  of  the  rabbit*  shows  that 
ova  may  degenerate,  and  that  one  of  the  chief  causes  is 
nutrition.     It   is   possible   that   nutrition   has  a    selective 

*  'Ovulation  and  Degeneration  of  Ova  in  the  Rabbit,'  Proc.  Eoyal 
Societij,  vol.  B.,  76,  1905. 


GENERA.TION  AND  DEVELOPMENT  593 

action  on  ovarian  ova,  and  so  effects  a  variation  in  the 
proportion  of  the  sexes  of  the  ova  produced ;  where  no 
such  selective  action  occurs  in  the  ovary,  the  proportion  of 
the  sexes  of  ovarian  ova  produced  is  governed  by  the  laws 
of  heredity  (Heape). 

Impregnation. — The  male  element  meets  with  the  female 
in  the  narrow  passage  of  the  Fallopian  tube.  Bearing  in 
mind  the  enormous  number  of  spermatozoa  in  even  only  a 
small  amount  of  the  secretion,  it  is  easy  to  understand  why 
the  ovum  cannot  escape  coming  in  contact  with  them.  The 
ovum  is  transported  towards  the  uterus  by  the  cilia  of  the 
tube,  the  spermatozoa  travel  in  the  opposite  direction  by 
means  of  their  own  vibratile  motion.  One  spermatozoan 
suffices  for  impregnation ;  it  passes  through  the  zona 
radiata,  reaches  the  vitelline  membrane,  and  when  within 
it  the  tail  is  lost,  and  only  the  head  and  part  of  the  middle 
piece  remain.  The  two  pro-nuclei,  male  and  female,  come 
together,  and  a  single  nucleus  results.  The  fusion  of  the 
nuclear  element  of  these  two  different  cells  results  in  a  new 
structure,  the  ovum  is  fertilized,  and  from  this  is  developed 
a  new  being  embodying  the  hereditary  properties  of  both 
parents. 

In  the  sheep*  the  impregnated  egg  enters  the  uterus 
on  the  fourth  or  fifth  day  and  travels  slowly  along  it  until 
the  ninth  day.  On  the  ninth  day  the  zona  radiata  ruptures 
and  the  blastocyst  (that  is  the  external  cover  of  the  cellular 
mass)  lies  in  contact  with  the  uterine  epithelium. 

On  the  twelfth  day  the  ovum  has  reached  nearly  to  the 
lower  limit  of  the  horn  in  which  it  lies,  the  glands  of  the 
uterus  enlarge,  and  the  blastocyst  rapidly  elongates  so  that 
each  end  grows  out  to  the  tip  of  each  horn  of  the  uterus. 
If  one  embryo  only  be  present  it  extends  through  both 
horns  of  the  uterus ;  if  there  are  two  they  are  each  con- 
fined to  one  horn.  On  the  seventeenth  and  eighteenth  day 
the  first  attachment  of  the  embryo  to  the  uterus  is  effected, 

*  '  The  Morphology  of  the  Ungulate  Placenta,'  by  R.  Assheton,  M.A., 
whom  in  the  above  account  of  the  sheep  we  have  entirely  followed. 
Phil.  Trans.,  B.  vol.,  198.     1905. 

38 


594     A  MANUAL  OF  VETERINAKY  PHYSIOLOGY 

a  very  important  period  in  embryonic  life.  Up  to  this  time 
the  only  nourishment  available  is  that  furnished  by  the 
juices  poured  into  the  uterine  cavity  by  the  glands,  and 
until  the  twentieth  day  the  ovum  receives  no  other  source 
of  supply  but  this.  On  the  twenty-eighth  day  villi  on  the 
external  covering  of  the  embryo  are  well  developed,  and  on 
the  maternal  cotyledons  are  little  depressions  into  which 
they  fit.  The  allantoic  (see  later)  has  grown  rapidly,  and 
the  yolk  sac  (which  see)  has  become  reduced  as  the  allantois 
increases.  By  the  forty-fourth  day  the  foetal  cotyledons 
are  scattered  over  the  whole  surface  of  the  embryonic 
covering.  On  the  seventy-eighth  day  the  general  character 
of  the  placenta  is  established.  As  the  uterus  swells  owing 
to  the  increase  in  size  of  its  contents,  it  does  so  generally 
excepting  the  upper  part  of  the  horns,  which  are  but  little 
longer  than  normal,  and  are  engaged  in  active  secretion. 

We  have  given  this  condensed  account  from  Assheton  of 
the  development  of  the  embryo  of  the  sheep,  as  we  have 
previously  had  but  little  exact  knowledge  how  the  embryo 
of  a  ruminant  comports  itself  during  the  early  days  of 
development.  The  development  of  the  embryo  of  the  horse 
has  been  dealt  with  by  Ewart,*  not  with  the  same  degree 
of  fulness  as  the  above,  as  that  is  practically  impossible, 
but  sufficiently  so  to  show  not  only  the  characteristic 
features  of  the  process,  but  their  profound  practical  bear- 
ing on  the  hygienic  care  of  brood  mares. 

We  shall  see  presently  that  the  human  decidua  grows 
over  the  ovum  on  its  arrival  in  the  uterus,  and  so  prevents 
its  escape.  No  such  pouch  is  formed  in  the  ungulates, 
and  the  escape  of  the  ovum  before  it  is  securely  fixed  to  the 
wall  of  the  uterus  is  not  unlikely,  especially  in  the  horse, 
where  the  connection  between  the  embryonic  sac  and  the 
uterus  is  easily  broken  down.  To  understand  how  this 
occurs  Ewart  points  out  that  the  remote  ancestors  of  the 
horse  were  probably  born  on  the  forty-seventh  or  forty- 
eighth  day  of  conception,  and  like  the  ancient  and  primi- 

*  '  A  Critical  Period  in  the  Development  of  the  Horse,'  by  T.  C. 
Ewart,  M.D.,  F.R.S.     1897. 


GENERATION  AND  DEVELOPMENT  595 

tive  mammals  the  opossum  and  kangaroo,  passed  from  the 
uterus  to  a  pouch  where  they  lay  securely  suspended  by  a 
teat  until  then-  perfect  development  was  completed.  The 
arrangement  by  which  the  equine  embryo  is  anchored,  as 
Ewart  calls  it,  to  the  wall  of  the  uterus,  is  in  the  first 
instance  by  some  of  the  cells  of  the  outer  layer  of  the 
embryo,  at  a  part  which  is  in  communication  with  the  yolk 
sac.  This  connection  (Fig.  144,  a,  h,  c)  is  of  a  very  slender 
kind  and  is  the  only  one  which  exists  up  to  the  fifth  week. 


Fig.  144. — Semi-diagrammatic  Representation  of  a  Four  Weeks 
Horse  Embryo  and  its  Fcetal  Appendages,  Natural  Size 
(Ewart). 

am.,  The  aixinion ;  y.s.,  the  j'olk  sac,  which  is  vascular,  v,  as  far  as  the 
circular  bloodvessel,  s.t.,  and  crowded  with  granules  which  have 
entered  by  the  absorbing  area,  a,  i,  c,  of  the  yolk  placenta;  all., 
the  allantois.  The  embryo  measures  nearly  three-eighths  of  an 
inch  in  length,  and  is  curved  so  that  the  tail  lies  under  the  head. 

At  the  fifth  week  additional  means  of  securing  the  embryo 
to  the  wall  are  evident,  by  an  increase  in  the  size  and 
strength  of  the  original  yolk  sac  adhesion.  There  is  also 
a  girdle  about  |  inch  wide,  not  hitherto  found  in  any 
mammal  (Ewart),  placed  around  the  equator  of  the  embryo 
(Fig.  144,  t.g.).  This  girdle  obtains  adhesion  to  the 
uterine  wall  and  so  strengthens  the  original  anchorage. 
About  the  end  of  the  sixth  week  the  attachment  of  embryo 
to  uterus  is  again  becoming  precarious,  for  the  yolk-sac 

38—2 


596     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

attachment  area  has  become  less  (Fig.  145,  a-c),  while  the 
girdle  has  shifted  from  the  equator  to  near  the  pole 
(Fig.  145,  t.g.).  It  is  at  this  period  Ewart  considers  the 
primitive  ancestors  of  the  horse  were  born. 

At  the  end  of  the  seventh  week  the  supply  of  nourish- 
ment through  the  medium  of  the  yolk  sac  has  nearly  come 
to  an  end,  the  absorbing  area  next  the  uterus  is  considerably 
reduced,  and  it  is  at  this  period  when  an  entirely  new 
source  of  supply  and  attachment  has  to  be  found.     The 


Fig.  145. — A  Seven  Weeks'  Horse  Embryo,  Half  Natural 

Size  (Ewart). 

all.,  Allantois ;  am.,  amnion ;  c.v.,  non-vascular  villi  between  the 
allantois  and  the  yolk  sac,  not  hitherto  found  in  any  mammal, 
and  function  unknown ;  y.s.,  yolk  sac ;  a-c,  absorbing  area  of  the 
yolk  placenta  ;  ■w\  vascvilar  villi  or  allantois  ;  t.t.,  external  vascular 
villi  over  the  surface  of  the  embryonic  sac. 

supply  is  furnished  by  means  of  the  allantois,  while  the 
additional  attachment  is  furnished  by  the  girdle  becoming 
folded  into  ridges,  which  fit  into  grooves  and  depressions 
in  the  mucous  membrane  of  the  uterus.  The  outer  cover 
of  the  embryo  beyond  the  girdle  is  now  dotted  with  numerous 
minute  points,  which  subsequently  become  villi ;  the  villi  are 
derived  from  a  sprouting  of  the  allantoic  sac,  and  as  they 
grow  are  accommodated  in  pits  in  the  uterine  wall.  By 
the  end  of  the  eighth  week  this  has  been  accomplished. 
The  villi  are  not  more  than  ^  inch  long,  even  when  full 


GENERATION  AND  DEVELOPMENT  597 

grown,  and  at  birth  they  are  withdrawn  from  the  uterine 
pits.  Once  the  villi  have  become  established  the  question  of 
nourishment  becomes  no  longer  a  difficulty,  and  the  critical 
stage  in  the  development  of  the  horse  is  passed. 

The  cause  of  mares  '  breaking  service  '  from  the  sixth  to 
the  ninth  week  is  answered  by  Ewart  in  the  light  of  his 
inquiries.  At  the  third,  sixth,  and  ninth  week  the  physio- 
logical changes  associated  with  restrus  are  likely  to  super- 
vene and  shake  the  reproductive  system.  At  the  third 
week  the  risk  of  casting  off  the  embryo  is  not  so  great,  as 
the  area  by  which  it  is  attached  to  the  uterine  wall  is 
sufficiently  large  to  render  it  moderately  secure,  but  at  the 
sixth  week  a  change  from  yolk  sac  to  placental  nourishment 
is  being  effected,  and  the  yolk  sac  area  is  less  than  it  was 
at  the  third  week.  At  such  a  time  a  contraction  of  the 
uterine  horn  will  be  followed  by  expulsion  of  the  embryo. 
At  the  ninth  week  the  question  of  not  '  breaking  service ' 
depends  on  whether  the  villi  have  appeared  in  time,  and 
obtained  a  sufficiently  intimate  relation  with  the  uterine 
vessels  to  supply  the  embryo  with  the  additional  nourish- 
ment its  development  requires,  that  through  the  yolk  sac 
being  insufficient. 

Ewart  says  that  the  embryo  of  the  mare  usually  occupies 
the  right  horn  of  the  uterus,  and  in  the  early  days  is 
suspended  by  the  yolk  sac  from  the  upper  wall  of  the 
organ,  the  head  being  towards  the  body  of  the  womb. 
Later  the  foetus  may  lie  in  the  body  of  the  uterus,  but  the 
hind  limbs  remain  to  the  last  in  the  right  horn. 

Development  of  the  Ovum. — Ova  are  Iwlohlastlc  or  uierohlastic 
according  to  the  method  of  segmentation.  This  depends  upon  the 
amount  of  yolk  contained  in  the  egg ;  if  very  little  or  none  the  seg- 
mentation is  holoblastic  and  complete  as  in  the  eggs  of  mammals  ; 
if  abundant  as  in  birds,  the  segmentation  is  meroblastic  and  partial. 

After  fusion  of  the  two  pronuclei  the  resulting  nucleus  begins  to 
divide,  and  there  first  results  two  cells  which  are  not  equal  in  size. 
These  also  divide,  each  into  two,  and  the  inequality  of  size  of  the  first 
generation  is  impressed  upon  the  second,  and  after  the  third  division, 
when  eight  cells  have  resulted,  we  find  four  large  cells  and  four  some- 
what smaller.  From  this  time  the  smaller  cells  divide  more  rapidly  than 


598     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

the  larger  and  become  superficial,  the  larger  cells  remaining  in  the 
centre  (Fig.  146,  I.).  Thus  as  the  result  of  repeated  division  there 
results  a  mulberry  mass  of  cells,  the  small  cells  being  external,  the 
large  cells  internal.  It  is  at  this  stage  that  the  segmenting  ovum 
enters  the  uterine  horn  from  the  Fallopian  tube ;  this  has  been 
observed  in  the  rabbit. 


Fig.  146. — Section  of  a  Rabbit's  Ovum  at  the  Close  of  Segmenta- 
tion. II.,  III.,  IV.,  Stages  in  the  Formation  of  the 
Blastodermic  Vesicle  (E.  v.  Benedenj. 

Z.B.,  Zona  radiata  ;  Ex.L.,  External  layer  of  cells  ;  I.M.,  Inner  mass 
of  cells  ;  I.L.M.,  Inner  lenticular  mass  of  cells  ;  s.c,  Segmenta- 
tion cavity. 

The  next  change  observable  is  the  appearance  of  the  segmentation 
cavity  (Fig.  146,  II.).  This  first  appears  as  a  cleft  between  the  inner 
large  cells  and  the  outer  small  cells ;  this  rapidW  increases  in  size  at  the 
expense  of  the  inner  cells,  which  are  pressed  together  forming  a  disc- 
like patch  within  the  now  hollow  sphere  of  small  external  cells.  This 
is  the  blastodermic  vesicle,  and  is  much  larger  than  the  original  ovum. 


GENERATION  AND  DEVELOPMENT  599 

The  lens-like  mass  of  inner  cells  flattens  somewhat,  still  however 
remaining  thicker  in  the  centre  ;  this  central  thickening  being  the 
first  sign  of  the  embryonic  or  embryonal  area  (Fig.  146,  III.  and  IV.). 
The  spherical  blastodermic  vesical  rapidly  becomes  ellipsoidal,  and  the 
membranes  or  coverings  of  the  ovum  become  thin  and  attenuated ;  the 
vitelline  membrane  indeed  may  have  disappeared. 

The  next  stages  in  the  blastodermic  vesicle  are  not  clearly  under- 
stood, but  it  appears  that  the  wall  of  the  vesicle  is  one  cell  thick  except 
at  the  embryonal  area,  where  two  layers  are  to  be  seen.  This  is  the 
hilaminar  blastoderm,  the  superficial  layer  of  which  is  epiblast,  the 
inner  hypoblast.  If  the  embryonal  area  be  examined  from  a  surface 
view  it  is  seen  to  be  pj'riform  in  outline,  and  in  its  posterior  part  the 
2)rimifive  streak  appears.  This  streak  is  due  to  a  thickening — to  the 
appearance  of  the  third  permanent  layer  of  cells — of  the  mesoblast, 
which  is  derived  probably  from  both  epiblast  and  hypoblast.  These 
three  layers  constitute  the  trilaminar  blastoderm,  from  which  the 
various  organs  and  tissues  are  developed. 

From  the  ejnblast  the  following  develop  :  The  whole  of  the  nervous 
sj'stem,  including  the  brain,  spinal  cord,  peripheral  nerves,  and  sympa- 
thetic system.  The  epithelial  structures  of  the  organs  of  special  sense. 
The  epidermis  and  its  appendages,  including  hairs,  hoofs,  and  nails. 
The  epithelium  of  all  glands  opening  upon  the  surface  of  the  skin, 
including  mammary  glands,  sweat  glands  and  sebaceous  glands.  The 
epithelium  of  the  mouth  (except  that  covering  the  tongue  and  posterior 
part  of  the  mouth  which  is  hypoblastic)  and  glands  opening  into  it. 
Epithelial  covering  of  anus.  The  enamel  of  the  teeth.  The  epithelium 
of  the  nasal  passages,  upper  part  of  pharynx,  and  cavities  and  glands 
opening  into  the  narial  passages,  e.g.,  sinuses  of  head,  etc. 

From  the  mesoblast  :  The  urinary  and  generative  organs  (except 
epithelium  of  urinary  bladder  and  urethra).  All  the  voluntary  and 
involuntary  muscles  of  the  body.  The  whole  of  the  vascular  and 
lymphatic  system,  including  serous  membranes  and  spleen.  The 
skeleton  and  all  connective  tissues.  The  amnion  is  partly  epiblastic 
and  partly  mesoblastic. 

From  the  hyjJoblast :  The  epithelium  of  the  alimentary  tract  from 
the  back  of  the  mouth  to  the  anus,  and  all  the  glands  opening  into  this 
part  of  the  tract,  such  as  the  liver,  pancreas,  etc.  The  epithelium  of 
the  Eustachian  tube  and  tympanum.  The  epithelium  of  the  bronchial 
tubes  and  air-sacs  of  the  lungs.  The  epithelium  lining  the  vesicles  of 
the  thyroid  body.  Epithelial  nests  of  the  thymus.  Epithelium  of  the 
urinary  bladder  and  urethra.  The  aUantois  is  partly  hypoblastic  and 
partly  mesoblastic* 

At  the  stage  mentioned  in  a  previous  paragraph,  viz.,  the  appearance 


*  Schafer,  Quain's  'Anatomy,'  vol.  i.,  Part  I.,  p.  44. 


()00     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

of  the  mesoblast,  the  hypoblast  has  grown  along  the  inner  surface  of 
the  epiblastic  layer,  and  nearly  lines  the  whole  blastodermic  vesicle, 
which  now  becomes  ellipsoidal  and  filled  with  a  coagulable  fluid.  In 
front  of  the  primitive  streak,  the  ^J^^wi^'i^'c  groove  appears  as  a  linear 
depression  bounded  by  two  ridges,  known  as  the  medullary  ridges,  the 
groove  is  the  medullary  groove.  The  ridges  continue  to  grow  upwards, 
and  then  to  curve  inwards  and  approximate  in  the  middle  line  from 
before  backwards,  forming  a  tube  the  foundation  of  the  cerebro-spinal 
nervous  system.  If  a  section  of  the  embryo  be  taken  at  this  stage 
across  the  medullary  groove  and  ridges,  we  find  placed  beneath  the 
groove  and  derived  from  the  hypoblast  a  mass  of  ceUs  circular  in 
section,  the  notochord  or  chorda  dorsalis  (Fig.  147).  The  notochord, 
which  is  rod-like,  gives  rise  to  nothing,  but  around  it  the  vertebral 
column  develops,  and  rudiments  of  it  are  found  even  in  adult  life  in 
the  pulpy  centre  of  the  intervertebral  disc.  The  mesoblast  has  been 
rapidly  growing  as  a  sheet  between  the  epiblast  and  hypoblast,  and  if 
the  young  embryo  be  examined  from  above,  it  is  seen  to  be  broken  up 
into  '  quadrangular  masses '  the  protovertebr*  or  somites.  These  somites 
give  rise  to  portions  of  the  vertebrae  and  to  the  muscles  of  the  trunk. 
During  the  growth  of  the  mesoblast  the  embryo,  which  is  developing 
in  front  of  the  primitive  streak,  is  being  gradually  lifted  from  off  the 
blastodermic  vesicle.  This  is  brought  about  by  a  process  of  tucking  or 
folding  off,  and  first  appears  at  the  tail-end  of  the  embryo,  and  extends 
along  either  side  to  the  head  ;  as  a  result  there  is  a  distinct  depression 
or  '  sulcus '  surrounding  the  embryo.  The  remainder  of  the  blasto- 
dermic vesicle  is  filled  with  fluid,  and  forms  the  yolk  sac  (Fig.  147),  and 
this  may  persist  in  some  animals,  as  the  dog,  until  birth.  Many 
believe  that  this  yolk  sac,  which  in  mammalia  contains  no  yolk,  but  is 
abundant  in  birds  and  reptiles,  points  to  the  fact  that  the  ancestors  of 
mammals  had  large  eggs  even  as  the  monotremes  (prototheria)  have 
to-day.  The  eggs  of  the  Ornithorhynchus  or  duck-mole  are  as  large  as 
a  hazel-nut. 

The  medullary  or  neural  groove,  which  has  now  been  converted  into 
a  canal,  becomes  dilated  and  vesicular  in  the  head  region.  These 
vesicles  are  at  first  three  in  number,  then  five,  and  give  rise  to  various 
parts  of  the  brain.  The  lumen  of  the  canal  and  vesicles  persists,  and 
we  see  them  in  the  adult  as  the  minute  central  canal  of  the  cord,  and 
the  ventricles  of  the  brain.  The  nervous  structures  of  the  eyeball  are 
derived  as  outgrowths  of  the  brain  ;  the  organ  of  smell  is  the  nasal  pit 
innervated  from  the  fore  part  of  the  brain  ;  the  ear  is  an  involution 
of  the  epiblast  that  also  speedily  receives  a  nervous  supply  from  the 
brain.  The  mesoblast  about  the  time  of  the  formation  of  the  cerebral 
vesicles  splits  into  two  laminae,  and  the  space  between  becomes  the 
conlom  or  body  cavity  (Fig.  147).  The  upper  lamina,  consisting  of 
epiblast  and  mesoblast,  is  known   as  the   somatopleure ;    the   lower 


GENERATION  AND  DEVELOPMENT 


601 


lamina,  consisting  of  mesoblast  and  hypoblast,  becomes  the  splanchno- 
pleure. 

Arising  from  the  somatopleure,  at  first  posteriori}'  and  then  at  the 
sides  of  the  embryo,  are  ridges  that  grow  upwards  over  the  embryo 
towards  the  head  region,  to  fuse  and  form  the  amnion  (Fig.  147).  In 
front  of  the  head  the  mesoblast  has  not  as  yet  extended,  and  the 
epiblast  and  hypoblast  are  united  forming  the  pro-amnion.  This 
however  soon  disappears,  and  a  ridge  developed  here  grows  over  the 


FctlseAntnion  or  Chorio?i 


Villi    of 
Chorion's 


\//       ytoria.  "■ 

'^  Mid  ^ui:" 


"^Amnion.- 


f-NotochoJ'd. 

,0 

"Coelont. 


Fig.  147. — Diagram  of  a  Transverse  Section  of  a  Mammalian 
Embryo,  showing  the  Mode  of  Formation  of  the  Amnion 
(Schafer). 

The  amnion  folds  have  nearly  united  in  the  middle  line. 

head  of  the  embryo  to  meet  those  advancing  from  behind.  This  fuses 
with  those  from  the  tail  and  sides,  and  as  a  cavity  appears  in  the  ridges 
the  embryo  has  a  dorsal  covering  (Figs.  147  and  148)  of  two  layers,  that 
next  the  embryo  being  the  true  amnion,  and  this  is  separated  from  the 
outer  or  false  amnion  (the  chorion)  by  a  cavity  into  which  the  allantois 
grows.  Thus  the  amnion  arises  from  the  same  portion  of  the  embryo 
which  gives  rise  to  the  body  wall.  The  outer  membrane  of  the  embryo 
is  an  organ  of  paramount  importance,  known  to  morphologists  in  its 
early  stages  as  the  troj^hoblast,  and  to  the  anatomist  in  its  complete 


602     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

form  as  the  chorion.  It  brings  about  the  connection  between  the 
fffitus  and  the  mother  through  the  medium  of  the  viUi.  These  viUi  are 
received  into  folds  of  the  uterine  mucous  membrane,  or  into  uterine 
crypts,  and  thus  attachment  to  the  mother  is  secured. 

The  development  of  the  organs  of  the  body  does  not  enter  into  a 
work  of  this  kind ;  the  student,  for  fuller  information,  is  referred  to 
special  works   on   Embryology.     Eeference  however   may  be  briefly 


Fig.  148.— Diagram  of   a  Longitudinal   Section  of  a  Mammalian 
Ovum,  after  the  Completion  of  the  Amnion  (Schafee). 


made  to  the  so-called  Chestnuts  and  Ergots  of  the  horse,  both  of 
which  are  ancestral  remains,  the  former  being  distinctly  seen  in  the 
foetus.  Both  the  chestnuts  and  ergots  are  considered  to  be  the  remains 
of  hoofs,  belonging  to  digits  long  since  lost  by  the  horse.  The  ergot 
grows  from  the  back  of  the  fetlock.  The  chestnut  is  found  on  the  inside 
of  the  arms  and  hocks,  and  is  always  larger  in  the  former  position. 
In  the  heavy  type  of  horse  it  may  grow  to  a  considerable  size. 
The  horn  of  which  it  is  composed  presents  microscopically  a  tubular 


GENERATION  AND  DEVELOPMENT  603 

structure,  and  is  produced  by  the  papillae  of  the  skin.  After  growing 
a  certain  size  it  drops  or  is  pulled  off.  Both  ergots  and  chestnuts  are 
found  larger  in  horses  wanting  in  quality  than  in  those  better  bred. 

The  Decidua. — At  every  monthly  period  in  the  human 
female  the  mucous  lining  of  the  uterus  undergoes  certain 
changes  which  result  in  the  formation  of  a  membrane 
known  as  the  decidua  ;  this  is  in  shape  a  counterpart  of  the 
interior  of  the  uterus.  The  membrane  is  shed  during 
menstruation ;  if  the  woman  becomes  pregnant  the  decidua 
is  not  exfoliated,  but  undergoes  further  development  in 
connection  with  the  ovum.  The  latter  on  its  arrival  in  the 
uterus  becomes  embedded  in  the  folds  of  mucous  membrane 
which  grow  up  around  and  anchor  it  to  the  wall  of  the 
uterus.  That  portion  of  the  mucous  membrane  which 
grows  over  and  envelops  the  ovum  is  known  as  the 
decidua  rejiexa,  that  which  lines  the  interior  of  the  uterus 
is  known  as  the  decidua  vera.  Through  the  decidua  vera 
the  uterine  glands  grow,  and  later  on  in  embryonic  life 
when  the  final  circulation  is  established  between  the  fcetus 
and  the  mother,  by  means  of  the  placenta,  the  latter  on  the 
maternal  side  is  attached  to  a  portion  of  the  decidua  vera, 
and  to  this  part  the  term  decidua  serotiua  is  given.  After  the 
birth  of  the  child  the  membrane  covering  it,  the  placenta, 
and  the  uterine  decidua,  are  all  cast  off,  with  the  result  that 
the  interior  of  the  uterus  is  converted  into  a  large  raw  wound. 

Placenta. — No  domesticated  animal  has  a  decidua ;  the 
ovum  is  attached  in  quite  another  way  to  the  uterine 
wall,  and  though  a  placenta  exists  it  is  differently  arranged 
to  that  of  the  human  female.  This  has  led  to  the  primary 
classification  of  placenta  into  deeiduate  and  )ion-deciduate, 
but  these  terms  in  the  light  of  recent  enquiry  are  not 
appropriate,  for  it  is  no  longer  a  matter  of  importance  from 
a  morphological  point  of  view,  whether  a  portion  of  the 
maternal  tissue  comes  away  with  the  afterbirth  or  not. 
The  most  recent  work  on  the  placenta  of  animals  suggests 
another    classification.*      Assheton    proposes     to     group 

*  '  On  the  Morphology  of  the  Ungulate  Placenta,'  by  R.  Assheton, 
M.A.     Phil.  Trans.,  B.  vol.,  198.     1905. 


604    A  MANUAL  OF  VETERINAEY  PHYSIOLOGY 

placentas  into  two  great  types,  placenta  cuiiudata  and 
placenta  iMcata,  these  terms  being  based  on  the  arrange- 
ment of  a  certain  group  of  cells  (the  tropkohlast)  in  the 
outer  layer  of  the  embryo,  through  which  the  embryo  is 
secured  to  the  wall  of  the  uterus.  Whatever  form  the 
placenta  may  be,  or  whatever  the  attachment  between  the 
fcetus  and  the  mother,  it  is  always  originated  by  the  tropho- 
blast  cells. 

Li  the  cumulate  type  of  placenta  the  trophoblast  cells 
heap  themselves  up  and  destroy  the  uterine  epithelium,  and 
form  spaces  into  which  the  maternal  blood  escapes,  while 
in  the  plicate  there  is  no  heaping  up,  but  a  process  of 
folding  and  ingrowth  takes  place,  the  uterine  epithelium  in 
most  cases  being  left  intact.  The  pig  is  the  extreme  type  of 
plicate  placenta,  then  follows  the  mare,  cow,  sheep,  while 
the  placenta  of  man  and  carnivora  is  of  the  cumulate  type. 
It  must  not  be  supposed  that  these  types  are  sharply 
divided — for  instance,  the  sheep  has  a  plicate  placenta 
which  contains  cumulate  features,  and  the  placenta  of  the 
dog  though  cumulate  has  features  of  a  plicate  type. 

Besides  recognising  placentae  as  deciduate  and  non- 
deciduate,  or  plicate  and  cumulate,  they  are  further  classified 
according  to  the  disposition  of  the  chorionic  villi.  If  the 
villi  are  scattered  over  the  whole  surface  of  the  chorion  the 
placenta  is  diffuse,  as  seen  in  the  sow,  mare,  and  camel. 
The  only  parts  of  the  chorion  in  these  animals  destitute  of 
villi  are  the  poles,  and  the  smooth  patch  is  very  minute. 
If  the  villi  are  gathered  into  tufts  upon  the  surface  of  the 
chorion,  and  these  tufts  correspond  to  elevations  of  the 
mucous  membrane  of  the  uterus,  the  placenta  is  cotyle- 
donary  or  polycotyledonary.  The  tufts  and  elevations  are 
the  foetal  and  maternal  cotyledons  respectively,  and  number 
sixty  more  or  less.  If  the  villi  are  disposed  in  a  strap-like 
manner  around  the  envelopes,  leaving  the  poles  for  some 
distance  free  from  villi,  the  placenta  is  zonary,  and  such  a 
condition  is  found  in  the  placenta  of  the  dog  and  cat. 
In  the  rabbit  and  woman  the  placenta,  from  its  shape,  is 
discoidal  or  metadiscoidal. 


GENEEATION  AND  DEVELOPMENT  605 

.  It  is  not  known  how  the  material  passes  from  the 
maternal  to  the  foetal  tissues  ;  the  blood  of  the  two  as 
previously  mentioned  does  not  come  in  contact,  but  active 
changes  occur  between  them  through  the  villi  of  the 
placenta.  Proteid,  fat,  carbohydrate,  and  oxygen  are 
received  by  the  fcetus,  and  carbon  dioxide,  nitrogenous 
waste  products,  etc.,  delivered  to  the  mother.  The  nourish- 
ment of  the  mother  directly  influences  that  of  the  embryo, 
and  pregnant  animals  imperfectly  fed  can  only  produce 
puny  offspring.  That  material  may  pass  from  mother  to 
fcetus  is  proved  by  the  bones  of  the  embryo  being  stained  if 
madder  be  administered  to  the  parent.  Yet  we  know  that 
the  placenta,  under  other  circumstances,  is  an  efficient 
filter  for  certain  pathological  substances,  and  that  the 
tuberculous  mother  does  not  convey  tuberculosis  to  the 
foetus.  It  has  been  suggested  that  the  passage  of  water, 
salts,  and  sugar  from  mother  to  foetus  may  occur  by 
diffusion,  the  passage  of  fat  and  proteid  being  perhaps 
connected  with  special  enzymes.  The  presence  of  glycogen 
in  all  the  embryonic  tissues  points  to  it  as  an  important 
material  in  the  nutrition  of  the  foetus.  Gradually,  as 
development  proceeds,  the  glycogenic  function  becomes 
largely  centred  in  the  liver  and  placenta. 

Fcetal  Membranes. — If  the  egg  of  the  hen  be  examined 
after  incubating  nine  days,  the  appearance  seen  in  Fig.  149 
presents  itself.  A  chick  in  an  advanced  stage  of  develop- 
ment is  bound  within  a  thin  tough  skin,  containing  fluid ; 
this  water-jacket  is  known  as  the  amnion,  and  its  use  is  to 
prevent  jar  when  the  egg  is  moved  ;  an  identical  arrange- 
ment exists  in  mammals.  The  supply  of  food  required  by 
the  embryo  chick  during  development  is  contained  in  a  sac 
known  as  the  yolk  sac,  to  this  food- supply  the  embryo  is 
connected  by  a  stalk  through  which  the  nourishment  enters 
its  body.  The  walls  of  the  yolk  sac  are  vascular  and  con- 
nected with  the  vessels  of  the  embryo ;  it  is  through  the 
medium  of  the  vascular  wall  that  the  altered  yolk  is  taken 
up.  A  modified  yolk  sac  is  found  in  mammals  (Figs.  145, 
147,  148) ;  it  does  not  contain  yolk,  but  it  takes  up  the 


606     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


nourishment  secreted  by  the  uterine  glands,  and  for  a  time 
this  suffices  for  the  needs  of  the  embryo.  The  chick  has 
another  fcetal  appendage  known  as  the  allantois,  it  grows 
out  from  the  body,  being  connected  to  the  embryo  by 
means  of  a  stalk,  and  forms  a  vascular  sac  through  which 
blood  from  the  chick's  body  circulates.  The  allantois  in 
the  chick  is  a  breathing  organ,  the  air  enters  through  the 
pores  of  the  shell,  and  the  blood  takes  up  oxygen  from  the 
air  surrounding  the  allantoic  sac ;  an  air  space  also  exists 
at  the  end  of  the  shell.  An  allantois  exists  in  the  mammal 
(Figs.  145,  148)  ;  unlike  that  of  the  bird  it  does  not  obtain 

aJ^,^^l:s  amotion. 


azrspou:^' 


yolk-socc 


Fig.   149. — Hen's  Egg  at  the  Ninth  Day  of  Incubation   (Ewaet 
after  milnes  marshall). 

oxygen  from  the  air,  though  it  is  a  breathing  organ  in  the 
sense  that  it  furnishes  oxygen  to  the  foetus. 

We  have  seen  that  immediately  enveloping  the  mam- 
malian embryo  is  the  amnion,  Figs.  145  and  148  ;  this  sac 
contains  a  fluid  in  which  the  foetus  lives.  The  fluid,  or 
liquor  anmii,  is  alkaline  in  reaction,  and  yellowish  in  colour 
during  the  early  days  of  gestation,  but  reddish  towards  the 
end  of  it,  probably  due  to  discoloration  with  meconium. 
The  amniotic  fluid  contains  proteids,  mucin,  urea,  sugar, 
lactic  acid,  keratin,  and  some  salts ;  besides  these  there  are 
also  portions  of  hoof,  epithelium,  etc.,  derived  from  the 


GENEEA.TION  AND  DEVELOPMENT 


607 


foetus.  The  source  of  this  fluid  is  probably  by  transudation 
from  both  the  foetus  and  mother.  Indigo  bhie  injected  into 
the  vessels  of  the  mother  tinges  the  amniotic  fluid,  though 
it  does  not  stain  the  foetal  tissues. 

The  function  of  this  fluid  is  protective  to  mother  and 
foetus ;  the  latter  lies  on  it  as  on  a  water-bed,  and  during 


Amnicn 


Amniotic  nartion^ 

ofUrnbilical^^  ^, 
Cord.  ,».>^.^'. 


h:.i,.p 


^^!>s^on  Amnion. 

''^..<^-...,C/ior/on  L 
"•  '"^.^^//alsn  amnion  .\ 

%  '^'^'iAUantois 

''•  i\  A  Inner  <x 

.•  "''\\\y  outer 

tntois  fiortion''y\  latjers  cf 
miiticat  cord.  '}<    .,,     ^    . 


of  Ujnbilicci.1  Cord 
to  the   en  ve/c/i e  s. 


Fig.  150. — Diagram  of  the  Fcetal  Envelopes  of  a  Five  Months 
Horse  Embryo  (Bonnet). 

parturition  it  assists  in  dilating  the  os  and  lubricating  the 
maternal  passage.  The  allantois  grows  out  from  the  body 
of  the  embryo  at  the  future  umbilicus ;  the  part  within  the 
body  forms  the  bladder,  that  outside  it  forms  a  sac  which 
in  the  mare  completely  envelops  the  amnion  (Fig.  150), 
but  in  ruminants  only  partly  so  (Fig.  151)  ;  the  bladder 
and  the  cavity  of  the  allantois  are  connected  by  a  canal  in 
the  umbilical  cord  known  as  the  nrachus.     The  fluid  found 


608     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

in  the  allantois  is  derived  from  the  fcBtal  urine  ;  in  the  first 
instance  it  is  colourless  or  turbid,  later  on  it  becomes  brown 
in  tint.  This  fluid  contains  urea,  and  a  substance  allied  to 
it,  allantoin,  albumin,  sugar,  lactic  acid,  and  certain  salts. 
The  allantois  is  the  organ  of  respiration,  and  to  a  limited 
extent  of  nutrition.  During  early  fcetal  life  the  vascular 
wall  of  the  allantois  is  able  to  bring  the  blood  of  the 
embryo  sufficiently  near  to  that  of  the  uterus  to  effect  an 
exchange  of  gases.  Later  on,  as  we  have  seen,  p.  596,  it 
furnishes  the  villi  which  penetrate  into  the  walls  of  the 
uterus  through  the  chorion. 

Floating  in  the  allantoic  fluid  of  the  mare,  or  attached 
to  the  wall  of  the  sac,  are  certain  peculiar  masses  termed 
hippomanes  ;  their  origin  and  use  are  quite  unknown.  It  is 
usually  considered  that  these  bodies,  which  may  be  multiple, 
are  found  in  the  foal's  mouth  at  birth,  but  we  are  assured 
by  a  close  and  reliable  observer*  that  this  is  a  fallacy. 
Hippomanes  have  also  been  observed  in  the  cow.f 

The  chorion  envelops  the  two  previous  coverings.  Through 
the  umbilical  cord  it  forms  the  vascular  connection  between 
the  foetus  and  the  mother,  and  the  villi  on  its  surface  pro- 
ject themselves  into  the  mucous  membrane  of  the  uterus, 
not  through  the  medium  of  a  decidua  as  in  the  woman, 
but  directly  into  the  uterine  wall.  The  bloodvessels  of  the 
chorion  and  those  of  the  uterus  do  not  anastomose,  but 
the  foetal  villi  are  bathed  in  the  blood  contained  within 
sinuses  in  the  uterus  into  which  they  run,  and  in  this  way, 
through  the  endothelial  lining  of  the  vessels  of  mother  and 
embryo,  the  blood  of  the  foetus  receives  oxygen  and  gets 
rid  of  carbonic  acid. 

Umbilical  Cord. — After  the  formation  of  the  foetal  envelopes 
the  body-walls  rapidly  close  in,  the  splanchnopleure  being 
received  up  into  the  body  to  form  the  primitive  gut  and  its 

*  Mr.  T.  B.  Goodall,  F.R.C.V.S.,  Christchurch. 

f  It  is  a  curious  fact  that  even  at  the  present  day,  in  some  country 
districts,  hippomanes  are  sought  for  in  virtue  of  the  properties  they 
have  been  supposed  to  possess  from  time  immemorial,  viz.,  for  use  as 
love  philtres. 


GENEEATION  AND  DEVELOPMENT 


609 


39 


GIO     A  MANUAL  OF  VETEKINARY  PHYSIOLOGY 

derivatives,  the  somatopleure  forming  the  body-wall  and 
the  limbs.  The  embryo  or  fcptus  retains  its  connections 
with  the  placenta  by  means  of  the  umbilical  cord,  which  is 
composed  as  follows  :  Structures  in  connection  with  the 
amnion  and  the  body-wall  at  the  umbilicus  ;  structures  in 
connection  with  the  allantois  and  the  urachus,  the  latter 
being  a  funnel-shaped  body  connected  with  the  urinary 
bladder,  and  the  remains  of  which  may  be  seen  as  a  scar 
on  the  fundus  of  that  organ,  even  in  the  adult ;  the 
umbilical  arteries  and  vein,  or  veins  (ruminants).  All 
these  are  cemented  together  by  an  embryonic  connective 
tissue,  the  Whartonian  jelly. 

Foetal  Circulation. — With  the  formation  of  the  fcetal 
envelopes  and  the  development  of  the  heart,  the  circulation 
takes  on  a  course  altogether  different  from  that  in  the 
vascular  area  in  early  embryonic  life.  The  placenta  acts 
as  the  foetal  respiratory  and  food-absorbing  organ.  Im- 
pure blood  that  has  circulated  through  the  tissues  of  the 
developing  young  is  brought  to  the  placenta  by  the 
umbilical  arteries,  these  acting  to  the  fcetus  as  the 
pulmonary  arteries  to  the  adult.  After  an  interchange 
of  gases  and  a  renewal  of  food  supply,  the  blood  is 
carried  away  to  the  foetus  by  means  of  the  umbilical  vein 
or  veins  found  in  the  cord.  The  vein  enters  the  body 
at  the  navel  or  umbilicus,  and  passes  forward  along  the 
floor  of  the  abdomen,  reaches  the  falciform  ligament  of  the 
liver,  travels  along  the  free  edge  of  that  structure,  and 
empties  itself  into  the  portal  vein.  After  birth  the  remains 
of  the  umbilical  vein  are  found  as  a  thickening  at  the  free 
edge  of  the  falciform  ligament,  and  is  named  the  round 
ligament  of  the  liver.  In  ruminants  the  umbilical  veins 
are  two  in  number,  but  they  unite  to  form  a  single  vessel 
on  entering  the  body.  The  vessel  thus  formed  passes 
along  the  abdominal  floor  towards  the  falciform  ligament 
to  occupy  the  same  position  as  in  other  animals,  but  before 
reaching  it,  it  detaches  a  large  branch,  the  ductus  venosus 
(Fig.  152,  d  r),  which  passes  upwards  to  join  the  posterior 
vena  cava.     After  the  blood  has  circulated  in  the  liver  it 


GENEEATION  AND  DEVELOPMENT  611 

leaves  by  the  hepatic  trunks,  and  is  poured  into  the  posterior 
vena  cava,  where  it  meets  with  the  blood  in  that  vessel 
and  is  thus  conducted  to  the  heart.  In  the  horse  the 
whole  of  the  foetal  blood  passes  through  the  liver  before 
reaching  the  heart  through  the  posterior  cava  ;  in 
ruminants  part  of  the  blood  passes  through  the  liver,  and 
part  goes  direct  to  the  systemic  circulation  of  the  foetus 
through  the  ductus  venosus. 

In  the  fcetal  heart  the  cavities  of  the  right  and  left 
auricles  are  in  communication  by  means  of  a  foramen,  the 
foramen  ovale.  This  opening  in  many  animals  is  provided 
with  a  valve,  the  Eustachian  valve,  that  stretches  from  the 
mouth  of  the  posterior  vena  cava  to  the  annulus  or  thickened 
border  of  the  foramen  ovale ;  it  is  absent  from  the  heart 
of  the  foetal  horse  and  pig.  The  function  of  this  valve  is 
to  direct  the  blood-stream  into  the  left  auricle  ;  the  blood 
in  this  way  gets  into  the  left  auricle,  passes  into  the  left 
ventricle,  and  thence  into  the  aorta.  The  greater  portion 
is  driven  into  the  vessels  that  supply  the  head,  neck, 
and  fore-limbs  (anterior  aorta  and  branches),  and  is 
conveyed  to  the  head  and  anterior  portion  of  the  body ; 
the  remainder  passes  backwards  in  the  posterior  aorta. 
The  head,  it  will  be  noticed,  receives  almost  pure  blood. 
After  the  fluid  has  circulated  in  this  part  of  the  body, 
it  is  returned  to  the  right  auricle  of  the  heart  by  the 
anterior  vena  cava.  From  the  right  auricle  it  passes  to 
the  right  ventricle,  and  from  this  cavity  it  is  pumped 
into  the  pulmonary  artery.  The  lungs,  however,  are 
not  functional,  and  are  more  or  less  solid  organs, 
consequently  they  are  not  yet  prepared  to  receive  the 
blood  as  they  will  be  after  birth,  when  they  become 
distended  with  air  and  have  taken  on  their  duties 
as  breathing  organs.  The  blood  must  therefore  take 
another  course  than  through  the  lungs.  This  course 
is  provided  by  the  ductus  arteriosus  (Fig.  152,  d  a),  a 
short  vessel  uniting  the  pulmonary  artery  to  the  aorta, 
and  thus  bringing  their  lumina  into  communication. 
By   this    conduit  the    blood    enters   the   posterior   aorta, 

39—2 


Fig.  152. — Diagram  of  the  Fcetal  Circulation  (Ellenbeeger). 

n.  v..  Umbilical  vein ;  d.  v.,  ductus  venosus  ;  2>f-  v.,  portal  vein  ;  Z,,  liver  ; 
V.  h,,  hepatic  veins  ;  p.  v.  c,  posterior  vena  cava ;  r.  a.,  right  auricle ; 
/.  0.,  foramen  ovale  ;  r.  v.,  right  ventricle  ;  p.  a.,  pulmonary  artery ; 
d.  a.,  ductus  arteriosus  ;  I.  a.,  left  auricle  ;  I.  v.,  left  ventricle  ;  a.,  the 
aorta ;  a.  a.,  arch  of  aorta ;  ant.  a.,  anterior  aorta  ;  i.v.,  innominate 
veins  ;  a.  v.  c,  anterior  vena  cava ;  ^jo.  a.,  posterior  aorta  ;  i.a.,  iliac 
artery  ;  h.  a.,  hypogastric  artery;  u.  a.,  umbilical  arteries;  i.  ve., 
iliac  veins  ;  h.  v.,  hypogastric  veins ;  u.  c,  umbilical  cord. 

The  diagram  actually  represents  the  fcetal  circulation  in  ruminants  ;  to 
make  it  applicable  to  the  horse  the  ductus  arteriosus  {d.  v.)  must 
be  supposed  to  be  removed,  the  whole  of  the  blood  then  traverses 
the  liver  by  the  union  of  the  umbilical  vein  {u.  v.)  with  the  portal 
vein  {2)t.  v.).  The  arrows  indicate  the  course  taken  by  the  blood  : 
observe  that  the  stream  entering  the  right  auricle  divides,  part 
passing  into  the  right  ventricle,  and  part  into  the  left  auricle 
through  the  foramen  ovale  (/.o.)- 


GENERATION  AND  DEVELOPMENT  G13 

and  is  conveyed  to  the  hinder  parts  of  the  body  and  to  the 
placenta. 

The  allantoic  or  umbilical  arteries  convey  the  blood  from 
the  foetus  to  the  placenta.  These  arteries  are  branches  of 
the  internal  pudics,  or  of  the  parent  vessels  the  internal 
iliacs,  and  during  intra-uterine  life  they  are  larger  than 
the  parent  vessels.  Soon  after  birth,  however,  their  walls 
become  thickened,  and  their  lumina  are  lost,  and  they 
become  impervious  to  the  passage  of  blood.  In  the  adult 
they  are  recognised  as  the  thickened  cords  found  in  the 
lateral  ligament  of  the  bladder.  The  ductus  arteriosus  just 
prior  to  birth  has  a  lumen  easily  receiving  an  ordinary 
cedar  pencil,  but  it  steadily  diminishes  until,  at  about  a 
month  after  birth,  it  is  no  greater  than  the  diameter  of  a 
knitting-needle.  It  is  probable  that  little  blood  passes 
this  way  after  birth,  but  the  exact  period  of  total  occlusion 
is  unknown.  Similarly  the  foramen  ovale  is  blocked  up  by 
the  development  of  a  membrane,  which  may  be  pulled  out 
with  the  forceps  shortly  after  birth,  and  then  resembles  in 
shape  an  old-fashioned  lace  nightcap  or  cowl.  When 
undisturbed  it  lies  in  a  heap  filling  up  the  foramen. 

The  short  cuts  in  the  foetal  circulation,  viz.,  the  dactua 
cenosus,  ductus  arteriosas,  and  foramen  ovale,  exist  mainly 
with  the  object  of  ensuring  that  the  purest  blood  reaches 
those  organs  which  require  it  the  most.  The  heart,  head, 
and  fore  limbs  receive  blood  which  is  mucli  purer  than  the 
blood  circulating  through  the  hind  limbs  and  abdominal 
viscera,  for  the  brain  must  be  well  fed.  The  fact  is  that 
the  foetal  blood  at  its  best  is  far  below  the  level  of  the 
arterial  blood  of  the  mother,  and  this  is  explained  by  saying 
that  the  demand  of  the  foetus  for  oxygen  is  small  owing  to 
the  low  rate  of  its  metabolism.  From  the  blood  of  the 
umbilical  artery  and  vein  of  the  foetal  sheep  the  following 
gases  have  been  extracted,  and  may  be  compared  with  the 
arterial  blood  of  the  mother  : 

Umbilical  Umbilical  Maternal  Arterial 
Artery.                    Vein.  Blood. 

Oxygen     2-3  6*3  20-0 

Carbon  dioxide    ...     47-0  40-5  40-0 


614    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  liver  is  a  very  active  organ  in  the  foetus,  and  is 
abundantly  supplied  with  a  mixture  of  blood,  the  worst 
and  the  best  in  the  body,  the  best  predominating.  Early 
in  intra- uterine  life  the  liver  begins  to  secrete  bile,  which 
is  discharged  into  the  intestines  as  meconium  (see  p.  212). 

Uterine  Milk. — If  the  villi  of  the  chorion  be  separated 
from  the  tubular  depressions  of  the  mucous  membrane  of 
the  uterus,  a  fluid  may  be  expressed  known  as  uterine  milk. 
This  is  particularly  observable  in  separating  the  fcetal  and 
maternal  cotyledons.  Uterine  milk  is  of  a  white  or  rosy- 
white  colour,  creamy  consistence,  and  contains  proteids, 
fat,  and  a  small  proportion  of  ash.  Examined  micro- 
scopically it  is  found  to  contain  globules  of  fat,  leucocytes, 
rod-like  crystals,  and  structureless  masses  of  proteid.  The 
use  of  the  fluid  is  for  the  nourishment  of  the  embryo,  and 
in  the  mare,  cow,  and  sheep  the  uterine  glands  take  a  pro- 
minent part  in  providing  nourishment  throughout  foetal 
life,  pouring  their  secretion  into  special  depressions  in  the 
placenta  (Assheton). 

The  Duration  of  Pregnancy  appears  to  be  based  on  no 
fixed  law.  Judging  from  the  length  of  time  the  elephant 
is  in  gestation  it  might  appear  that  body  size  had  an 
influence,  but  against  this  is  the  fact  that  the  ass  carries  her 
young  longer  than  the  horse,  while  whether  it  be  a  toy 
terrier  or  a  Newfoundland,  a  dog  goes  from  fifty-nine  to 
sixty-three  days.  It  certainly  does  appear  that  among 
animals  of  the  same  species  breed  has  an  influence  in  the 
matter ;  different  herds  of  cows  vary  from  277  to  288  days. 
Merino  sheep  average  150  days,  Southdowns  144  days.  It 
is  not  clear  why  the  guinea-pig  should  require  a  period  of 
gestation  twice  as  long  as  the  rabbit,  which  is  also  a 
rodent. 

The  following  are  average  periods  of  gestation  : 

Elephant     ...         2  years  (nearly). 

Mare  11  months,  and  liable  to  vary 

within  relatively  vv'ide  limits. 

Ass  358  to  385  days. 

Zebra  ...         ...         ...         ...     13  months  (and  over). 

Cow...         40  weeks. 


GENERATION  AND  DEVELOPMENT 


615 


Sheep 
Camel 
Pig  ... 
Dog  ... 
Cat  ... 
Kabbit 
Guinea-pig 


21  weeks  (average). 

45  weeks. 

16  weeks. 

59  to  63  days. 

56  days. 

32  days. 

63  days. 


Parturition. — The  foetus  having  reached  its  full  stage  of 
development,  changes  of  an  obscure  nature  take  place 
which  lead  to  its  expulsion.  During  uterine  life  the 
equine  ftetus  is  lying  on  its  back  on  the  floor  of  the 
mother's  abdomen,  with  its  chin  on  its  chest,  the  fore-legs 


Fig.  153. — The  Position  occupied  by  the  Equine  Fcetus  duking 
Intra-Uterine  Life  (Franck), 

bent  at  the  knee,  and  the  hind-legs  in  the  right  horn 
(Fig.  153).  Preparatory  to  birth  the  foetus  changes  position 
and  turns  on  its  side,  so  as  to  assume  first  a  lateral  posi- 
tion (Fig.  154),  and  lastly  an  upright  one  (Fig.  155),  by 
which  the  foetal  and  maternal  spines  are  brought  nearer 
together.  To  assume  this  position  the  ftetus  has  had  to 
make  a  complete  revolution ;  it  is  now  brought  with  the 
muzzle  and  fore-legs  in  the  direction  of  the  pelvis  (Fig.  155), 
and  dilatation  of  the  passage  follows.  In  the  cow  the 
fcetus  lies  on  its  back  on  the  floor  of  the  abdomen  as  in  the 
mare,  but  lies  somewhat  crooked,  viz.,  the  head  inclining 
towards  one  side,  and  the  hind  extremities   towards  the 


(il(i     A  MANI^AL  OF  VETERINAUY  PHYSIOLOGY 

other  ;  in  all  other  respects  it  resembles  the  mare.  The 
alteration  in  the  position  of  the  foetus  does  not  occur 
through  its  own  movements,  but  by  the  contraction  of  the 
uterus ;  on  the  other  hand,  the  stretching  of  the  limbs  is 
the  result  of  fcetal  movement.*  There  can  be  little  doubt 
that  the  revolution  of  the  foetus  prior  to  birth  is  the 
explanation  of  the  complete  torsion  of  the  neck  of  the 
uterus  and  vagina  which  is  sometimes  found  in  both  the 
cow  and  mare. 

The  dilatation  of  the  os  is  assisted  by  the  amniotic  and 


Fig.  154. — The  First  Stage  ix  the  Revolution  of  the  Fcetus  ; 
Lateral  Position.  The  Os  is  Dilated  by  the  Membranes 
which  have  not  yet  Euptured  (Franck.) 

allantoic  fluids.  Each  contraction  of  the  uterus  is  accom- 
panied by  a  pain  ;  the  pains  last  from  15  to  90  seconds, 
and  the  interval  between  them  is  from  2  to  4  minutes. 

The  contractions  of  the  uterus  occur  under  the  influence 
of  a  centre  in  the  lumbar  portion  of  the  cord ;  they  are 
not  under  the  control  of  the  will,  and  occur  even  though 
the  animal  be  unconscious,  or  the  spinal  cord  divided  in 
the  lower  cervical  region  (dog). 

The  mare  is   remarkable   for   the   rapidity  with  which 

*  This  description  of  the  change  in  the  position  of  the  foetus  pre- 
paratory to  birth  is  taken  from  Ellenberger's  '  Physiologie.' 


GENEEATION  AND  DEVELOPMENT  617 

delivery  is  effected  ;  ruminants,  on  the  other  hand,  are 
often  very  slow  and  in  labour  for  hours.  Parturition  in 
the  mare  is  accompanied  by  a  complete  separation  of  the 
chorion  from  the  uterine  wall ;  this  is  the  explanation 
why  any  difficulty  in  foaling  invariably  sacrifices  the  life  of 
the  foal.  In  ruminants,  on  the  contrary,  the  circulation 
between  the  mother  and  fcetus  is  kept  up  to  the  last  by  the 
gradual  separation  of  the  cotyledons,  so  that  though  the 
process  may  be  delayed  several  hours,  the  animal  is 
generally  born  alive. 


Fig.  155. — The  Eevolution  Completed,  Membranes  Ruptured,  and 
Foal  in  the  Normal  Position  for  Delivery  (Franck). 

The  cause  of  the  first  respiration  of  the  fcetus  is  dealt 
with  at  p.  112. 

The  Secretion  of  Milk. 

As  the  period  of  parturition  approaches,  the  mammary 
glands  become  swollen  owing  to  active  changes  occurring 
in  them,  and  at  or  shortly  after  the  birth  of  the  animal 
milk  is  formed. 

Two  processes  contribute  to  the  formation  of  milk ;  in 
one  the  cells  lining  the  alveoli  of  the  gland  are  bodily  shed 
and   form    the   fat  of   the  milk,   while   in   the   other  the 


618     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

water,  proteids,  salts,  etc.,  are  formed  from  the  lymph  in 
the  gland  by  the  ordinary  process  of  secretion.  We  must 
examine  the  first  of  these  at  somewhat  greater  length.  If 
the  mammary  gland  of  an  animal  which  has  never  been 
pregnant  be  examined,  the  alveoli  it  contains  are  much 
smaller  and  less  numerous  than  those  of  a  secreting  gland. 
The  alveoli  of  the  first-mentioned  gland  are  found  to  be 
packed  with  small  rounded  cells  of  very  slow  growth ; 
when  the  animal  becomes  pregnant  the  gland  enlarges, 
the  alveoli  increase  in  number,  but  remain  packed  with 
cells  until  parturition  approaches  or  occurs.  The  solid 
masses  of  cells  are  now  cast  off,  and  leave  behind  them 
alveoli  lined  with  a  single  layer  of   secretory  epithelium, 


Loaded. 


Discharfred. 


Fig.  156. 


-Mammary    Gland    of    Dog    during    Lactation. 
Heidenhain  (Waller). 


After 


the  function  of  which  is  to  furnish  the  milk.  The  shedding 
of  the  mass  of  cells  which  originally  occupied  the  alveoli 
supplies  the  colostrum  or  first  milk. 

The  appearance  presented  by  the  single  layer  of  cells 
lining  the  alveolus  of  the  secretory  gland,  depends  upon 
whether  the  gland  is  loaded  or  discharged.  If  the  gland 
is  loaded,  viz.,  active  secretion  occurring,  the  cells  are 
found  to  be  large  and  columnar  in  shape,  possessing  two 
or  more  nuclei,  one  being  at  the  base  of  the  cell,  and 
the  other,  giving  indications  of  degeneration,  placed  near 
the  apex  (Fig.  156).  In  the  apex  or  free  portions  of  the 
cell  fat  globules  can  be  seen,  which  may  even  have  partly 
extruded  themselves  from  the  cell,  and  besides  these  there 
are  other  particles.     Further,  the  cell  gives  the  appearance 


GENERATION  AND  DEVELOPMENT    619 

of  the  apex  or  free  border  being  separated  from  the  base  by 
a  process  of  constriction. 

If  the  gland  be  examined  when  discharged,  viz.,  after 
the  milk  has  been  drawn  off,  the  cells  lining  the  alveolus 
are  cubical  or  flattened,  each  containing  a  nucleus ;  the 
lumen  of  the  alveolus  is  also  increased  in  size,  and  within 
it  may  be  seen  some  of  the  elements  of  the  milk  (Fig.  156). 

It  is  evident  that  the  cells  in  the  active  gland  are 
loaded  with  material,  much  of  it  being  fat,  and  these  cells 
break  off  leaving  behind  them  the  parent  cell  containing  a 
nucleus  from  which  another  cell  grows.  In  spite  of  this  the 
formation  of  fat  in  milk  is  really  a  process  of  cell  secretion, 
and  this  is  supported  by  the  fact  that  animals  such  as 
carnivora,  whose  food  is  deficient  in  fat,  produce  a  fat- 
containing  milk,  and  the  fat  is  elaborated  by  the  mammary 
cell  from  the  proteid  of  the  body.  A  fat  diet  does  not 
increase  the  fat  in  milk,  though  a  proteid  diet  has  this 
effect. 

The  proteids,  sugar,  and  salts,  found  in  milk  are  secreted 
in  the  ordinary  way  from  the  blood,  or  rather  the  lymph 
circulating  in  the  gland,  the  cells  lining  the  alveolus  being 
the  active  factor  in  the  matter,  and  that  these  substances 
are  really  elaborated  by  the  cell,  is  supported  by  the  fact 
that  neither  caseinogen  nor  milk  sugar  exists  in  any  other 
tissue  of  the  body.  It  has  been  supposed  that  the 
secretion  of  milk  is  influenced  by  the  nervous  system, 
but  there  is  no  experimental  evidence  which  places  this 
beyond  doubt. 

Composition. — The  milk  of  herbivora  has  an  alkaline 
reaction  which  may  readily  turn  acid  ;  in  carnivora  the 
reaction  is  acid.  Fresh  cow's  milk  is  amphoteric,  viz.,  it 
gives  both  an  acid  and  an  alkaline  reaction  to  test  paper  ; 
this  is  due  to  the  presence  of  acid  and  alkaline  salts.  In 
the  cow  the  specific  gravity  is  1028  to  1034.  The  secretion 
contains  proteids  (caseinogen  and  albumin),  sugar  (lactose), 
fats,  and  salts.  An  average  secretion  of  milk  from  a  cow 
may  be  taken  at  6  quarts  (6*8  litres)  per  diem  for  forty 
weeks  in  the  year. 


H20    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


In  the  following  table  is  given  an  analysis  of  the  milk  of 
different  animals : 


Co'w. 

Alare. 

Sheep 

Water      . 

.     84-28     . 

..     92-5       .. 

82-84 

Solids 

,     15-72     . 

..       7-5       .. 

.       15-17 

Casein 

.       3-57     . 

..       1-3  } 

4-7 

Albumin  . 

-75     . 

..         -3f    •• 

Fat 

.       6-47     . 

•6       .. 

4-8 

Lactose    . 

.       4-34     . 

.       4-7       .. 

3-4-6  } 

Salts 

-63     . 

•3       .. 

•6l 

Ass. 

90-5 

9-5 

1-7 

1-4 

6-4 


Dorf. 

76-0 

24-0 

10-0 

10-0 

<i  3-5 
\     -5 


It  will  be  observed  that  the  milk  of  the  cow,  dog,  and 
sheep  is  remarkable  for  the  high  percentage  of  fat  it  con- 
tains ;  the  caseinogen  of  mare's  milk  is  much  less  than  that 
found  in  the  cow,  and  more  like  that  of  the  human.  The 
milk  of  the  dog  is  rich  in  caseinogen,  fat,  and  calcium,  but 
poor  in  lactose. 

Under  the  influence  of  rennin  caseinogen  becomes  in- 
soluble, and  the  milk  is  coagulated,  resulting  in  a  clot  and 
wliei/ ;  the  clot  or  insoluble  casein  is  now  termed  tyrein. 
Neither  the  albumin  nor  the  caseinogen  in  milk  is  pre- 
cipitated by  boiling;  on  the  other  hand,  colostrum  is 
precipitated  by  heating,  and  this  is  due  to  the  fact  that 
it  contains  globulin.  The  albumin  of  milk  offers  some 
peculiarities  as  compared  with  ordinary  serum  albumin, 
and  has  been  termed  lactalhiuiiui. 

The  fats  in  milk  are  olein,  stearin,  and  palmitin,  and 
the  proportion  of  these  differs  in  various  animals.  The 
fat  is  contained  within  fat  globules,  and  these  form  in  milk 
a  true  emulsion,  each  globule  being  separated  by  a  layer  of 
milk  plasma.  On  standing  the  globules  rise  to  the  surface 
of  the  fluid  and  form  cream  ;  by  the  process  of  churning 
the  emulsion  is  destroyed,  and  the  fat  is  obtained  as  butter. 
Butter  consists  of  68  per  cent,  of  palmitin  and  stearin, 
30  per  cent,  of  olein,  and  2  per  cent,  of  specific  butter  fats. 

JMilk  sugar  or  lactose  is  very  liable  to  undergo  fermentation, 
resulting  in  the  production  of  lactic  acid  and  the  curdling 
of  milk.  It  is  not,  however,  capable  of  undergoing  direct 
alcoholic  fermentation,  which    would  appear  to  be  a  pro- 


GENERATION  AND  DEVELOPMENT  n21 

vision  against  fermentative  decomposition  occurring  either 
in  the  gland  or  in  the  alimentary  canal  (Lea).  The  milk 
of  the  mare  in  the  presence  of  suitable  ferments  may 
undergo  alcoholic  fermentation,  as  in  the  production  of 
koumiss. 

The  salts  of  milk  are  principally  calcium  phosphate,  and 
salts  of  sodium  and  potassium.  In  the  composition  of  the 
milk  we  obtain  an  insight  into  the  nature  and  quantity  of 
the  salts  required  by  growing  animals.  Bunge  gives  the 
following  ash  analysis  of  mare's  and  cow's  milk  : 


1 

Mare's  Mill-. 

Cow's  Milk 

Potassium 

..     104 

...     1-76 

Sodium 

..       -14       ... 

...     Ill 

Calcium 

..     1-23       ... 

...     1-59 

Magnesium 

..       -12       ... 

...       -21 

Iron  ... 

..       -015     ... 

...       -003 

Phosphoric  acid 

..     1-31       ... 

...     1-97 

Chlorine 

..       -31       ... 

...     1-69 

Total  ash  per  1,000 

..     4-17       ... 

...     7-97 

The  phosphates  are  increased  by  those  contained  in 
the  proteids ;  they  are  employed  mainly  in  the  construc- 
tion of  the  skeleton.  The  excess  of  potassium  over  sodium 
salts  is  a  feature  common  to  many  of  the  secretions  of  the 
herbivora,  but  in  milk,  probably  in  all  animals,  the  ash 
contains  more  potassium  than  sodium.  Bunge  states  that 
this  is  due  to  the  fact  that  as  the  animal  grows  it  becomes 
richer  in  potassium  and  poorer  in  sodium  salts,  depending 
upon  the  relative  increase  in  the  muscular  structure  which 
is  rich  in  potassium,  and  the  relative  decrease  in  the  carti- 
laginous material  which  is  rich  in  sodium. 

Bunge  compared  the  ash  of  a  puppy  with  the  milk  of  the 
mother,  and  the  milk  with  the  blood.  It  was  remarkable 
how  closely  the  composition  of  the  puppy's  system  agreed 
with  the  salts  it  was  receiving  in  the  milk,  though  when 
the  ash  of  the  milk  was  compared  with  the  ash  of  the  blood 
of  the  mother,  the  greatest  diversity  in  composition  was 
apparent.  In  comparing  Bunge's  analysis  of  the  ash  of 
cow's  and  mare's  milk,  one  is  struck  by  the  fact  that  the 
calf  requires  much  more  salts  for  its  nutrition  than  the  foal. 


622    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  first  milk  secreted  is  termed  Colostrum.  The  source 
of  colostrum,  and  some  peculiarities  in  its  composition,  have 
already  been  dealt  with.  In  appearance  it  is  a  yellowish- 
white  fluid  of  an  alkaline  reaction,  sweetish  taste,  and 
remarkable  for  the  amount  of  proteid  it  contains,  as  much 
as  15  per  cent.,  whilst  ordinary  milk  only  contains  4  per 
cent,  or  5  per  cent.  Examined  microscopically  colostrum 
is  found  to  contain  bodies  termed  '  colostrum  corpuscles.' 
These  are  large  granular  corpuscles  containing  fat. 

The  use  of  colostrum  is  to  act  as  a  natural  purge,  by 
which  means  the  intestinal  canal  of  the  newly  born  animal 
is  cleared  out. 


CHAPTER  XIX 

GROWTH,  DECAY,  AND  DEATH 

Growth. — The  young  of  the  herbivora  very  rapidly  shake 
off  the  helpless  condition  in  which  they  first  find  them- 
selves in  this  world.  This  is  largely  due  to  the  fact  that 
they  are  born  with  a  nervous  system  in  a  high  state  of 
development ;  in  the  course  of  a  few  hours  thej  learn  to 
stand  and  walk,  and  in  a  day  or  two  can  skip  and  run. 
The  young  animal,  moreover,  is  born  in  full  possession  of 
its  senses,  such  as  sight,  touch,  hearing,  smell,  taste,  and 
with  an  amount  of  intelligence  which  nearly,  if  not  quite, 
equals  its  parents  ;  it  practically  has  nothing  to  learn  but 
obedience  to  man.  Not  only  is  the  nervous  system  in  an 
advanced  condition,  but  also  the  locomotor :  the  legs  of  the 
foal  are  remarkably  long,  some  of  the  bones  being  nearly 
their  full  length,  though,  of  course,  not  their  full  weight ; 
such  joints  as  the  knee  and  hock  have  very  little  to  grow. 
We  can  understand  the  reason  of  this  development  of  the 
limb  from  what  we  have  previously  said,  while  the  length 
of  leg  in  the  foal  is  undoubtedly  for  the  purpose  of  enabling 
the  animal  to  reach  the  mammary  gland. 

The  limb,  however,  is  only  partially  developed ;  from  the 
knee  and  hock  to  the  ground  it  is  nearly  the  length  of  the 
adult ;  from  the  knee  to  the  elbow  and  the  hock  to  the 
stifle  it  is  decidedly  below  the  adult ;  whilst  from  the  elbow 
to  the  withers,  and  the  stifle  to  the  croup,  the  body  has  a 
considerable  amount  to  grow.  It  has  been  said,  and  the 
statement  appears  to  be  true,  that  the  future  height  of  the 

623 


624     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

foal  may  be  ascertained  by  measuring  the  fore  limb  from 
the  fetlock  to  the  elbow  and  multiplying  it  by  two. 

Table  showing  the  Length  of  the  Bones  of  the  Limbs  of 
THE  Foal  and  Adult  Horse. 


J 
Adult  Horse. 

Foal  of 
Six  Weeks. 

Difference. 

Scapula    - 

15  in. 

8jin. 

6|in. 

Humerus  - 

12  in. 

Bin. 

4  in. 

Radius  and  ulna  - 

18  in. 

12  in. 

6  in. 

Knee-joint 

'61  X  3^  in. 

3  X  3  in. 

h  in. 

Metacarpal 

91  in. 

Bfin. 

|in. 

Suffraginis 

3:^  in. 

3  in. 

h  in. 

Femur 

17  in. 

lOi  in. 

6Mn. 

Tibia 

13i  in. 

Q^in. 

4  in. 

Calcis  to  metatarsal  bone 

6  in. 

5  in. 

1  in. 

Metatarsal 

11  in. 

10  in. 

lin. 

Sufifraginis 

3^  in. 

3  in. 

i  in. 

The  hind  quarters  of  the  foal  are  in  a  more  advanced 
state  of  development  than  the  fore ;  the  shoulders  are  very 
oblique,  the  chest  contracted  and  shrunken-looking,  and 
neither  contains  much  muscle.  The  oblique  position  of  the 
scapula  is  due  to  the  weight  of  the  body  on  the  limbs,  the 
weakness  of  the  muscles  at  this  part  allowing  the  angle 
formed  by  the  scapula  and  humerus  to  be  considerably 
closed,  and  the  shoulder-joint  to  bulge.  The  head  of  the 
foal  is  prominent  over  the  brain  and  depressed  over  the 
nasal  bones.  The  hair  is  fully  developed  but  woolly,  that 
of  the  mane  being  scanty,  and  of  the  tail  curly,  while  the 
colour  of  the  body-hair  is  light  of  its  kind.  A  similar 
deficiency  of  pigment  is  observed  in  the  iris. 

The  rate  at  which  the  foal  increases  in  weight,  and  other 
circumstances  connected  with  its  nutrition,  were  made  the 
subject  of  inquiry  by  Boussingault.*  He  found  that  the 
mean  weight  at  birth  was  112  lbs.,  that  during  the  first 
three  months  the  daily  increase  in  weight  was  2'2  lbs.  ; 
from  three  up  to  six  months  the  increase  was  1*3  lbs.,  and 

*  Quoted  by  Colin. 


GROWTH,  DECAY,  AND  DEATH  625 

from  six  months  up  to  three  years  of  age  the  increase  was 
at  the  rate  of  "7  lb.  per  diem. 

The  influence  of  feeding  on  development  is  most  remark- 
able ;  not  only  does  the  body  increase  in  size  and  weight, 
but  the  animal  presents  the  appearance  of  the  adult,  so 
that  a  thoroughbred  at  two  years  old  is  '  furnished  '  and 
looks  as  old  as  an  ordinary  horse  at  four  years  old. 

Calves,  according  to  Torcy,*  have  a  mean  weight  at  birth 
of  77  lbs.,  the  daily  increase  during  the  first  two  years 
being  1*5  lbs.  With  sheep  the  daily  increase  in  weight  is 
more  rapid ;  a  lamb  will  in  ten  days  gain  50  per  cent,  on 
its  original  weight,  will  double  its  weight  at  the  end  of  the 
first  month,  and  treble  it  at  the  end  of  the  second.  Swine 
present,  however,  the  most  rapid  increase  in  weight,  for, 
according  to  the  authorities  quoted,  a  pig  will  increase 
20  per  cent,  in  its  weight  per  diem  during  the  first  week, 
and  up  to  the  end  of  the  first  year  will  add  '44  lb.  daily  to 
its  body  weight. 

The  relative  rate  of  growth  of  each  part  is  not  the  same. 
The  eyes,  ears,  brain,  kidneys,  and  liver  grow  less  rapidly 
than  the  other  parts,  owing  to  their  relatively  large  size  at 
birth ;  the  greatest  increase  is  in  the  skeleton  and  muscles, 
and  to  the  rate  of  this  increase  we  have  just  alluded  ;  the 
least  increase  is  in  the  eyes  and  the  ears,  and  the  limbs 
below  the  knee  and  hock.  Few  observations  have  been  made 
on  the  rate  of  growth.  Percival  f  many  years  ago  drew  up  a 
table,  which  he  considered  very  imperfect,  as  to  the  rate  at 
which  some  horses  of  his  regiment  grew,  from  which  he 
showed  that  the  increase  in  height  between  2  years  and 

3  years  was  on  an  average  one  inch,  between  3  years  and 

4  years  one-third  of  an  inch,  and  between  4  years  and 

5  years  one-third  of  an  inch.     Some  of  the  horses  did  not 
grow  : 

Of  35  two-year-olds,  2  did  not  grow  during  the  year. 
Of  144  three-year-olds,  17  did  not  grow  during  the  year. 
Of  48  four-year-olds,  7  did  not  grow  during  the  year. 
Of  11  five-year-olds,  2  did  not  grow  during  the  year. 

*  Quoted  by  Colin.  f  '  Lectures  on  Form  and  Action.' 

40 


626     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

There  can  be  no  doubt  that  many  horses  grow  much 
more  than  two-thirds  of  an  inch  between  three  and  five 
years  old.  It  is  probable  that  many  grow  up  to  their  sixth 
year. 

During  the  time  the  calf  and  foal  are  receiving  their 
mother's  milk  the  urine  is  acid,  for  the  reason  that  the 
animal  is  practically  carnivorous  ;  once  a  vegetable  diet  is 
taken  the  urine  becomes  alkaline,  and  it  is  probable  de- 
creases in  quantity.  The  activity  of  certain  glands,  such 
as  the  thymus,  becomes  considerably  reduced  as  the  animal 
grows,  and  finally  they  disappear  at  the  adult  period.  One 
characteristic  of  the  young  animal  is  the  necessity  for 
sleep  ;  it  is  probably  during  slumber  that  the  tissues  make 
the  immense  strides  noticeable  during  the  first  few  weeks 
of  life. 

Dentition  commences  immediately  at  birth,  if  it  has  not 
already  commenced  in  ntero ;  the  following  tables  show 
the  period  at  which  changes  take  place  in  the  teeth  from 
birth  to  adult  age  : 

Horse. 


Eruption. 

Change. 

Jncisors  : 

Central 

At  birth. 

2|  years. 

Lateral 

1  to  2  months. 

3^  years. 

Corner 

7  to  8  months. 

4|  years. 

Molars  : 

First    - 

I 

C  2\  years. 

Second 

\  At  birth. 

<  3  years. 

Third  - 

J 

(  About  '6\  years. 

Fourth 

About  1  year. 

Fifth    - 

About  2|;  years. 

Sixth   - 

About  Si  to  4  years. 
About  4|  years. 

Canines  - 

GROWTH,  DECAY,  AND  DEATH 
Ox* 


627 


Eruption. 

Change. 

Incisors  : 

Central 

■* 

(1^  years. 

1 2^^  to  2/^  years.f 

Middle 

At  or   soon   after 

Lateral 

j       birth. 

"  2j«o-  to  3  years.f 

Corner 

, 

i2i§  to  3  I'V  years.f 

Molars  : 

First 

] 

C About  2/o  years. 

Second 

I  At  birth. 

4  About  2^^  years. 

Third 

j 

(About  2  Y*V  years. 

Fourth 

6  months. 

Fifth 

About  12  months. 

Sixth 

21  months. 

Sheep. 


Eruption. 

Change. 

Incisors  : 

Central 

\ 

'About  1  year. 
About  2  years. 

Middle 

At   birth   or   soon 

Lateral 

after. 

Soon  after  2  years.  J 
lAbout  3  years.J 

Corner 

; 

Molars  : 

Fu-st 
Second 

1  At   birth   or   soon 
j        after. 

rSoon    after     18 
J.      months. 

Third 

f^ About  2  years. 

Fourth 

3  months. 

Fifth 

9  months. 

Sixth 

18  months. 

*  The  age  of  the  ox,  sheep,  and  pig  is  tabulated  from  the  data  given 
by  Professor  Brown  in  his  '  Dentition  as  Indicative  of  the  Age  of 
Animals.' 

t  There  is  considerable  variation  in  the  development  of  these  teeth. 

J  These  teeth  are  liable  to  great  variation  in  their  development. 


40—2 


628     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 

Pig. 


Eruption. 

Change. 

Incisors  : 

Central 

1  month. 

12  months. 

Lateral 

2  months. 

18  months. 

Corner 

At  birth. 

8  months. 

Molars  : 

First 

-  1  month. 

) 

Second 

}  About  15  months. 

Third 

i 

Fourth 

5  months. 

Fifth 

10  to  12  months. 

Sixth 

18  months. 

Premolars  - 

5  months. 

Tusks 

At  birth. 

9  months. 

In  all  these  tables  the  periods  given  are  those  of  erup- 
tion only  ;  the  teeth  are  not  fully  developed  for  some  time 
later,  which  varies  from  four  to  six  months  in  the  horse  to 
a  month  in  the  pig  and  ruminant. 

The  completion  of  dentition  marks  the  age  of  maturity ; 
the  uncastrated  animal  presents  very  distinctive  features 
as  compared  with  the  female,  viz.,  greater  bulk,  a  heavy 
crest  and  neck,  and  a  harsher  voice ;  the  castrated  horse 
more  closely  resembles  the  mare.  No  such  difference  as  is 
observable  in  the  human  family  exists  between  the  male 
and  female  of  the  horse  tribe  ;  the  mare  arrives  at  maturity 
at  the  same  time  as  the  horse,  and  the  castrated  animal  is 
not  deficient  in  stamina,  strength,  or  capacity  for  work ; 
moreover,  castration  in  the  horse  does  not  lead  to  a  deposi- 
tion of  fat  in  the  body. 

Decay. — It  is  doubtful  to  what  age  a  horse  would  live  if 
not  subjected  to  domestication,  but  we  may  safely  say  that 
at  seventeen  years  old  the  powers  of  life  in  the  majority  of 
them  are  on  the  wane,  though  at  this  period  some  may  be 
found  in  full  possession  of  life  and  vigour.  These  are 
probably  cases  of  a  survival  of  the  fittest,  and  cannot  be 
taken  as  a  general  guide.  As  a  broad  rule  it  may  be  stated 
that  an  old  horse  is  liable  to  be  killed  by  a  hard  day's 


GROWTH,  DECAY,  AND  DEATH  629 

work,  and  in  this  sense  he  is  certainly  old  at  seventeen. 
Arterial  degeneration  is  not  marked  at  this  period  of  life, 
and  few  horses  live  long  enough  for  their  arteries  to  become 
rigid. 

Doubtless  the  work  performed  by  horses  is  the  chief 
cause  of  their  rapid  decay,  for  their  legs  always  wear  out 
before  their  bodies ;  but  apart  from  this,  changes  in  their 
teeth,  such  as  the  wearing  away  of  the  molars,  appear  to 
preclude  many  of  them  from  reaching  a  ripe  old  age, 
though  instances  are  on  record  of  horses  attaining  the  age 
of  thirty-five,  forty-five,  fifty,  and  one  animal  is  known  to 
have  lived  to  sixty-two  years  of  age.  Blaine*  appears  to 
have  gone  very  carefully  into  the  question  of  old  age  in 
equines,  and  he  drew  the  following  comparison,  which  is 
doubtless  very  close  to  the  truth  : 

'  The  first  five  years  of  a  horse  may  be  considered  as 
equivalent  to  the  first  twenty  years  of  a  man  ;  thus,  a  horse 
of  five  years  may  be  comparatin'Jy  considered  as  old  as  a 
man  of  twenty ;  a  horse  of  ten  years  as  a  man  of  forty  ;  a 
horse  of  fifteen  as  a  man  of  fifty ;  a  horse  of  twenty  as  a 
man  of  sixty  ;  of  twenty- five  as  a  man  of  seventy  ;  of 
thirty  as  a  man  of  eighty ;  and  of  thirty-five  as  a  man  of 
ninety.' 

Death.— Death  from  natural  causes  in  the  horse  is  a 
matter  of  rare  occurrence ;  it  is  seldom  that  an  animal  is 
taken  such  care  of  that  the  tissues  are  worn  out  by  age 
and  decay,  or  that  he  is  allowed  to  live  until  the  breath  of 
life  passes  gradually  from  the  body.  Sentiment  plays  no 
part  in  horse  management ;  a  useless  mouth  is  one  to  be 
got  rid  of.  In  consequence,  the  majority  of  horses  meet 
either  with  a  violent  death  or  one  the  result  of  disease. 

Natural  death  is  described  as  commencing  either  at  the 
heart,  lungs,  brain,  or  blood.  Probably  in  the  main  most 
cases  of  natural  death  may  be  attributed  to  a  failure  of  the 
heart's  action ;  but  from  what  we  know  of  the  physiology 
of  the  heart,  respiration,  and  blood,  it  is  very  difficult  to 
separate  these  in  discussing  the  causes  of  natural  death, 
*  '  Encyclopaedia  of  Rural  Sports.' 


630     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

knowing  as  we  do  how  completely  one  is  dependent  on  the 
other.  The  cessation  of  the  heart's  action  may  be  looked 
upon  as  the  termination  of  life. 

We  cannot  enter  upon  the  cause  of  death  the  result  of 
disease,  excepting  to  notice  the  interesting  fact  that  horses 
seldom  die  quietly  ;  a  large  majority  of  them  leave  this 
world  in  powerful  convulsions,  fighting  or  struggling  to 
the  last,  lying  on  their  side,  and  galloping  themselves  to 
death.      Especially   is   this    marked   in   acute   abdominal 


Fig.  157. — Convulsive  Limb  Movements  at  the  Moment  of 
Brain  Destruction. 

Note  the  tail  is  affected  as  well  as  the  limbs.     The  bandages  are  put 
on  to  assist  the  plate. 

trouble.  The  struggles  at  the  end  should  not  be  mistaken 
for  pain ;  the  animal  is  quite  unconscious.  The  violent 
convulsions  which  occur  at  the  last  moment  are  not  present 
in  death  from  acute  chest  diseases ;  such  cases  stand  per- 
sistently to  the  last,  and  either  drop  dead  or  die  very 
shortly  afterwards. 

In  violent  death  by  destruction  of  the  brain  in  horses, 
remarkable  muscular  contractions  of  the  limbs  occur ; 
these  cannot  be  seen  with  the  unaided  eye  as  they  are  so 
rapid,  but  are  readily  revealed  by  the  camera  (Fig.  157). 


GROWTH,  DECAY,  AND  DEATH 


631 


In  spite  of  their  rapidity,  a  marked  interval  between  brain 
destruction  and  muscular  contractions  occurs ;  in  Fig.  158 
the  brain  was  destroyed  by  a  charge  of  large  shot,  yet  the 
horse  is  still  standing,  the  impulses  relaxing  muscle  tonus 
not  yet  having  had  time  to  pass  out.  At  the  moment  of 
death  the  bladder  and  rectum  are  emptied,  the  horse 
sweats  on  the  inside  of  the  thighs,  the  pupil  dilates  widely, 
and  occasionally  the  panniculus  is  called  into  play  and  the 
animal  may  shake  the  skin  as  if  to  dislodge  a  fly. 


Fig.  158. — Brain  Destroyed  by  a  Charge  of  Shot. 

The  muscles  of  the  quarters  are  preparing  to  contract,  as  may  be  seen 
by  their  outHne ;  the  tail  is  also  turned  to  one  side,  and  the  heel 
of  one  limb  has  left  the  grovmd.  There  is  nothing,  however,  to 
indicate  the  fact  that  the  horse  is  dead. 


Soon  after  death  rigor  mortis  appears  (see  p.  379),  and 
within  a  short  time  tympany  of  the  abdomen  is  apparent  in 
the  herbivora,  reaching  such  a  degree  in  a  few  hours, 
especially  during  warm  weather,  that  post-mortem  rup- 
tures of  the  diaphragm  and  other  viscera  are  exceedingly 
common.  The  explanation  of  the  tympany  is  the  con- 
siderable amount  of  gas  generated  by  the  fermentative 
decomposition  of  vegetable  food. 


CHAPTEK  XX 

THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY* 

A  LAKGE  number  of  elements  enter  into  the  composition  of 
the  body.  Oxygen,  hydrogen,  carbon,  nitrogen,  sulphur, 
phosphorus,  chlorine,  fluorine,  silicon,  potassium,  sodium, 
calcium,  magnesium,  and  iron  are  found,  not,  it  is  true,  in 
a  free  state  or  only  to  a  very  slight  extent,  but  brought 
together  in  such  a  way  as  to  form  compounds,  and  these 
may  be  divided  into  two  classes,  organic  and  inorganic. 

Carbon  is  present  in  the  atmosphere  in  small  amounts 
in  the  form  of  carbonic  acid,  viz.,  united  to  oxygen ;  it  is 
only  in  this  form  that  it  can  be  taken  up  by  plants,  which 
in  their  special  laboratory  split  off  the  oxygen  molecule 
and  store  up  the  carbon,  returning  the  oxygen  to  the  air, 
and  thus  supply  to  the  atmosphere  that  element  of  which 
animals  are  continually  depriving  it. 

Carbon  enters  the  animal  system  with  the  carbon  of  the 
food,  and  leaves  it  either  as  carbonic  acid  or  in  compounds, 
such  as  urea ;  as  carbonic  acid,  therefore,  it  is  again  taken 
up  by  the  plant. 

Hydrogen  does  not  occur  in  a  free  state  in  nature,  but 
principally  as  water,  and  a  very  small  quantity  as  ammonia, 

*  This  brief  outline  of  the  chemistry  of  the  body  was  originally 
based  on  a  summary  of  the  principal  facts  contained  in  Bunge's 
'  Physiological  and  Pathological  Chemistry,'  and  Sheridan  Lea's 
appendix  to  Foster's  '  Physiology,'  '  The  Chemical  Basis  of  the 
Animal  Body.'  This  chapter  is  in  no  way  intended  to  be  a  complete 
statement  as  to  the  chemical  constituents  of  the  animal  body,  but 
elucidatory  and  supplementary  to  the  chemical  statements  scattered 
throughout  the  preceding  chapters. 

632 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY   633 

and  it  is  in  these  forms  that  hydrogen  is  taken  up  by  plants. 
Animals  give  off  hydrogen  as  water  and  ammonia,  or  as 
substances  which  may  be  readily  made  to  yield  these. 

Oxiigen  is  the  most  widely  distributed  of  the  elements, 
forming  one-quarter  by  weight  of  the  atmosphere,  and 
eight-ninths  by  weight  of  water ;  it  also  forms,  by  means 
of  its  compounds,  one-half  the  weight  of  the  earth's  crust. 
It  is  the  only  element  which  enters  the  animal  or  vegetable 
body  in  a  free  state. 

Nitrogen  exists  largely  in  a  free  state,  since  it  forms  no 
less  than  four-fifths  of  the  atmosphere ;  it  has  but  little 
affinity  for  other  elements.  In  the  form  of  ammonia, 
nitrous  and  nitric  acids,  it  enters  the  plant ;  as  proteid 
it  enters  the  animal,  leaving  it  as  urea,  etc.,  which 
by  decomposition  readily  yields  ammonia.  The  animal 
cannot  utilize  free  nitrogen  any  more  than  the  plant  can, 
though  leguminous  plants  appear  to  utilize  the  atmospheric 
nitrogen  by  symbiotic  co-operation  with  certain  bacteria. 
As  a  gas  it  is  found  dissolved  to  a  slight  extent  in  some 
of  the  fluids  of  the  body. 

Sulphur  exists  largely  in  nature  in  combination  as 
sulphates  of  alkalis  and  alkaline  earths ;  in  this  form  it 
is  taken  up  by  plants,  and  becoming  a  part  of  their  proteid 
molecule  is  taken  into  the  body  of  the  animal,  where  by 
splitting  up  and  oxidation  it  yields  sulphuric  acid,  in  which 
form  it  is  excreted  in  the  urine  as  sulphates  or  colligated 
with  certain  organic  substances  (see  p.  300). 

Phosphorus  enters  plants  as  phosphoric  acid  united  with 
alkalis ;  in  soils  it  exists  in  only  small  quantities,  hence 
the  necessity  of  phosphates  as  manure.  In  the  plant 
phosphoric  acid  forms  a  part  of  the  complicated  compounds 
known  as  lecithin  and  nuclein,  in  which  condition  it  enters 
the  animal  body,  forming  a  part  of  both  the  solid  and  fluid 
tissues. 

Chlorine  does  not  exist  in  a  free  state  in  nature,  but 
combined  with  potassium  and  sodium,  in  which  form  it 
enters  plants,  and  from  these  passes  in  the  same  com- 
pounds into  animals. 


634     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Neither  sodium,  potassium,  nor  magnesium  enter  or  leave 
the  body  or  plant  in  any  organic  form,  but  simply  as  in- 
organic salts.  On  the  other  hand,  Bunge  considers  that 
calcium  does  enter  the  body  as  an  organic  compound. 

Iron  does  not  occur  free  in  nature,  but  chiefly  as  com- 
pounds with  oxygen  in  a  ferrous  and  ferric  state.  In  the 
animal  it  occurs  chiefly  in  the  highly  complex  body  haemo- 
globin, which  acts  as  an  oxygen-carrier.  Iron  furnishes 
the  plant  with  its  colouring  matter,  for  chlorophyll  cannot 
be  formed  without  its  aid.  It  is  not  known  in  what  form 
iron  leaves  the  body. 

Silicon,  in  the  form  of  silicic  acid,  is  taken  up  by  plants. 
From  the  plant  it  is  taken  into  the  body  and  passes  into 
the  tissues.  It  is  largely  of  use  in  the  growth  of  hair,  and 
much  of  it  passes  out  of  the  body  of  herbivora  with  the 
urine ;  in  sheep,  according  to  Bunge,  it  sometimes  causes 
stone  in  the  bladder. 

Bunge  draws  a  contrast,  in  the  following  terms,  between 
the  methods  employed  by  the  vegetable  and  animal  organism 
in  the  utilization  of  the  various  elements  and  compounds 
presented  to  them : 

1.  The  plant  forms  organic  substances ;  the  animal 
destroys  organic  substances.  The  vital  process  in  the  plant 
is  preponderatingly  synthetic,  in  the  animal  analytic. 

2.  The  life  of  the  plant  is  a  process  of  reduction ;  the  life 
of  the  animal  a  process  of  oxidation. 

3.  The  plant  uses  up  kinetic  energy  and  produces 
potential  energy  ;  the  animal  uses  up  potential  energy  and 
produces  kinetic  energy. 

The  organic  compounds  in  the  body  may  be  broadly 
divided  into  nitrogenous  and  non-nitrogenous. 

NITROGENOUS  BODIES. 

Proteids. — This  term  is  applied  to  a  large  number  of 
substances  more  or  less  closely  allied,  which  in  one  form 
or  other  go  to  make  up  by  far  the  largest  portion  of  the 
animal  body. 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY   635 

Proteids  are  highly  complex  substances,  possessing  as 
yet  no  definite  chemical  formula  owing  to  the  difficulty  in 
obtaining  them  in  a  sufficiently  pure  state  for  analysis,  and 
to  the  magnitude  of  their  molecule.  With  some  few  excep- 
tions they  have  never  been  obtained  in  a  crystalline  con- 
dition, and  their  nature  is  colloidal,  for  they  do  not  diffuse 
through  an  animal  membrane,  not  even  those  which  can 
be  obtained  in  a  crystalline  form. 

All  proteids  contain  carbon,  hydrogen,  oxygen,  nitrogen, 
and  very  variable  amounts  of  sulphur. 


Carbon 

51*5  to  54-5  per  cent 

Hydrogen 

6-9  to    7-3 

Oxygen 

20-9  to  23-5       „ 

Nitrogen 

15-2  to  17-0       „ 

Sulphur 

0-3  to    2-0 

In  spite  of  the  fact  that  proteids  exist  in  several  forms  in 
all  animal  and  vegetable  bodies,  and  that  it  is  quite  im- 
possible to  maintain  life  without  them,  yet  very  little  is 
known  of  them  owing  to  their  extreme  complexity.  Neither 
their  chemical  formula  nor  their  molecular  weight  is  known, 
though  the  latter  must  certainly  be  very  large,  and  the 
chemist  has  never  as  yet  been  able  to  build  them  up 
synthetically,  though  the  work  now  being  carried  on  by 
Emil  Fischer  and  his  pupils  is  full  of  promise  for  a  future 
synthesis. 

The  decomposition  products  of  proteids  are  very 
numerous  and  very  varying  in  nature,  according  to  the 
methods  employed.  In  the  body  carbonic  acid,  water,  urea, 
and  uric  acid  are  the  final  end  products,  but  between  these 
and  proteids  are  glycine,  leucine,  and  other  substances. 
From  the  non-nitrogenous  portions  of  the  proteid  glycogen 
and  fat  may  be  obtained,  as  we  have  previously  seen. 
Proteids,  when  acted  upon  outside  the  body  by  means  of 
heat,  putrefaction,  acids,  alkalis,  and  oxidizing  agents, 
yield  a  large  and  numerous  class  of  substances. 

In  the  absence  of  adequate  chemical  knowledge,  all 
classification  of  proteids  must  necessarily  be  artificial,  and 


G8fi     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

is  at  present  based  on  their  varying  solubilities  in  water, 
saline  solutions,  acids,  and  alkalis. 

In  the  following  table  the  proteids  are  thus  roughly 
classified,  and  the  distinguishing  characteristics  of  each 
class  given. 

Classification  of  the  Proteids. 

A.  Simple  Proteids. 

Class  I.  Native  Albumins. 

These  are  soluble  in  distilled  water,  and  the  solutions 
are  coagulated  by  heat  at  70°  to  75°  C,  especially  in  the 
presence  of  dilute  acetic  acid.  They  are  not  precipitated 
by  saturating  their  solutions  with  neutral  salts  other  than 
sodio-magnesium  sulphate  and  neutral  ammonium  sulphate. 
Examples  of  this  class  are  egg  and  serum  albumin,  cell, 
muscle,  and  milk  albumin,  or  lactalbumin  (p.  G'20). 

Class  II.  Globulins. 

These  are  insoluble  in  distilled  water,  but  soluble  in 
dilute  saline  solutions  ;  from  these  they  are  precipitated  by 
saturation  with  common  salt  or  magnesium  sulphate.  In 
this  class  are  found  the  globulin  of  the  crystalline  lens 
(crystallin),  the  globulin  of  the  blood,  para-  or  serum- 
globulin,  the  fibrinogen  of  the  blood-  and  myosin  of  muscle. 

Class  III.  Derived  Albumins  {Alhuminates). 

These  are  obtained  by  the  action  of  acids  or  alkalis 
on  albumins  or  globulins.  They  are  insoluble  in  distilled 
water  or  in  dilute  neutral  saline  solutions,  but  soluble  in 
acids  and  alkalies,  and  the  solution  is  not  coagulated  by 
boiling,  though  it  is  precipitated  by  careful  neutralization. 
Examples  of  this  class  are  acid  albumin,  syntonin,  and 
alkali  albumin.  Caseinogen  (casein),  which  was  at  one  time 
placed  in  this  class,  is  now  known  to  be  a  nucleo-proteid. 

Class  IV.  Fibrins. 

These  are  insoluble  in  water,  but  soluble  by  the  pro- 
longed action  of  strong  neutral  saline  solutions,  whereby 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY   637 

they  are  largely  changed  mto  globulms  (Class  II.),  and 
with  difficulty  in  strong  acids  and  alkalis,  being  at  the 
same  time  converted  into  acid  or  alkali  albumin  (Class  III.). 
Examples  are  the  fibrin  formed  during  the  clotting  of  the 
blood  and  lymph. 

Class  V.  Coa(jalated  Proteids. 

Any  of  the  above  which  have  been  coagulated  either  by 
heat  or  the  prolonged  action  of  alcohol. 

Class  VI.  Alhumoses  (Proteoses)  and  Peptones. 

Both  of  these  are  very  soluble  in  water,  albumose  being 
precipitated  by  saturation  with  ammonium  sulphate,  while 
peptones  are  not.  Peptones  are  not  precipitated  by  any 
ordinary  proteid  precipitant  excepting  alcohol,  and  even 
the  prolonged  action  of  alchol  does  not  coagulate  them. 
Albumoses  may  be  precipitated  by  the  careful  addition  of 
nitric  acid  in  the  cold,  and  the  precipitate  characteristically 
disappears  on  heating  and  reappears  on  cooling. 

Albumoses  (or  proteoses)  are  formed  as  the  primary 
product  of  the  action  of  the  gastric  and  pancreatic  enzymes 
on  proteids.  Three  well-marked  forms  of  albumose  are 
known,  characterized  by  their  varying  solubilities  and 
their  precipitability  by  neutral  salts  or  acetic  acid  and 
potasssium  ferrocyanide.  Peptones  are  the  Jinal  product 
of  the  action  of  gastric  and  pancreatic  enzymes  on  proteids. 
One  of  their  most  interesting  characteristics  is  that  they, 
alone  among  proteids,  are  diffusible  through  membranes. 
They  differ  from  albumoses  by  the  fact  that  they  are  not 
precipitated  when  their  solution  is  saturated  with  neutral 
ammonium  sulphate  or  any  other  neutral  salt. 

B.  Compound  Proteids. 

Class    I.    Niicleo-proteids. 

The  nucleo-proteids  are,  as  the  name  implies,  compounds 
of  a  proteid  with  nuclein,  the  characteristic  constituent  of 
nuclei.     They  form  the  bulk  of   the   proteids   present  in 


638     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

most  cell  protoplasm  and  their  solubilities  are  closely 
similar  to  those  of  the  globulins.  Their  compound  nature 
is  shown  by  the  fact  that  when  digested  with  gastric  juice 
they  yield  albumoses  and  peptones,  together  with  an  undis- 
solved residue  of  nuclein.  Ordinarily  the  nuclein  thus 
obtained  is  the  true  nuclein,  which  yields  substances  of  the 
xanthine  (purin)  series  when  hydrolyzed  by  acids.  In 
other  cases  the  nuclein  residue  (pseudo-nuclein)  does  not 
yield  xanthine  bodies  by  hydrolysis,  and  ty^Dical  examples 
of  this  form  of  nucleo-proteid  are  found  in  the  caseinogen 
(casein)  of  milk  and  the  vitellin  of  egg-yolk.  They  all 
contain  phosphorus,  since  this  element  is  characteristically 
a  constituent  of  nuclein. 

Class  II.  Glyco-proteids. 

These  forms  of  proteid  are  characterized  by  yielding,  on 
hydrolysis,  some  kind  of  (carbohydrate)  substance  which 
reduces  Fehling's  fluid  and  gives  osazones  with  phenyl- 
hydrazine.  This  reducing  substance  frequently  contains 
nitrogen,  and  is  probably  in  many  cases  glucosamine 
(CeHijOj.NHg),  or  amido-glucose. 

The  characteristic  members  of  the  glyco-proteid  group 
are  the  various  kinds  of  mucin.  Of  these  the  mucin  of 
saliva  may  be  regarded  as  the  truest  and  most  typical 
form.  Mucin  confers  on  its  solutions  their  well-known 
viscidity  or  'ropiness.'  From  these  it  is  readily  precipi- 
table  by  the  addition  of  acetic  acid,  and  is  resoluble  in 
alkalis. 

The   Albuminoids. 

Under  this  name  a  number  of  substances  are  grouped 
together,  which,  while  closely  allied  to  the  proteids,  differ 
from  them  in  some  important  particulars,  and  differ  also 
in  many  respects  the  one  from  the  other.  The  best  known 
members  of  the  group  are  collagen  and  gelatin,  chondrin, 
elastin,  and  keratin. 

Collagen  is  the  ground  substance  of  which  the  fibres  of 
connective  tissue  are  formed  and,  under  the  name  of  ossein, 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY   639 

forms  a  large  part  of  the  organic  basis  of  bones.  It  is 
insoluble  in  water,  salt  solutions,  and  dilute  alkalis  and 
acids,  though  it  swells  up  as  a  gelatinous  mass  by  the 
action  of  the  latter.  Prolonged  boiling  with  water,  espe- 
cially in  presence  of  dilute  acids,  converts  it  into  gelatin, 
and  the  latter  can  be  reconverted  into  collagen  by  a  dry 
heat  of  130°  C. 

Gelatin. — A  common  and  well-known  substance  of  which 
isinglass  is  a  typically  pure  form  and  glue  an  impure. 
Insoluble  in  cold  water,  it  swells  up  by  its  action,  and  now 
dissolves  readily  when  heated,  the  solution  forming  a  jelly 
on  cooling,  even  when  it  contains  only  1  per  cent,  of 
gelation.  When  digested  with  pepsin  or  trypsin,  gelatin 
yields  diffusible  substances  known  as  gelatin-peptones.  By 
hydrolysis  it  yields  leucine  and  glycine,  but  no  tyrosine  or 
any  member  of  the  aromatic  series,  and  hence  gives  no  red 
reaction  with  Millon's  reagent  (see  below). 

Chondriu. — This  is  obtained  from  hyaline  cartilage  by 
those  processes  which,  when  applied  to  connective  tissue  or 
bones,  yield  gelatin.  Chondrin  resembles  gelatin  in  that  its 
solutions  gelatinize  on  cooling,  but  it  differs  chemically  in 
many  respects  from  gelatin.  Thus  it  is  precipitable  by 
acetic  acid,  and  when  hydrolyzed  yields  a  substance  which 
reduces  Fehling's  fluid. 

Elastin. — This  is  the  ground  substance  of  the  fibres  of 
elastic  tissue.  It  is  extraordinarily  insoluble  and  resistant 
to  ordinary  reagents,  and  is  hence  obtained  by  treating  a 
tissue  such  as  ligamentum  nuchse  with  a  succession  of 
reagents  which  dissolve  out  everything  except  the  elastin. 
By  digestion  with  pepsin  or  trypsin  or  by  hydrolysis, 
elastin  yields  products  similar  to  many  of  those  similarly 
obtainable  from  true  proteids. 

Keratin. — The  characteristic  constituent  of  epidermal 
structures  such  as  hair,  nails,  feathers,  and  horn.  From 
these  it  is  obtained  as  a  residue  by  their  extraction  with 
a  series  of  reagents,  such  as  water,  alcohol,  ether,  dilute 
acids,  etc.  Its  elementary  composition  is  closely  allied  to 
that  of  the  true  proteids,  but  it  differs  from  them  by  the 


640     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

large  amount  of  loosely  combined  sulphur  it  contains, 
5  per  cent.  It  yields  in  hydrolysis  large  amounts  of 
leucine  and  tyrosine  and  other  substances  similar  to  those 
thus  obtainable  from  proteids. 

The  various  albumins  we  have  spoken  of  belong  to  the 
animal  body,  but  in  the  vegetable  kingdom  proteids  are 
found  which  do  not  differ  in  any  essential  particular  from 
animal  proteids.  The  amount  of  proteid  matter  in  plants 
is  less  than  that  found  in  animals,  and  globulins  exist  in 


Fig.  159. — Albumin  Crystals  from  Horse-Serum  (Gurbee). 

larger  amounts  than  albumins,  in  fact  there  are  food 
substances  used  by  animals,  oats,  maize,  peas,  etc.,  in 
which  it  is  said  that  the  whole  of  the  proteid  occurs  as 
globulin  and  none  as  albumin. 

Some  of  the  plant  proteid  matter  crystallizes  readily, 
vitellin  for  example.  It  is  this  substance  which  has 
furnished  the  so-called  '  crystallized  albumin,'  the  existence 
of  which  has  been  known  for  some  time.  Egg  albumin 
may  be  readily  crystallized  and  the  serum-albumin  of 
horse's  blood  is  remarkable  for  the  ease  with  which  it  may 
be  obtained  in  a  crystalline  form  (Fig.  159). 

Both  albuminates  and  proteoses  occur  in  plants,  but 
peptone  does  not  appear  to  be  found  in  them. 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY   641 

The  process  by  which  plants  form  proteids  is  that  of 
synthesis ;  it  is  possible  that  such  substances  as  asparagine, 
leucine,  tyrosine,  etc.,  which  are  met  with  in  the  plant  are 
on  their  way  to  tissue  construction,  and  are  not,  as  in 
animals,  the  result  of  proteid  destruction. 

A  very  remarkable  fact  about  proteid  substances  is  that, 
though  they  constitute  the  mainspring  of  organic  life,  yet 
they  number  amongst  them,  or  amongst  their  decomposition 
products,  some  of  the  most  powerful  poisons  known. 
Snake  poison  is  a  proteid,  and  even  the  albumose  formed 
during  the  peptic  digestion  of  albumin  is  highly  poisonous 
if  injected  into  the  circulation. 

The  principal  tests  employed  to  detect  the  presence  of 
proteids  are  as  follows  : 

Proteid   Reactions. 

1.  Xanthoproteic  Reaction. — Solutions  heated  with  strong 
nitric  acid  turn  yellow,  and  on  the  addition  of  ammonia  or 
caustic  soda  are  changed  to  orange. 

2.  Millon's  Reaction. — With  Millon's  reagent*  they  give 
a  precipitate  which  turns  red  on  heating. 

3.  Piotrowski's  Reaction. — To  the  solution  of  proteid  is 
added  in  excess  a  strong  solution  of  caustic  soda,  and  one 
or  two  drops  of  a  weak  (1  per  cent.)  solution  of  sulphate  of 
copper ;  this  gives  a  violet  colour  which  deepens  in  tint 
on  boiling.  This  test  is  also  used  to  determine  the  presence 
of  albumoses  and  peptones  ;  the  colour  reaction  given  by 
these  is  rose-red  on  the  first  careful  a)ul  limited  addition  of 
the  sulphate  of  copper,  turning  to  violet  at  once  on  the 
addition  of  any  excess  of  the  copper  salt,  and  is  termed  the 
biuret  reaction. 

4.  Adamkieicicz's  Reaction. — To  a  solution  of  the  proteid 
is  added  strong  sulphuric  acid  and  glacial  acetic  acid  ;  a 
violet  colour  and  slight  fluorescence  occur. 

*  A  mixture  of  mercurous  and  mercuric  nitrates  in  presence  of 
nitric  acid. 

41 


642     A  MANUAL  OF  VETERINAPtY  PHYSIOLOGY 

5.  Acetic  acid  and  a  solution  of  ferrocyanide  of  potassium 
give  a  precipitate,  except  in  the  case  of  true  peptones  and 
some  forms  of  albumose. 

6.  Acetic  acid  and  sulphate  of  soda  give  a  precipitate  on 
boiling,  except  in  the  case  of  peptones. 

7.  Saturation  of  the  solution  with  neutral  ammonium 
sulphate  precipitates  proteids  other  than  peptones. 

8.  To  a  neutral  or  faintly  acid  solution  of  proteid  abso- 
lute alcohol  is  added  in  large  excess,  and  a  precipitate 
obtained. 

9.  Heating  a  solution  of  proteid  (albumins  and  globulins) 
causes  a  coagulum  to  form.  The  solution  should  be  ren- 
dered faintly  acid  with  acetic  acid,  any  excess  of  acid  being 
avoided,  as  otherwise  no  precipitate  may  be  produced. 

The  first  three  alone  of  the  above  reactions  suffice  to 
detect  the  smallest  traces  of  any  proteid  in  solution. 

There  are  many  other  tests  for  proteids,  mercuric  chloride, 
lead  acetate,  etc.,  but  the  above  are  those  which  are  princi- 
pally employed  either  to  determine  their  presence,  or  to 
free  a  solution  entirely  from  proteid. 

Ferments. — The  term  '  fermentation  '  was  originally  ap- 
plied to  the  characteristic  phenomena  which  occur  during 
the  action  of  yeast  in  solutions  of  sugar,  whereby  the  latter 
is  actively  and  rapidly  converted  into  alcohol,  and  the 
agent  which  gave  rise  to  the  phenomena  was  hence  called 
the  ferment.  Pasteur  showed  that  in  the  case  of  the 
alcoholic  fermentation  of  sugar  the  active  agent  is  the 
yeast- cell,  the  process  being  dependent  (as  also  in  putre- 
factions) on  the  activity  of  the  cell  as  an  organized  living 
structure.  Previously  to  this  soluble  substances,  such  as 
diastase  from  malt  and  pepsin  from  gastric  juice,  had  been 
obtained,  and  since  the  conditions  under  which  they  worked 
best,  and  many  of  the  phenomena  attending  their  action 
were  closely  similar  to  those  holding  good  in  the  case  of 
yeast,  they  also  came  to  be  called  ferments.  As  more  and 
more  of  these  '  substances '  were  discovered  and  their 
supreme  importance  ascertained,  as  the  causative  agents  in 
the  chemical  changes  of  digestion  and  numberless  other 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY  643 

physiological  processes,  and,  since  they  were  all  soluble  in 
water  and  therefore  devoid  of  any  organized  structure,  they 
were  called  the  unorganized  ferments  or  enzymes.  This 
division  of  the  ferments  into  two  classes  held  good,  and  is 
even  now  convenient,  until  a  few  years  ago,  when  Buchner 
showed  that  a  soluble,  unorganized  substance  (zymase) 
can  be  extracted  from  yeast-cells,  and  is  able  to  produce 
alcohol  by  its  action  on  sugar.  He  obtained  similar  results 
with  other  (bacterial)  organisms,  so  that  we  cannot  now 
speak  of  any  essential  differences  in  the  activities  of  the 
living  (organized)  ferments  and  the  non-living  (unorganized) 
ferments  or  enzymes. 

Ferments  are  remarkable  substances  whose  mode  of 
action  is  still  a  mystery.  The  outcome  of  their  action  is 
usually  hydrolytic — that  is  to  say,  they  lead  to  the  assump- 
tion of  water  by  the  substance  on  which  they  are  working, 
and  its  decomposition  into  simpler,  more  stable  bodies  of 
smaller  potential  energy.  We  cannot,  however,  enter  here 
into  the  details  of  ferment  action,  but  must  be  content  to 
point  out  the  chief  characteristics  of  their  activity  and 
properties.  1.  They  are  inactive  at  sufficiently  low  tempera- 
tures, and  work  best  at  some  given  medium  temperature, 
such  as  40°  to  45°  C.  Above  this  temperature  the  animal 
enzymes  show  a  gradually  diminishing  activity,  which  is 
finally  and  irretrievably  destroyed  at  70  °C.,  or  at  once  when 
their  solutions  are  boiled.  2.  Their  activity  is  closely 
dependent  on  the  reaction  of  the  solution  in  which  they 
work,  whether  it  be  acid,  alkaline  or  neutral,  as  in  the  case 
of  pepsin,  trypsin,  and  ptyalin.  3.  Their  action  is  tem- 
porarily lessened  or  even  stopped  by  the  presence  of  an 
excess  of  the  products  of  their  activity,  to  begin  again  when 
these  products  are  removed.  This  is  well  seen  in  a  diastatic 
conversion  of  starch  into  sugar.  4.  In  some  cases  their 
action  appears  to  be  reversible ;  as,  for  instance,  the  in- 
verting enzyme  (maltase)  of  the  intestinal  juice  can,  if 
allowed  to  act  on  concentrated  solutions  of  dextrose,  recon- 
vert a  part  of  this  into  maltose.  5.  They  are  all  soluble 
in  water,  and  conveniently  so  in  glycerin.      From  these 

41—2 


644     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

solutions  they  may  be  precipitated  by  a  sufficiency  of 
absolute  alcohol  or  by  saturation  with  neutral  ammonium 
sulphate.  When  purified  they  resemble  proteids  in  com- 
position and  reactions.  6.  They  are  all  non-diffusible 
through  membranes.  7.  They  are  not  apparently  used  up 
in  the  changes  they  produce,  and  they  therefore  influence 
the  velocity  of  any  given  conversion,  not  its  total  amount. 
Thus,  a  trace  of  enzyme  will  in  time  effect  the  conversion 
of  an  unlimited  amount  of  the  substance  on  which  it  is 
working :  more  of  the  enzyme  merely  hastens  the  rate  at 
which  the  final  result  is  reached. 

The  enzymes  in  tissues  do  not  always  exist  in  a  free  and 
active  state,  but  as  an  inactive  antecedent  to  which  the 
term  Zymogen  has  been  applied ;  a  zymogen  by  appro- 
priate means  may  be  converted  into  an  active  enzyme 
(see  p.  234). 

Of  the  Pigments  of  the  body  comparatively  little  is  known, 
though  they  are  widely  distributed  and  perform  important 
functions.  The  best  known  animal  pigment  is  haemoglobin, 
the  red  colouring  matter  of  the  blood ;  it  is  of  a  proteid 
nature,  yet  crystallizable,  and  it  also  contains  iron.  It 
acts  as  an  oxygen  carrier,  and  is  often  spoken  of  as  a  re- 
spiratory pigment;  it  has  several  derivatives  (see  pp.  8-12), 
which  supply  the  colouring  matter  of  the  bile,  urine,  and 
partly  that  of  the  fseces. 

The  next  pigment  widely  distributed  is  the  black  pigment 
of  the  body  or  melanin ;  it  occurs  in  the  skin,  hair  (p.  274), 
eye,  horn,  and  is  the  chief  constituent  of  the  melanotic 
tumours  so  common  in  the  horse. 

Both  in  the  faeces  and  in  the  dandruff  from  the  skin  of 
the  horse  chlorophyll  is  found  (p.  283)  ;  its  function  in  the 
body  is  quite  unknown. 

The  bile  ingments  have  been  sufficiently  dealt  with  on 
p.  220. 

There  are  several  other  pigments,  but  none  so  important 
as  the  above. 

Nitrogenous  Fats. — Though  true  fatty  substances  contain 
no  nitrogen,  yet  there  are  certain  complex  nitrogenous  fats 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY   645 

and  their  derivatives  which  are  found  distributed  in  the 
body ;  the  most  important  of  these  is 

Lecithin,  which  is  found  in  the  white  blood-corpuscles, 
the  white  matter  of  the  brain,  nerves,  and  spinal  cord, 
semen,  etc.,  and  also  in  yolk  of  egg,  where  it  is  united  with 
vitellin.  Decomposition  products  of  lecithin  are  (jlycero- 
phosphoric-acid  and  choline.  The  latter  substance  is 
poisonous,  and  by  oxidation  with  nitric  acid  yields  the 
extremely  poisonous  substance  muscarine.  Lecithin  is 
largely  introduced  into  the  body  by  means  of  the  food ; 
the  poisonous  action  of  the  choline  it  contains  is  probably 
prevented  by  the  substance  being  broken  up  by  the  bacteria 
of  the  intestines  into  carbonic  acid,  marsh  gas,  and 
ammonia. 

Neurine  is  a  substance  closely  related  to  choline,  but 
much  more  poisonous ;  it  is  the  active  principle  in  the 
poisonous  alkaloids  produced  by  putrefactive  decomposition 
of  animal  matter. 

Amides  and  Amido  -  Acids.  —  Many  of  the  substances 
belonging  to  this  series  are  of  considerable  importance,  and 
very  interesting  from  the  point  of  view  of  their  probable 
relationship  to  the  formation  of  urea  in  the  animal  body. 

Glycine,  CoH-NOo  (also  known  as  glycocoll  and  glyco- 
cine),  is  amido-acetic  acid,  CHo(NHo)COOH.  It  does  not 
exist  in  the  free  state  in  the  body,  but  in  union  with 
benzoic  acid,  to  form  hippuric  acid  (p.  298),  and  with 
cholalic  acid  to  form  the  glycocholic  acid  of  bile  (p.  222). 
It  is  very  soluble  in  water,  the  solutions  having  an  acid 
reaction  but  sweet  taste,  and  it  crystallizes  readily. 

^•arcos/» e',  CgH^NOo, is  methyl-glycine, CH2NH(CH3)COOH. 
Chemically  it  closely  resembles  glycine,  and  though  not 
found  in  the  body  is  an  interesting  substance,  owing  to 
its  chemical  relationship  to  creatine  and  its  discussional 
relationship  to  the  question  of  how  urea  is  formed  in  the 
body  (p.  295). 

Taurine,  C2H7NSO3,  or  amido-isethionic  acid,  is  one  of 
the  constituents  of  the  bile  acid  of  carnivora,  viz.,  tauro- 
cholic  acid.     It  is  a  substance  with  a  neutral  reaction  and 


646     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

is  very  stable,  even  when  exposed  to  a  high  temperature 
and  boiling  dilute  acid  and  alkalis.  In  the  intestinal 
canal  taurine  in  some  animals,  as  man,  is  absorbed  and 
reappears  in  the  urine;  in  dogs  a  large  part  is  excreted 
unaltered ;  in  herbivora  part  is  excreted  and  part  oxidized, 
leading  to  an  increase  of  sulphates  in  the  urine.  It  is 
found  in  small  amounts  in  horseflesh. 

Creatine,  C^H^NgO^,. — This  is  the  chief  and  characteristic 
'  extractive '  of  muscle-substance,  in  which  it  is  present  to 
the  extent  of  0-2  to  0*3  per  cent.  It  is  hence  present  in 
large  amount  in  'meat-extracts,'  from  which  it  may  there- 
fore most  conveniently  be  prepared,  and  is  easily  obtain- 
able, since  it  crystallizes  readily.  When  boiled  with 
baryta-water  it  takes  up  a  molecule  of  water  and  splits 
into  sarcosine  and  urea  (p.  295).  When  heated  with 
mineral  acids  it  loses  a  molecule  of  water,  and  is  thereby 
converted  into 

Creatinine,  C4H^N30. — It  is  present  in  urine  as  a  con- 
stant and  characteristic  constituent,  varying  greatly  in 
amount,  according  to  the  amount  of  proteid  in  the  food. 

Lysatine  (or  lijsatinlne),  C^HiyNgO.,,  or  CgHjiNgO,  is  a 
homologue  of  either  creatine  or  creatinine.  It  is  an  interest- 
ing substance  because  it  is  obtained  among  the  products 
of  decomposition  of  proteids  by  means  of  boiling  hydro- 
chloric acid  and  zinc  chloride,  and  readily  yields  urea 
when  it  is  itself  heated  with  baryta-water.  In  this  way 
the  long-sought-for  production  of  urea  from  proteids  hy 
purely  chemical  means  became  for  the  first  time  an  accom- 
plished fact. 

Leucine,  GJl^^Oo,  or  amido-caproic  acid,  is  a  charac- 
teristic product  of  the  pancreatic  digestion  of  proteids,  and 
is  physiologically  interesting,  as  a  probable  step,  by  the 
changes  it  undergoes  in  the  liver,  in  the  formation  of  urea 
in  the  body  (pp.  231,  293).  It  may  also  be  obtained  in 
large  quantities  by  boiling  horn  shavings  with  sulphuric 
acid.  It  crystallizes  readily,  and  in  forms  so  easily  re- 
cognizable and  so  characteristic  that  they  afford  an  in- 
fallil)le  means  of  determining  the  presence  of  leucine  in 
the  minutest  quantities  (Fig.  160). 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY   647 

Aspartic  Acid,  C^H7N04,  or  amido- succinic  acid,  may  be 
obtained  by  the  decomposition  of  proteids  during  pancreatic 
digestion,  or  their  hydrolysis  with  acids.  It  is  also  found 
in  plants,  but  forms  no  part  of  the  animal  body.  Closely 
related  to  this  acid  is  Asparagine,  C^HgNoOg,  which  is 
principally  of  interest  in  the  proteid  metabolism  of  plants, 
though  it  does  not  occur  in  animals.  When  taken  into  the 
body  of  the  carnivora,  asparagine  is  wholly  converted  into 
urea ;  with  herbivora  it  would  appear  that  a  part  of  the 
nitrogen  of  the  asparagine  can  take  the  place  of  proteid 
and  be  stored  up.  Considering  the  frequency  with  which 
asparagine  exists  in  plants,  the  conversion  of  asparagine 
into  proteid  is  a  valuable  provision. 


®.^  ? 


Fig.  160. — Leucine  Crystals  (Krukenberg). 

The  Urea  and  the  Uric  Acid  Group. —  Urea,  or  carhamide, 
is  the  end  product  of  proteid  decomposition,  and  the  chief 
nitrogenous  constituent  of  the  urine.  It  has  the  formula 
(NH2).3CO,  and  is  found  in  minute  quantities  in  some  of 
the  tissues  of  the  body,  though  it  is  never  found  in  muscle. 
In  a  pure  state  it  crystallizes  in  long  needles,  but  in  the 
form  of  nitrate  it  separates  out  as  six-sided  tables  arranged 
in  piles  (Fig.  68,  p.  "294),  and  as  oxalate  in  crystals  re- 
sembling the  nitrate,  but  of  prismatic  form.  Urea  is  very 
soluble  in  water,  soluble  in  alcohol,  but  insoluble  in  ether. 
The  crystals  have  a  bitter  taste  somewhat  resembling  salt- 
petre. It  may  be  easily  obtained  in  quantity  by  con- 
centrating urine  to  a  syrupy  state  and  extracting  this  with 
alcohol.  The  alcoholic  extract  then  yields  urea  by  slow 
crystallization. 

Two  modes  of  the  artificial  preparation  of  urea  outside 


648     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

the  body  are  peculiarly  interesting.  When  drji  ammonia 
and  carbon  dioxide  are  brought  together  they  form  car- 
bamic  acid,  which  at  once  unites  with  ammonia  to  form 
ammonium  carbamate,  2NH3  +  COo=NH4NH2C02.  Simple 
dehydration  converts  this  at  once  into  urea;  hence  the 
name  carbamide,  as  being  an  amide  of  carbonic  acid. 
Ammonium  carbamate  readily  takes  up  one  molecule  of 
water  to  become  ammonium  carbonate.  Urea  may  simi- 
larly be  converted  into  ammonium  carbonate  by  the 
assumption  of  two  molecules  of  water,  a  change  quickly 
completed  by  heating  it  in  sealed  tubes.  The  above  purely 
chemical  facts  are  important  to  the  question  of  how  urea 
is  formed  in  the  body  (see  p.  293). 

The  second  interesting  synthesis  of  urea  is  by  the  action 
of  ammonium  sulphate  on  potassium  cyanate ;  this  yields 
ammonium  cyanate,  NH^.CNO,  which  by  mere  evapora- 
tion to  dryness  is  molecularly  rearranged  into  urea, 
NH2.CO.NH2.  The  interest  which  attaches  to  this  is 
that  it  was  the  first  instance  (in  1828)  of  the  preparation 
by  purely  artificial  means  of  a  substance  till  then  known 
only  as  a  product  of  the  living  animal  body. 

When  urine  is  allowed  to  stand  it  rapidly  becomes  highly 
alkaline,  due  to  the  conversion  of  the  urea  it  contains  into 
ammonia  and  carbonic  acid  under  the  influence  of  organisms 
such  as  the  Micrococcus  urets. 

When  urea  is  heated  in  a  dry  state  for  some  time  to 
150°  C.  it  gives  ofjf  ammonia,  and  is  largely  converted  into 
biuret.  This  substance  yields  a  bright  pink  colour  by  the 
addition  of  sulphate  of  copper  and  caustic  soda  to  its  solu- 
tions. Since  peptones  (and  some  albumoses)  yield  a  similar 
pink  colour  with  the  same  reagents,  it  has  come  to  be 
spoken  of  as  the  '  biuret  reaction.' 

Uric  Acid  has  the  formula  C5H4N4O3.  It  is  the  chief 
nitrogenous  constituent  of  the  urine  of  birds  and  reptiles, 
but  only  occurs  in  small  quantities  in  the  urine  of  the  dog, 
and  is  absent  from  that  of  the  herbivora.  It  is  a  crystal- 
line substance  (Fig.  68,  p.  297),  odourless,  tasteless,  and 
extremely  insoluble  in  water,  very  slightly  soluble  in  ether 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY  649 

and  alcohol,  but  readily  soluble  in  caustic  potash.  Uric 
acid  does  not  occur  free  in  the  urine,  but  as  urates  com- 
bined with  bases.  Apart  from  its  characteristic  crystalline 
appearance,  uric  acid  is  readily  recognizable  by  evaporating 
a  fragment  of  the  suspected  substance  carefully  to  dryness 
on  a  piece  of  white  porcelain  with  a  few  drops  of  strong 
nitric  acid.  If  the  substance  is  uric  acid,  the  residue  thus 
obtained  may  be  yellow,  but  is  frequently  pink,  and  is 
certain  to  turn  to  a  bright  reddish  purple  under  the 
influence  of  the  fumes  of  ammonia.  This  is  the  well- 
known  murexid  test  for  uric  acid,  the  colour  being  due  to 
ammonium  purpurate. 

There  is  a  very  close  chemical  relationship  between  urea 
and  uric  acid,  but  there  is  nothing  to  account  for  the  fact 
that  snakes  and  birds  turn  out  the  nitrogenous  end-products 
of  their  metabolism  as  uric  acid,  while  mammals  get  rid 
of  it  as  urea.  The  chemical  relationship  of  uric  acid  to 
urea  is  at  once  apparent  on  mere  inspection  of  the  consti- 
tutional formula  of  the  acid — 

NH— CO 

CO      C— NH. 

I       I!  >co 

NH— C— NH/ 

the  groups  on  the  right  and  left  of  the  formula  containing 
the  obvious  potentiality  of  becoming  urea.  In  accordance 
with  this  we  find  that  in  nearly  every  possible  decomposi- 
tion of  uric  acid,  whatever  other  substances  are  obtained, 
urea  is  constantly  present  among  them. 

We  have  previously  dealt  with  the  probable  mode  of 
origin  of  uric  acid  in  the  body,  on  p.  296,  and  have  there 
also  indicated  its  relations  to  the  xanthine  bodies  as  allied 
members  of  the  purin  group. 

Allantoin,  C4H^N403,  is  a  substance  found  in  the  allan- 
toic fluid,  especially  that  of  the  calf,  and  in  foetal  urine  and 
amniotic  fluid.  It  can  be  obtained  from  urine  after  the 
administration  of  uric  acid,  and  from  uric  acid  by  oxidation 
with  potassium  permanganate. 


650     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  Aromatic  Series. — Many  members  of  this  series  occur 
in  the  urine  and  some  in  the  digestive  canal. 

Benzoic  Acid,  C-H^Oo,  is  found  principally  in  the  urine 
of  herbivora,  and  more  commonly  in  stale  than  in  the  fresh 
secretion.  In  stale  urine  it  is  derived  from  the  decom- 
position of  hippuric  acid.  This  acid  does  not  exist  free  in 
the  urine,  but  is  combined  with  alkalis.  It  may  be  ob- 
tained as  fine  glistening  needles  which  give  microscopically 
the  appearance  presented  in  Fig.  71,  p.  300. 

This  acid  is  not  very  soluble  in  water,  but  readily  dis- 
solves in  alcohol  and  ether ;  on  heating  it  sublimes,  in 
which  respect  it  difiers  considerably  from  hippuric  acid. 
The  source  of  benzoic  acid  in  the  body  is  discussed  on 
p.  298. 

Hippuric  Acid,  C9H9NO3.  —  This  acid  exists  largely  in 
the  urine  of  the  herbivora ;  it  is  formed  within  the  body  by 
the  union  of  benzoic  acid  with  glycine,  and  may  readily  be 
found  in  fresh  urine,  though  when  decomposition  occurs  it 
breaks  up  into  its  constituents. 

Hippuric  acid  is  found  in  the  urine  united  to  an  alkali, 
but  may  be  obtained  as  a  crystalline  substance  (Figs.  69, 
70,  p.  299).  The  acid  is  not  very  soluble  in  water,  but  is 
readily  dissolved  by  alcohol ;  it  is  insoluble  in  petroleum 
ether,  a  fluid  in  which  benzoic  acid  is  soluble.  When 
heated  dry  in  a  small  tube  it  yields  a  characteristic  odour 
of  new  hay.  The  source  of  this  acid  in  the  body  is  dis- 
cussed on  p.  298. 

Tyrosine,  CciH^iNOg. — This  is  found  in  many  plants,  and 
also  in  the  intestinal  canal  as  the  result  of  the  pancreatic 
digestion  of  proteids.  It  is,  in  fact,  the  close  companion 
of  leucine  in  nearly  all  the  decompositions  of  proteids 
and  other  substances.  In  some  ways  it  is  less  interesting 
physiologically  than  is  leucine,  since  there  is  no  evidence 
that  it  is  in  any  way  a  forerunner  of  urea  in  the  body,  as  is 
so  often  said  to  be  the  case.  On  the  other  hand,  it  is  of 
great  interest  as  indicating  the  presence  or  absence  of 
aromatic  groups  in  substances  which  do  or  do  not  yield 
tyrosine  by  hydrolysis.     Thus  gelatin  yields  no  tyrosine  by 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY  651 

pancreatic  digestion  ;  it  lacks  therefore  the  aromatic  group 
ill  its  molecule,  and  consequently,  unlike  its  allies  the 
true  proteids,  gives  no  reaction  with  Millon's  reagent  (see 
p.  641). 

Tyrosine  crystallizes  in  fine  needles  which  are  sparingly 
soluble  in  water,  insoluble  in  alcohol,  but  soluble  in  acids 
and  alkalis.  Tyrosine  yields  a  very  brilliant  reddish  pink 
colour  when  heated  with  Millon's  reagent,  if  present  even 
in  minute  traces,  so  that  its  identification  is  easy. 

PJu'iiol  and  Crcsol  are  formed  in  the  animal  body  during 
the  putrefactive  decomposition  of  proteids,  and  are  excreted 
by  the  bowels  and  urine,  in  the  latter  being  found  as  an 
ethereal  salt  of  sulphuric  acid.  This  phenyl-sulphuric  acid 
is  also  formed  from  the  aromatic  compounds  in  the  food, 
especially  that  taken  by  the  herbivora  (p.  301). 

Pyrocatechin  is  found  largely  in  the  urine  of  the  horse 
and  other  herbivora,  and  also  after  the  administration  of 
benzene  or  phenol.  The  dark  colour  of  urine  on  standing, 
such  as  is  well  seen  in  the  horse,  is  due  to  the  oxidation  of 
pyrocatechin.  The  source  of  this  substance  is  from  the 
phenol  of  the  intestinal  canal,  and  it  may  probably  be 
introduced  with  certain  constituents  of  the  food  (p.  301). 

Indigo  Series. — This  contains  several  substances  found  in 
the  urine  and  digestive  canal. 

Indol  is  the  substance  which  gives  the  odour  to  fseces. 
It  is  present  during  the  decomposition  of  proteids,  and  may 
be  readily  obtained  from  an  artificial  jJidref active  pancreatic 
digestion,  the  odour  of  which  is  due  to  this  substance. 

Part  of  the  indol  leaves  the  body  by  the  urine  as  a 
potassium  salt  of  indoxyl-sulphuric  acid,  and  if  this  be 
oxidized  it  may  be  made  to  jield  indigo  blue;  if  indigo 
blue  be  acted  upon  by  powerful  reducing  agents  it  yields 
indol. 

Indol  administered  to  animals  increases  the  output  of 
indican,  and  whatever  increases  intestinal  putrefaction 
increases  the  output  of  this  substance;  this  is  the  reason 
why  it  is  found  more  largely  in  herbivora  than  in  carni- 
vora  (p.  300). 


652    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

The  presence  of  indican  in  the  urine  of  the  horse  can 
readily  be  demonstrated  by  mixing  the  urine  with  an  equal 
volume  of  hydrochloric  acid,  and  adding  a  solution  of  hypo- 
chlorite of  calcium  until  a  blue  colour  appears. 

Skatol  is  a  substance  closely  allied  to  indol ;  it  has  much 
the  same  odour,  and  if  excreted  with  the  urine  it  passes  off 
as  a  potassium  salt  of  skatoxyl-suiphuric  acid. 

The  Bile  Acids. — These  have  been  sufficiently  dealt  with 
on  p.  222. 

THE  NON-NITROGENOUS  BODIES. 

Fats  and  Fatty  Acids. — The  fats  met  with  in  the  animal 
body  are  compounds  formed  by  substituting  the  radicles 
of  certain  acids  of  the  acetic  and  acrylic  series  for  the 
hydroxyls — OH — in  the  triatomic  alcohol  glycerin.  The 
acids  in  question  are  the  sixteenth  and  eighteenth  in  the 
acetic  series  —  namely,  palmitic  and  stearic  —  and  the 
eighteenth  of  the  acrylic  series — oleic  acid.  The  fats  thus 
formed  are  therefore  known  as  palmitin,  stearin,  and  olein, 
and  the  mode  of  their  formation  is  at  once  made  clear  by 
the  following  typical  equation  : 

Palmitic  Acid.        Glycerin.  Palmitin. 

3(Ci,H3i.COOH)+C3H,(OH)3  =  C3H.(Ci5H3i.CO.O)3  +  3H20. 

A  certain  proportion  of  the  fats  in  milk,  and  hence  in 
butter,  are  formed,  as  above,  from  acids  lower  down  in  the 
acetic  series,  such  as  caproic,  caprylic,  and  capric  acid. 

Fat  is  insoluble  in  water  and  only  slightly  so  in  alcohol, 
but  freely  soluble  in  ether,  chloroform,  and  benzene.  When 
pure  it  is  neutral  in  reaction,  tasteless  and  colourless,  and 
by  the  action  of  caustic  alkalis  or  superheated  steam  may 
be  decomposed  into  its  respective  fatty  acid  and  glycerin, 
the  process  being  simply  a  reversal  of  the  equation  given 
above  for  the  formation  of  a  fat.  When  this  splitting  is 
brought  about  by  an  alkali  the  base,  sodium  or  potassium, 
at  once  unites  with  the  free  fatty  acid,  and  forms  a  salt 
which  is  what  is  known  as  soap.  This  decomposition  and 
saponification  take   place  to  a   greater  or   less   extent  in 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY  653 

the  intestine  under  the  influence  of  the  pancreatic  juice 
and  bile. 

The  solid  fat  of  the  body  is  composed  principally  of 
stearin,  such  as  is  found  in  the  ox  and  sheep  ;  the  more 
liquid  fat,  such  as  is  found  in  the  horse  and  carnivora, 
contains  more  palmitin,  but  in  all  cases  a  mixture  of  the 
three  fats  is  obtained.  Fat  as  it  exists  in  the  cells  of  the 
living  body  is,  of  course,  in  a  liquid  condition.  Since  the 
melting-point  of  palmitin  is  45°  C.  and  that  of  stearin 
55°  to  60°  C,  it  is  evident  that  the  fluidity  of  living  fat  is 
due  to  the  olein  it  contains,  whose  melting-point  is  —  5°  C. 

The  amount  of  fat  in  the  body  must  depend  upon  the 
feeding  of  the  animal,  and  will  obviously  vary  within 
extreme  limits.  In  individual  tissues  marrow  has  the 
largest  amount ;  nerve,  brain,  milk,  muscle,  liver,  bone, 
bile,  and  blood,  have  proportions  which  decrease  in  the 
order  given. 

The  change  which  the  fats  undergo  in  the  alimentary 
canal  is  discussed  in  the  chapter  on  the  Pancreas  (p.  236), 
whilst  the  origin  of  fat  in  the  body  and  its  function  is  dealt 
with  under  Nutrition  (p.  324). 

Butyric  Acid  is  found  in  the  intestines,  and  in  milk,  it 
exists  in  union  with  glycerin  as  a  neutral  fat,  and  on  the 
decomposition  of  this  fat  gives  the  odour  to  rancid  butter. 
It  may  also  be  produced  by  the  second  stage  of  lactic  fer- 
mentation in  the  stomach  and  alimentary  canal,  being 
derived  from  the  carbohydrate  matter  ingested. 

Glycerin,  which  since  it  is  an  alcohol  should  really  be 
known  as  glycerol,  is  a  viscid,  colourless,  sweet  fluid,  soluble 
in  all  proportions  in  water  and  alcohol,  but  insoluble  in 
ether.  When  heated  strongly  it  yields  acrolein,  a  substance 
which  gives  the  pungent  odour  to  burned  fat. 

Lactic  Acid  exists  in  two  forms  in  the  body :  ethylidene- 
lactic  acid  is  the  chief  product  of  the  lactic  fermentation 
of  sugars,  and  is  found  in  the  stomach  and  intestines 
especially  after  a  diet  containing  carbohydrate  ;  sarco-lactic 
acid  occurs  in  muscles,  and  is  the  cause  of  their  acidity 
after  activity. 


654     A  MANUAL  OF  VETEKINAKY  PHYSIOLOGY 

Cholesterin  is  a  peculiar  substance  extracted  originally 
from  gall-stones.  It  can  be  obtained  in  sparkling  crystals 
which  are  soapy  to  the  touch,  and  of  characteristic  micro- 
scopical shape.  Cholesterin  is  the  only  alcohol  which 
occurs  free  in  the  body ;  it  is  not  a  fat,  though,  as  a  matter 
of  convenience,  it  is  generally  dealt  with  in  speaking  of 
fats.  Being  an  alcohol  it  should  be  called  cholesterol. 
It  is  quite  insoluble  in  water  and  cold  alcohol,  but  readily 
soluble  in  solutions  of  bile  salts,  in  ether  and  in  chloroform. 
If  an  equal  volume  of  strong  sulphuric  acid  is  added  to  a 
solution  of  cholesterin  in  chloroform  the  latter  becomes  at 
first  blood-red,  and  then  passes  through  blue  and  green  to 
become  finally  yellow.  This  play  of  colours  is  very  similar 
to  that  observed  on  the  addition  of  nitrous  acid  to  bile 
pigments  (p.  220).  If  solid  cholesterin  be  treated  with 
strong  sulphuric  acid  it  turns  red  or  violet,  the  colour 
changing  additionally  to  blue  or  green  on  the  addition  of 
dilute  solution  of  iodine.  Cholesterin  is  thus  a  substance 
easily  recognizable  when  present  in  even  minute  amounts. 

Cholesterin  is  found  in  the  nervous  system,  and  is 
especially  common  in  the  pia  mater  of  the  cerebellum  and 
plexus  choroidea  of  the  horse,  where  it  may  give  rise  to 
tumours,  the  nature  of  the  growth  being  readily  recognized 
from  its  silvery  fish-scale-like  appearance.  It  is  also  found 
in  lanoline  or  wool  fat  and  in  dandruff,  where  it  replaces 
the  glycerin  in  the  fat. 

Carbohydrates. — This  important  class  is  of  the  greatest 
interest  to  the  physiologist,  inasmuch  as  the  bulk  of 
material  consumed  as  food,  especially  in  the  herbivora, 
consists  of  carbohydrate  matter.  It  is  an  extensive  group 
of  bodies  consisting  of  such  substances  as  starch  and  its 
derivatives,  the  various  forms  of  sugar,  and  cellulose. 
Though  so  much  carbohydrate  material  enters  the  body, 
but  little  can  be  found  in  the  tissues.  An  animal  starch 
(glycogen)  may  be  found  in  the  liver  and  other  organs, 
minute  amounts  of  sugar  may  be  found  in  the  blood,  and 
a  sugar  exists  in  milk ;  but  very  much  less  carbohydrate 
is  recoverable  from  the  body  than  enters  it  as  food,  for  the 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY  655 

reason  that  the  bulk  of  it  becomes  converted  into  fat 
(p.  325)  or  is  rapidly  oxidized  to  carbonic  acid  and  water 
as  a  source  of  heat  and  energy  to  the  body  (p.  323). 

The  carbohydrates  may  be  divided  into  the  starch  group 
or  polysaccharides,  the  cane-sugar  group  or  disaccharides, 
and  the  dextrose  group  or  monosaccharides. 

Polysaccharides,  (CeHiQOj)^. 

Starch. — The  formula  for  starch  is  unknown ;  it  is  con- 
sidered to  be  (CjjHjjjOj)^,  where  n  is  not  less  than  5  or  6, 
and  is  probably  very  much  larger. 

Starch  exists  in  plants  in  the  form  of  grains,  the  shape 
of  which  depends  upon  the  group  from  which  it  is  derived  ; 
thus  potato,  bean,  wheat,  and  other  starch  grains  have 
each  a  distinctive  shape.  The  grain  is  composed  of  two 
parts,  an  envelope  known  as  cellulose,  and  an  interior 
called  granulose.  The  granulose  is  the  true  starch  ;  the 
cellulose  is  not,  however,  identical  with  the  ordinary 
cellulose  of  plants. 

Starch  is  insoluble  in  cold  water,  but  when  boiled  the 
grains  burst,  and  a  viscid,  opaque,  pasty  mass  results 
which  is  not,  however,  a  true  solution  of  starch.  A  solution 
of  starch  can  be  obtained  from  this  mass  by  careful  and 
limited  digestion  with  an  enzyme,  such,  for  instance,  as 
human  saliva,  or  by  the  action  of  dilute  acid ;  when  this 
takes  place  the  material  becomes  watery,  perfectly  trans- 
parent, and  filters  readily,  while  previously  this  was  impos- 
sible. To  this  limpid  fluid  the  term  soluble  starch  has 
been  given. 

The  characteristic  test  for  starch  is  the  blue  colour 
produced  on  the  addition  of  iodine.  Starch  has  no  reducing 
action  on  Fehling's  solution. 

Dextrin. — When  starch  paste  is  acted  upon  by  dilute 
mineral  acid,  or  the  enzymes  found  in  the  saliva  and 
pancreatic  juice,  soluble  starch  is  first  formed  as  above 
described  ;  but  if  the  process  be  allowed  to  continue,  further 
changes  rapidly  occur,  leading  to  the  production  of  dextrin 


656     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

and  finally  of  sugar.  There  are  probably  several  dextrins, 
though  two  are  generally  more  particularly  described,  viz., 
erythro-dextrin  and  achroo- dextrin.  These  are  distin- 
guished from  starch  and  from  each  other  by  their  colour 
reactions  with  iodine,  erythro-dextrin  giving  a  reddish 
colour,  while  achroo-dextrin  gives  no  colour.  Much  the 
same  change  which  can  thus  be  brought  about  by  acting 
upon  starch  out  of  the  body,  takes  place  in  a  more  perfect 
and  complete  form  within  the  body. 

The  conversion  of  starch  into  dextrin  and  finally  into 
sugar  under  the  influence  of  certain  enzymes,  performs 
a  most  important  physiological  function ;  neither  starch 
nor  dextrin  is  capable  of  being  absorbed  as  such,  whereas 
the  sugar  which  results  from  this  conversion  is  readily 
assimilable. 

Glycogen  closely  resembles  starch ;  it  is  found  in  several 
of  the  tissues  of  the  body,  and  its  origin  and  use  in  the 
economy  have  been  previously  discussed  (see  p.  225).  It 
may  be  obtained  as  an  amorphous  white  powder,  readily 
soluble  in  water,  and  gives  with  iodine  a  port-wine  colour 
instead  of  blue.  By  the  action  of  acids  or  enzymes  it  is 
readily  converted  into  dextrin,  and  finally  into  sugar.  The 
sugar  resulting  from  the  action  of  acid  is  dextrose,  whereas 
that  produced  by  the  enzyme  is  maltose ;  in  the  liver  the 
sugar  produced  is  dextrose  and  not  maltose,  and  the  method 
by  which  this  conversion  is  obtained  has  been  previously 
dealt  with  (p.  229). 

Cellulose,  though  not  found  in  the  animal  body,  is  of 
great  interest  to  the  physiologist  from  its  intimate  relation 
to  the  feeding  of  the  herbivora.  The  food  substance  in 
plants  is  locked  up  in  a  cellulose  envelope,  and  until  this 
envelope  is  broken  down  the  material  within  cannot  be 
acted  upon  by  the  digestive  juices.  This  breaking  down 
is  accomplished  by  laceration  during  the  process  of  masti- 
cation, but  also  by  a  subsequent  digestion  of  the  covering, 
by  which  means  it  is  removed  and  the  food  substance 
exposed. 

The  digestion  of  cellulose  is  a  question  which  has  given 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY  657 

rise  to  great  discussion,  inasmuch  as  no  animal  is  known 
to  secrete  a  cellulose  enzyme,  although  many,  such  as  the 
herbivora,  are  known  to  digest  cellulose.  Bunge  states 
that  sheep  are  capable  of  digesting  30  to  40  per  cent,  of 
the  cellulose  of  sawdust  and  paper  when  mixed  with  hay. 

The  two  views  most  generally  held  at  the  present  time 
with  reference  to  the  digestion  of  cellulose  are  that  it  is 
either  due  to  putrefactive  organisms  or  to  a  specific 
enzyme. 

Cellulose  may  be  digested  outside  the  body  under  the 
influence  of  putrefactive  organisms,  with  the  evolution  of 
marsh  gas  and  carbonic  acid.  Every  condition  necessary 
for  this  change  exists  within  the  body  in  most  efficient 
form ;  for  example,  in  the  rumen  of  the  ox,  and  the  large 
intestines  of  the  horse ;  but  it  would  appear  to  be  more 
than  probable  that  a  cellulose-dissolving  enzyme  exists. 
Young  cellulose  is  more  easily  digested  than  old ;  it  is 
certain  that  the  older  parts  of  the  plant  are  converted  into 
lignin,  and  this  to  the  majority  of  animals  must  be  in- 
soluble. 

Cellulose  when  treated  with  strong  sulphuric  acid  is 
converted  into  a  dextrin-like  product,  and  is  finally 
converted  into  dextrose. 

Disaccharides,  Ci^HooOji. 

Saccharose,  or  cane-sugar,  is  not  found  as  part  of  the 
animal  body,  but  exists  largely  in  plants,  and  forms  a  well- 
known  supply  of  carbohydrate  to  the  system.  Cane-sugar 
does  not  give  some  of  the  characteristic  sugar  reactions, 
among  others  it  has  no  reducing  action  upon  salts  of 
copper,  but  by  boiling  with  dilute  mineral  acids  it  is 
converted  into  equal  parts  of  dextrose  and  levulose,  and  the 
same  change  may  be  efl:ected  by  enzymes  in  the  stomach 
and  small  intestines.  This  conversion  of  cane-sugar  is 
recognised  by  the  changed  action  of  the  solution  on 
polarized  light,  the  rotation  of  the  plane  of  polarization 
being  now  left-handed  instead  of  right-handed  as  it  was 

42 


658     A  MANUAL  OF  VETERINAKY  PHYSIOLOGY 

previously  to  the  conversion,  that  is  to  say  inverted ;  hence 
the  name  invert  sugar. 

If  cane-sugar  he  injected  into  the  circulation  it  passes 
out  unaltered ;  it  is  certain  that  hefore  this  sugar  can  he 
assimilated  it  must  be  converted  into  dextrose. 

Maltose  is  formed  by  the  action  of  malt  extract  (diastase) 
on  starch  paste,  also  by  the  action  of  saliva  and  pancreatic 
juice  upon  starch  paste  or  glycogen.  In  its  reactions  it 
corresponds  closely  to  dextrose,  but  it  has  a  one-third  less 
reducing  action  upon  Fehling's  solution,  and  it  does  not 
reduce  Barfoed's  reagent,*  which  dextrose  is  capable  of 
doing.  Its  specific  activity  in  rotating  the  plane  of 
polarized  light  is  considerably  greater  than  that  of  dex- 
trose, being  about  -»- 140°  as  against  +52°  for  dextrose. 
If  5  c.c.  of  a  i  per  cent,  solution  of  maltose  is  warmed  for 
half  an  hour  on  a  water-bath  together  with  1  decigramme 
of  phenyl-hydrazine  hydrochloride  and  2  decigrammes  of 
sodium  acetate,  a  yellow  compound  is  obtained  in  charac- 
teristically shaped  crystals.  These  are  phenyl-maltosazone, 
C.^Hg.N^O,,.  When  heated  the  crystals  melt  at  206°  C, 
and  this,  together  with  the  shape  of  the  crystals  and  their 
specific  solubility  in  75  parts  of  boiling  water,  renders  the 
identification  of  maltose  easy. 

Maltose  is,  like  cane-sugar,  non-assimilable,  for  if 
injected  into  the  circulation  it  is  excreted  unchanged, 
and  it  is  probable  that  before  absorption  it  has  to  be 
converted  into  dextrose. 

Lactose,  or  milk-sugar,  is  found  solely  in  milk.  It 
reduces  Fehling's  solution,  and  has  the  same  rotatory 
power  as  dextrose,  but  it  does  not  reduce  Barfoed's  reagent, 
nor  does  it  undergo  direct  alcoholic  fermentation  with 
yeast.  If  boiled  with  dilute  mineral  acids  it  is  converted 
into  equal  parts  of  dextrose  and  galactose. 

Lactose  readily  undergoes  lactic  fermentation,  as,  for 
instance,  in  souring  milk.  The  cause  of  this  is  a  micro- 
organism ;  but  there  are  reasons  for  believing  that  an 
enzyme  may  also  bring  it  about. 

*  A  solution  of  cupric  acetate  to  which  acetic  acid  is  added. 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY  659 

In  spite  of  the  fact  noted  above,  that  isolated  lactose  is 
unable  to  ferment  in  the  presence  of  yeast,  yet  an  alcoholic 
fermentation  is  capable  of  occurring  in  milk,  such,  for 
instance,  as  in  the  koumiss  from  mare's  milk,  and  kephir 
from  cow's  milk.  It  is  probable  that  the  changes  which 
bring  this  about  are  very  complex,  and  due  to  several 
organisms. 

Lactose,  like  saccharose  and  maltose,  is  non-assimilable 
as  such,  and  it  is  probable  that  it  is  changed  into  dextrose 
before  absorption,  not  necessarily  as  the  result  of  the 
action  of  any  digestive  secretion,  but  during  its  passage 
through  the  intestinal  wall. 

Like  maltose,  lactose  yields  an  osazone,  phenyl-lacto- 
sazone,  which  crystallizes  in  characteristic  rounded  clumps 
of  yellow  crystals.  These  crystals  melt  at  200°  C,  and  are 
soluble  in  80  to  90  parts  of  boiling  water. 

Monosaccharides,  CgHi^O^. 

When  the  members  of  the  preceding  group  of  sugars, 
the  disaccharides,  are  boiled  with  dilute  acids  or  otherwise 
hydrolyzed,  they  take  up  a  molecule  of  water  and  split  into 
two  molecules  of  a  new  sugar.  Thus  cane-sugar  yields 
dextrose  and  levalose,  maltose  gives  two  molecules  of 
dextrose,  and  lactose  yields  dextrose  and  galactose.  Of 
these  the  most  important  is  : 

Dextrose,  Glucose,  or  Grape  Sugar. — This  is  probably  the 
form  to  which  all  sugars  must  be  reduced  in  the  alimentary 
canal,  whether  before  or  during  absorption,  in  order  that 
they  may  be  assimilable  by  the  tissues. 

In  its  ordinary  reactions  dextrose  resembles  maltose,  but 
may  be  easily  distinguished  from  it  by  the  following  differ- 
ences in  behaviour. 

Its  specific  rotatory  power  is  only  +52°.  It  reduces 
Barfoed's  reagent  (see  Maltose).  The  osazone  it  forms, 
phenyl-glucosazone,  crystallizes  in  fine  yellow  needles ; 
these  melt  at  205°  C.  and,  unlike  the  corresponding  com- 
pound of  maltose,  are  almost  insoluble  in  water. 

42—2 


600     A  MANUAL  OF  VETERINAllY  PHYSIOLOGY 

Dextrose  is  capable  of  undergoing  three  fermentations, 
viz.,  alcoholic,  lactic,  and  butyric ;  the  two  latter  are 
probably  always  present  in  the  intestinal  canals  of  animals, 
especially  after  a  carbohydrate  diet. 

Leridose. — This  occurs  in  fruits  and  honey,  mixed  with 
glucose  ;  it  may  also  be  prepared  by  acting  upon  cane- 
sugar  with  sulphuric  acid,  by  which  means  the  cane-sugar 
is  converted  into  equal  parts  of  dextrose  and  levulose. 

Inosite,  CoHioO,.  (CH.OH)^. 

This  is  a  crystallizable  substance,  found  among  the 
'  extractives '  of  many  tissues,  usually  in  very  minute 
quantities,  though  it  is  markedly  present  in  heart-muscle 
and  in  horse-flesh,  which  may  contain  as  much  as  '003  per 
cent.  It  occurs  also  in  semen.  Inosite  is  found  abundantly 
in  vegetable  tissues,  especially  in  unripe  beans,  which  thus 
provide  a  convenient  source  for  its  preparation.  Possessed 
of  a  sweet  taste,  and  as  being  originally  found  in  muscles, 
inosite  has  at  times  been  called  '  muscle-sugar.'  But 
although  its  empirical  formula  is  the  same  as  that  of  a 
monosaccharide,  it  is  not  a  sugar  at  all :  its  solutions  exert 
no  rotatory  power  on  polarized  light,  do  not  reduce  metallic 
salts,  and  form  no  osazone  with  phenyl-hydrazine,  nor  are 
they  capable  of  undergoing  alcoholic  fermentation.  It  is 
in  fact  a  member  of  the  benzene  series,  and  consists  of  a 
closed  ring  of  six  CH.OH  groups. 

The  sugars  of  chief  physiological  importance  are,  as  we 
have  seen,  the  hexoses,  that  is  to  say  a  sugar  such  as 
dextrose  which  contains  six  atoms  of  carbon  in  the  molecule, 
or  the  disaccharides  which  contain  twelve.  But  the  recent 
progress  of  organic  chemistry  has  led  to  the  synthesis  not 
only  of  the  sugars  which  are  ordinarily  met  with,  but  of  a 
series  of  artificial  sugars  containing  three  (trioses),  four, 
five  (pentoses),  seven,  eight,  and  nine  atoms  of  carbon  in 
their  molecule.  Of  these  the  pentoses  alone  at  present 
possess  any  physiological  interest.     This  is  due  to  the  fact 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY  661 

that  a  pentose  may  be  obtained  by  the  decomposition  of 
the  nucleo-proteids  of  the  pancreas  and  of  yeast-cells. 
These  pentoses  are  not  assimilable,  as  shown  by  their 
rapid  appearance  in  the  urine  after  their  introduction 
into  the  body.  Pentose  yields  an  osazone  which  melts 
at  160°  C. 

Tests  for  Sugar. 

1.  Trommer's. — An  excess  of  caustic  potash  and  a  small 
amount  of  dilute  solution  of  copper  sulphate  is  added  to 
the  fluid  and  the  whole  heated.  The  copper  is  reduced  to 
suboxide  by  the  sugar  and  a  red  precipitate  falls.  Fehling's 
solution,  which  is  used  as  a  quantitative  test  for  sugar, 
consists  of  hydrated  cupric  oxide  in  caustic  soda,  and  the 
double  tartrate  of  sodium  and  potassium.  This  is  made  to 
contain  such  an  amount  of  the  cupric  oxide  in  each  cubic 
centimetre  as  is  exactly  reduced,  and  the  blue  colour 
destroyed,  by  0"005  gramme  of  dextrose.  The  principle  of 
this  test  is  the  same,  viz.,  the  reducing  action  of  the  sugar, 
which  robs  the  cupric  compound  of  its  oxygen. 

2.  Moore's. — A  solution  of  sugar  boiled  with  caustic 
potash  turns  brown. 

3.  Bottcher's. — Bismuth  oxide  and  excess  of  caustic 
potash  are  added  to  the  fluid  containing  sugar  and  heated ; 
the  solution  becomes  grey  and  then  l)lack,  from  the  deposi- 
tion of  metallic  bismuth. 

4.  Picric  Acid  Test. — Boil  the  solution  of  sugar  with  a 
little  picric  acid  and  caustic  soda  in  small  quantities ;  a 
brown-red  opaque  coloration  is  obtained. 

5.  Fermentation  Test. — The  fluid  containing  a  piece  of 
yeast  is  placed  in  a  tube  and  inverted  over  mercury ;  if 
sugar  is  present  it  undergoes  fermentation,  and  carbonic 
acid  is  given  ofl',  which  collects  in  the  tube. 

The  osazone  tests  have  already  been  described  under  the 
respective  sugars.  They  are  very  important  for  the  dis- 
crimination of  the  various  sugars,  as  well  as  for  their 
identification. 


662     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Inorganic  Constituents. 

The  inorganic  substances  found  in  the  body  are 
water,  gases,  and  salts.  Water  forms  about  60  per  cent, 
of  the  whole  body ;  it  is  taken  in  with  the  food  and 
drink,  and  a  small  quantity  may  be  formed  within  the 
system. 

The  amount  of  water  consumed  by  animals  depends  upon 
the  nature  of  their  food  and  the  class  of  animal.  Horses 
fed  on  dry  food  consume  more  water  than  cattle,  the  food 
of  which  contains  as  a  rule  a  considerable  amount  of 
water. 

An  excess  of  water  leads  to  body  waste  by  carrying  off 
the  solids  through  the  kidneys,  whilst  reduction  in  the 
amount  of  water  produces  thirst  and  loss  of  nutrition. 

The  Gases  found  in  the  body  are  oxygen,  nitrogen, 
hydrogen,  carbonic  acid,  sulphuretted  hydrogen  and  marsh 
gas.  The  two  former  are  taken  in  with  the  inspired  air, 
carbonic  acid  is  formed  in  the  tissues,  while  hydrogen  and 
its  compounds  are  formed  in  the  intestinal  canal  as  the 
result  of  cellulose  and  other  decompositions. 

The  largest  portion  of  the  inorganic  matter  consists  of 
the  various  Salts  of  sodium,  potassium,  calcium,  magnesium, 
and  iron,  in  the  form  of  chlorides,  sulphates,  phosphates, 
and  carbonates.  The  distribution  of  these  salts  throughout 
the  tissues  is  very  variable  ;  some,  like  bone,  are  excessively 
rich,  whilst  others  are  remarkably  poor  in  them.  Certain 
tissues  and  fluids  have  a  preponderance  of  some  salts  to  the 
exclusion  of  others. 

The  amount  of  the  salts  existing  in  the  body  depends 
upon  the  age  of  the  animal,  and  their  nature  is  modified  by 
the  character  of  the  food.  The  daily  quantity  ingested  and 
stored  up  is  largely  affected  by  the  rate  of  growth,  young 
growing  animals  storing  up  material  which  the  adult 
rejects. 

The  diet  of  the  herbivora  furnishes  considerably  more 
potassium  than  sodium  salts  to  the  system,  with  the  result 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY   663 

that  in  the  excretions  salts  of  potassium  are  in  excess  over 
those  of  sodium. 

Sodii())i  and  Potassium.— Omng  to  the  poorness  of 
vegetable  food  in  sodium  salts,  Bunge  believes  that  the 
administration  of  common  salt  with  the  food  of  herbivora 
is  a  necessity.  As  this  view  is  open  to  question  his  argu- 
ments in  the  matter  should  be  known. 

Bunge  says  that  in  spite  of  the  many  inorganic  salts 
found  in  the  food,  one  only,  viz.,  sodium  chloride,  is  taken 
separately  by  the  human  subject  in  addition  to  that  already 
existing  in  the  food.  But  carnivora  avoid  salted  food,  as 
sufficient  sodium  chloride  exists  in  the  blood  and  tissues 
in  the  raw  state  in  which  these  are  consumed  by  them. 
Herbivora,  on  the  other  hand,  have  been  known  to  travel 
considerable  distances  to  obtain  salt. 

According  to  Bunge  the  explanation  of  the  desire  shown 
by  herbivora  for  common  salt  lies  in  the  large  amount  of 
potassium  consumed  in  their  diet,  the  effect  of  potassium 
salts  in  the  blood  being  to  withdraw  sodium  salts  from  the 
system. 

Here  are  some  tables  given  by  him  to  show  the  pro- 
portion potassium  bears  to  sodium  in  various  articles  of 
diet. 

In  every  1,000  parts  of  dried  material : 


Potassium. 

Sodium 

Rice 

1 

•03 

Bullock's  blood 

2 

19 

Oats 

Wheat 

Rye       "      

5  to  6 

•1  to  -4 

Barley  ^ 

Dog's  milk 

5  to  6 

2  to  8 

Peas 

12 

•2 

Milk  of  herbivora 

9  to  17 

1  tolO 

Hay 

6  to  18 

•3  to  1-5 

Beef 

19 

3 

Beans             

21 

•1 

Clover             

28 

•I 

664    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

For  one  equivalent  of  sodium  the  equivalents  of  potassium 
are : 

Equivalent  K.p. 

Mangel-wurzel     ...         ...         ...         ...  2 

Milk  of  herbivora            -8  to  6 

Beef           4 

Wheat       12  to  23 

Barley       14  to  21 

Oats          15  to  21 

Eice           24 

Rye           9  to  57 

Hay          3  to  57 

Peas          44  to  50 

Clover       90 

Beans       ...         ...         ...         ...         ...  110 

The  preponderance  of  potassium  over  sodium  salts  is  here 
most  marked,  and  Bunge  considers  that  when  a  relation  of 
from  4  to  6  equivalents  of  potassium  to  1  equivalent  of 
sodium  is  obtained  in  a  diet  no  addition  of  sodium  chloride 
is  necessary ;  but  where  the  proportion  of  potassium  is 
higher  than  this  the  animal  instinctively  seeks  for  sodium, 
for  the  reason  previously  given. 

We  do  not  deny  the  stimulant  to  the  palate  which 
common  salt  may  afford  the  herbivora,  but  so  far  as  horses 
are  concerned,  and  we  think  the  same  argument  must 
apply  to  cattle,  it  is  quite  certain  that  no  addition  of 
common  salt  to  the  ordinary  diet  is  necessary,  and  that  the 
food  furnishes  ample  sodium  for  the  purposes  of  the  body. 

Calcium  forms  the  largest  mineral  deposit  in  the  body  ; 
it  is  taken  in  by  means  of  the  food.  Bunge  states  that  it 
is  probable  that  the  lime  salts  required  for  the  growth  of 
bone  in  young  animals  are  contained  in  some  organic 
compound,  and  that  the  administration  of  inorganic  com- 
pounds of  lime  in  rickets  is  irrational  and  useless. 

Lime  exists  largely  in  clover  and  hay,  but  only  in  small 
quantities  in  the  cereal  grains  ;  it  is  principally  in  the  hay 
that  the  amount  excreted  by  horses  through  the  kidneys  is 
supplied.  In  the  urine  it  passes  from  the  body  in  such 
quantities  that  it  cannot  be  held  in  solution  by  the  alkaline 


THE  CHEMICAL  CONSTITUENTS  OF  THE  BODY  665 

fluid,  and  the  urine  of  the  horse  is  therefore  always  turbid. 
In  the  body  calcium  exists  in  the  form  of  phosphate, 
sulphate,  and  carbonate,  in  the  urine  principally  as 
carbonate  and  oxalate. 

Magnesium  salts  occur  in  the  body  prnicipally  as  phos- 
phates, and  in  this  form  they  enter  largely  into  certain 
foods,  such  as  oats.  The  amount  of  magnesium  passing 
away  from  horses  through  the  kidneys  is  small,  but  con- 
siderable quantities  derived  from  the  food  pass  out  with 
the  faeces,  as  they  cannot  be  utilized  in  the  body.  By 
collecting  in  the  bowels  this  salt  produces  the  ammonio- 
magnesium  phosphate  calculi  so  common  in  horses. 

Phosphates  are  united  with  soda,  potash,  lime,  and 
magnesia.  They  are  principally  taken  in  with  the  food? 
but  may  also  be  formed  in  the  body  from  the  metabolism 
of  phosphorus-containing  compounds.  The  foods  richest 
in  phosphoric  acid  are  oilcake  and  bran,  while  hay  and 
straw  are  poorest  in  this  substance.  Phosphoric  acid  is 
principally  excreted  by  herbivora  with  the  f?eces,  only  small 
quantities  passing  away  with  the  urine. 

Carbonates  are  found  in  several  of  the  secretions  of  the 
body,  notably  in  the  urine,  where  they  cause  the  most 
intense  evolution  of  gas  on  the  addition  of  an  acid.  The 
carbonates  in  the  system  of  the  herbivora  result  from  the 
carbonates  of  the  food,  and  the  combustion  of  organic 
acids,  malic,  citric,  tartaric,  etc.  ;  these  enter  the  body  as 
salts  of  sodium  and  potassium,  and  the  bases  being  set  free 
unite  with  carbonic  acid  to  form  carbonates. 

The  Sulphur  in  the  body  is  derived  from  the  albumin  of 
the  food ;  in  the  system  it  is  converted  into  sulphuric  acid, 
and  in  this  form  80  per  cent,  of  the  ingested  sulphur 
appears  in  the  urine.  Sulphur  exists  in  horn,  hair,  and 
epidermis. 

The  importance  of  the  sulphates  in  the  urine  is  consider- 
able, as  they  afford  a  passage  out  of  the  body  for  the 
products  of  proteid  decomposition.  Phenol  and  allied 
compounds  are  in  this  way  got  rid  of  in  the  form  of  phenyl- 
sulphate  of  potassium. 


()()()     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 

Iron  is  an  important  constituent  of  the  complicated 
substance  hemoglobin.  It  is  also  found  in  the  hair,  skin, 
bile,  lymph,  most  body  fluids  and  tissues,  and  a  small 
quantity  in  the  ffeces.  Bunge  considers  that  the  iron 
which  enters  the  system  can  only  be  absorbed  when  in  the 
form  of  an  organic  compound.  Inorganic  iron,  though 
largely  used  in  the  treatment  of  certain  diseases,  is  not 
absorbed  from  the  intestinal  canal ;  food  contains  only 
organic  and  not  inorganic  iron,  and  the  haemoglobin  of  the 
blood  is  formed  from  the  complex  organic  compounds  of 
iron  which  are  produced  by  the  vital  process  of  the  plant. 


IKDEX 


Abdomixal  muscles  in  respiration,  S9 
Abducens  cranial  nerve.  428 
Aberration,  chromatic,  483 

spherical,  483 
Abomasum,  175 
Abscess  of  liver,  241 
Absorption,  242 

bands  of  hfemogloliin,  9 
of  CO  hasmoglobin,  10 

by  the  caecum,  260 

by  the  cellular  tissue,  259 

by  the  conjunctiva,  258 

by  the  peritoneal  cavity,  258 
of  strychnine,  258 
of  potassium  iodide,  259 

by  the  pleural  cavity,  258 

by  the  respiratory  passages,  257 

by  the  skin,  258 

by  the  stomach,  176,  259 

by  the  vagina,  258 

of  acetic  acid,  257 

of  alcohol,  257 

of  carbohydrates,  263 

of  chloroform,  257 

of  ether,  257 

of  fat,  261 

of  gases  in  liquids,  97 

of  hydrocyanic  acid,  259 

of  liquid  anmionia,  258 

of  morphia,  257 

of  pilocarpin,  257 

of  proteid,  263 

of  potassium  ferrocyanide,  257 

of  physostigmin,  257 

of  salts,  263 

of  turpentine,  258 

of  water,  257,  263 

from  the  cellular  tissue,  259 
the  stomach,  176 

anfesthetics  by  rectum,  231 

anthrax  and  conjunctiva,  258 

atropin,  from  conjunctiva,  258 

capillar}',  lymph,  243 

chyle,  253,  250 

chyme,  256 


Absorption,  chyme,  Colin's  oliserva- 
tion,  256 
conjunctiva,  258 
cow,  lymph  from,  246 
curare  and  conjunctiva,  258 
dialysis,  248 
diffusion   in    lymph    formation, 

247 
dog,  lymph  from,  247 

stomach,  259 
emulsification  of  fat,  261 
ether,  by  air-passages,  257 

per  rectum,  261 
fibrinogen  in  lymph,  245 
filtration   in    lymph    formation, 

247 
glands,  lymphatic,  244 
Heidenhain,   Ij'mph    production 

theory,  250 
herbivora,  stomach  in,  259 
horse,  257 

legs,  cedema  of,  251 
lymph  from,  247 
in  general,  257 
intestinal,  259 
lacteal  vessel,  254 
lymph.  242,  245 
capillary,  243 
Colin  on,  246 
formation  of,  247 
movement  of,  251 
Colin  on.  252 
Weiss  on,  253 
plasma,  246 

production,  Heidenhain  on, 
250 

physical  theory,  247 
secretory  theory,  250 
Starling  on,  249 
sinus,  245 
spaces.  242 
quantity  of,  246 
lympliagogues,  250 
lymphatic  glands,  244 
vessels,  243 


667 


608    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Absorption  of  methylene  blue  from 
pleura,  259 
nux  vomica  by  air-passages,  257 

in  cpecum,  260 
cedema,  production  of,  251 
osmosis  in  lymph  formation,  247 
osmotic  pressure,  248 
ox,  lymph  from,  247 
paraglobulin  in  lymph,  245 
peptonuria,  263 
Peyer's  patches,  255 
potassimn  ferrocyanide,  alisorp- 
tion   of,   by   air-i)as- 
sages,  257 
from  bowel,  259 
from  cellular  tissue,  258 
from  peritoneum,  259 
from  skin,  258 
iodide,  259 
serous  cavities,  243 
serum  albumin  in  lymph,  245 
sheep,  lymph  from,  247 
solitary  follicles,  255 
Starling  and  Tubby  on,  259 
strychnine,  by  pleura,  259 

by  peritoneum,  259 
thoracic  duct,  243 
villi,  the,  253 
Accelerator  centre  in  medulla,  49 
Accommodation,  eye,  466 

Helmholtz  on,  467 
Acid,    acetic,  absorption  of,  liy  air- 
passages,  258 
in  digestion,  207,  209 
in  stomach,  162 
aniido-acetic,  645 
-caproic,  646 
-isethionic,  645 
-succinic,  647 
aspartic,  647 

pancreas,  236 
urine,  293 
benzoic,  650 

Liebig  on,  298 
liver,  222 
urine,  292,  298 
butyric,  209,  653 

in  stomach,  162 
capric,  652 
caproic,  652 
caprylic,  652 
cholalic,  221 
ethylidene-lactic,  653 
formic,  209 
glutaminic,  236 
glycero-phosphoric,  645 
glycocholic,  222 
glycuronic,  301,  311 
hippuric,    222,    292,    298,    302, 
650 


Acid,  hydrochloric,  161 

hydrocyanic,  259 

hyoglycocholic,  221 

hyotaurocholic,  221 

indoxyl-sulphuric,  651 

lactic,  161,  175,  209,  653 

malic,  209 

oleic,  652 

oxalic,  301,  302 

palmitic,  652 

phenyl-proprionic,  199 

phosphate  of  soda,  305 

phosphoric,  210,  303 

sarco-lactic,  119,  374,  378,  379. 
653 

skatoxyl-sulphuric,  652 

stearic,  652 

sulphuric,  231,  300,  302,  303 

succinic,  209 

tannic,  210 

taurocholic,  645 

uric.  292.  295,  379 
Acids,  biliary,  212 

fatty,  212 

of  stomach,  161 
Achroo-dextrin,  656 
Acrolein,  653 
Acromegaly  in  man,  270 
Adamkiewicz's  reaction,  641 
Addison's  disease  in  man,  269 
Adenine,  296 
Adrenalin,  270 
Adrenals,  269 
Afferent  nerves,  382 

paths  in  the  cord,  405 
African  'horse-sickness,'  27 
Aids  to  the  circulation,  74 
Air.  alveolar,  116 

amount  of,  required,  113 

atmospheric,  95 

composition  of,  95 
moisture  in,  95 

complemental,  113 

entrance  of,  into  veins,  65 

reserve,  113 

residual,  113 

tidal,  113 
AUniinin  in  milk,  620 
Albuminates,  636 
Albuminoids,  638 
Albumins,  derived,  636 

native,  636 
Albumose,  pancreas,  236 
Albumoses.  637 

Alcohol,    absorption   of,    b}'   air-pas- 
sages, 257 
Allantoin.  608,  649 
Allantois,  596,  606 
Alveolar  air,  116 
Amble  of  liorse,  526 


INDEX 


669 


Amides,  6i'» 
Amido-acetic  acid,  645 
Aniido-acids.  digestion,  187,  199 
-bodies  in  urine,  293 
-caproic  acid,  646 
-;^lucose,  638 
-isethionic  acid,  645 
-succinic  acid,  647 
Ammonia,  liquid,  absorption  by  air- 
passages,  258 
in  urine,  304 
salts  in  urine,  297 
Amnionic  -  magnesium      phosphate, 

digestion,  210 
Ammonium  carbamate  in  urine,  293 
carbonate  in  urine,  304 
sulphate,  action  on  plasma,  3 
Amoeboid  movements  of  white  cor- 
puscles, 13 
Amount  of  air  required,  113 

respired  (Boussingault),  116 
of  food  required,  332 
of  heat  produced,  350 
Amnion,  601,  605 
Amphoteric  milk,  619 
Amylolytic  action,  143 
Araylopsin  (pancreas),  234,  236 
Anacrotic    limb     of    sphygmogram, 

69 
Amesthetics  per  rectum,  261 
Analyses  of  blood,  4 
Anelectrotonus  (nerve),  387 
Angle,  visual,  485 
Animal  heat,  336 

amount  produced,  350 
body  temperature,  337 
cattle,  temperature  of,  339 
chloroform    narcosis    and    heat 

production,  342 
clipping,  effect  on   temperature 

(Siedamgrotzky),  349 
Colin  on  blood  temperature,  33S 

on  heat  loss,  351 
colour  effect  of,  345 
conduction,  343 
corpus  striatum  (heat  puncture), 

342 
curare  and  heat  production,  342 
Despretz  on  heat  loss,  351 
Dieckerhofi'  on  temperature,  339 
dog,  amount  of  heat  produced, 
351 
loss  of  heat,  351 
temperature  of,  339 
dormouse,  hibernation  of,  350 
enzymes,  intracellular,  336 
evaporation,  343 
fatigue  fever,  341 
grey  horaes   and   loss   by    heat, 
346 


Animal  heat,  loss  of,  343 
production,  340 
puncture,  342 
regidation,  343 
hibernating  animals,  338 
hibernation,  349 
homoithermal  animals,  337 
horse,  amount  of  heat  produced, 
351 
temperature  of,  338 
influence  of  heat  and  cold,  346 
of  nervous  system  on  heat 
production,  341 
intracellular  enzymes.  336 
marmot,  hibernation,  350 
negroes'  skin,  346 
nervous  system,  influence  of,  on 

heat  production,  341 
normal  temperature  of  animals, 

338 
ox  heat,  loss  by,  351 
oxidases,  337 

oxidations  in  animal  heat,  336 
peroxidases,  337 
pig  heat,  lost  by,  351 
poikilothermal  animal,  337 
post-mortem   rises   of    tempera- 
ture, 351 
radiation,  343 

Siedamgrotzky  on  effect  of  clii»- 
ping,  349 
on  temperature,  338 
sheep,    amount    of    heat     pro- 
duced, 351 
temperature  of,  339 
swine,  temperature  of,  339 
temperature,  Colin  on,  338 
normal,  of  animals,  338 
variations  in  body  temperature, 

339 
varnishing  the  skin,  284,  346 
wet,  eff'ect  of,  347 
Wolff'  on  heat  loss,  351 
Wooldridge,  on  temperature,  339 
Ancestrous  period,  577 
Ancestrum,  577 
Auo-spinal  centre  in  cord,  424 
Anthrax  and  conjunctiva,  258 
Antibody,  26 

Anti  concussion  mechanisms  in  limbs, 
518 
in  foot,  568 
Antipepsin,  178 
Antiperistalsis,  203 
Aorta,  pressure  in,  63 
Aortic  valve,  29 
Apex  beat,  non-existent,  39 
Apncea,  107 
Apomorphia,  180 
Apoplexy  of  lungs,  horse,  120 


670     A  MANUAL  OF  YETERINAKY  PHYSIOLOGY 


Aiiueous  humour,  455 
Arginine,  318 

pancreas,  23d 
Argon,  95 
Arloing  on  sweating,  281 

on  symjiathetic  nerves,  448 
Aromatic  series,  tlie,  650 
Arrangement  of  food  in  the  stomacli, 
157 

of  the  spinal  cord,  392 
Arterial  blood,  20 

system,  55 

'tone,'  75 
Arteries,  55 

contractility  of,  56 

elastic  property  of,  55 

structure  of,  56 

pathological  conditions,  83 
Artery,  anterior  mesenteric,  parasites 

in,  S3 
Artificial  insemination,  591 
Arytenoid  cartilages,  94 
Arytenoideus  muscle,  122 
Ancaris  megalocephala,  589 
Ascending  tracts  (spinal  cord),  402 
Ash,  fieces,  composition  of,  210 

milk,  composition  of,  621 

muscle,  composition  of,  379 
Asparagine,  647  ^ 

Aspartic  acid,  236,  647 
Asphyxia,  106 
Ass,  amount  of  air  required,  116 

analysis  of  milk,  620 

braying  of,  129 

period  of  gestation,  614 

ps3chical  powers,  449 

subepiglottic  sinus,  129 
Assheton  on  development  of  embryo 
of  sheep,  594 

on  impregnation  in  sheep,  593 

on  uterine  glands,  614 
Astigmatism,  457 

horse,  469 
Astragalus,  screw  action  of,  508 
Atmospheric     air,    composition     of, 

95 
Atropin,   absorption    from    conjunc- 
tiva, 258 

effects  on  cat,  468 
on  dog,  468 
on  horse,  468 
on  iris,  459 

in  secretion  of  saliva,  146 

in  sweating,  279 
Auditory  sensations,  501 
Auriculo  ventricular  valves,  33 
Automatic  action,  423 
Axis-cylinder  (nerve),  383 
Axone  (nerve),  412 
Azoturia,  10,  321 


l>acteriolysis,  26 
llalfour  on  polar  bodies,  590 
Barfoed's  reagent,  658 
Bars  (hoof),  548 

use  of,  566 
Basophile  leucocytes,  13 
Bayliss  and  Starling  on   pancreatic 
secretion,  233 
on  peristalsis,  203 
Bear,  generation,  578 
Bell's  experiment,  horse,  427 
Bellini,  duct  of  (kidney),  289 
Bellowing  of  ox,  129 
Beneden,  Van,  on  polar  bodies,  589 
Benzoic  acid,  222,  650 
Berlin,  eye  measurements,  480 
Bernard,  Claude,  on  division  of  facial, 
429 
on  glycogen,  225 
Bezoar  stones,  210 
Bicuspid  valve,  33 
Bile  acids,  origin  of,  222 

Pettenkofer's  test,  222 

amount  of,  secreted,  223 

Ellenberger  on,  219 

Gmelin's  test,  220 

Hofmeistcr  on,  225 

horse,  218 

percentage  composition,  219 

pigments,  220 

ox,  218 

salts,  221 

sheep,  218 

use  of,  224 

Voit's  experiments,  225 
Biliary  calculi,  241 
Bilirubin,  11,  12,  220 

Hammarsten  on,  221 
Biliverdin,  220 
Binocular  vision,  473 
Bischoff  and  Voit  on  urine  of  dog,  31 1 
Biuret  reaction,  641 
Bladder,  urinary,  311 
Blaine  on  old  age  in  horses,  629 
Blastocyst,  593 
Blastoderm,  bilaminar,  599 
Bleating  of  sheep,  129 
Blind  spot,  retina,  463 
Blood,  1 

absorption  bands  of  CO  hemo- 
globin, 10 
of  haemoglobin,  9 

African  '  horse-sickness,'  27 

ammonium  sulphate,  action  on 
plasma,  3 

amoeboid    movements   of  white 
corpuscles,  13 

amount  of,  in  living  animal,  22 

analyses  of,  4 

antibody,  26 


INDEX 


671 


Blood,  arterial,  '20 

azoturia  of  horse,  10 
bacteriolysis,  26 
basophile  leucocytes,  13 
biliruliin,  11,  12,  220 
Bohr  on  COo  in,  25 

on  htemoglobin,  9 
'  buffy  coat,'  2,  15 
Buuge's  table  of  salts  in,  21 
butyric  acid,  odour  from,  2 
calcium  phosphate  in,  21 

salts,    effect    of,    on    milk- 
curdling,  20 
on  coagulation,  19 
camel  tribe,  corjiuscles  of,  4 
carbon  dioxide  in,  23,  24 
carboxy-hremoglobin,  10 
cause  of  coagulation,  17 
cholesterin  in,  5,  20 
in  leucocytes,  14 
in  red  corpuscles,  5 
clot,  composition  of,  Itj 
coagulation  of,  15 

average  time  of,  16 

cause  of,  17 

circumstances  affecting,  18, 

20 
effect  of  salts  of  alkalis,  19 
of  acetic  acid  dilut.,  18 
of  ammonium  salts,  19 
of  calcium  salts,  19 
of  citric  acid,  19 
of  CO,,,  19 
of  cold,  19 
of  leech  extract,  20 
of  lime  salts,  19 
of  potassium  oxalate,  19 
of  peptone,  20 
Hammarsten's  theory,  17 
living  test-tul)e  experiment, 
19 
colour  of,  1 

of  venous,  20 
composition  of,  2,  25 
corjniscles  of,  4 
colourless,  12 

amceboid  movements  of, 

13 
origin  of,  14 
varieties  of,  12,  13 
red.  4,  5 
crassamentum,  15 
creatin  in,  20 
defensive    mechanisms    of    the 

body,  25 
defibrinated,  16 

diapedesis  of  white  corpuscles,  13 
disease  in,  26 

distribution  of,  in  the  bod}',  23 
dog,  2,  21 


Blood,  dog,  time  of  clotting,  16 
Ellenberger's  calculations,  6,  8 
eosinophile  cells,  13 
extractives  of.  20 
fats  in,  20 
fibrin  in,  4.  17 

I'erment,  3,  18 
fibrinogen  in  liquor  sanguinis,  3 

precipitation  of,  3 
Kbrino-globulin,  3 
foot-and-mouth  disease,  27 
functions  of,  1 
gases  of,  23 
globin  from,  11 

globulicidal  action  of  serum,  25 
glycogen  in  leucocytes,  14 
grape-sugar  in,  20 
Grower's  method   for  number  of 

corpuscles,  5 
h;ematin,  11 

spectrum  of,  11 
hsematogen,  S 
hffimatoidin,  11 
hrematoporphyrin,  11 
hfemiu,  11 
lutmochromogen,  11 
hffimoglobin,  1,  8 

absorption  bands  of,  9 

amount  of,  in  horse's  body,  S 

carboxy-,  10 

compounds  of,  10 

decomposition  of,  11 

Ellenberger's  estimate,  8 

in  corpuscle,  5 

meth-,  10 

Miiller's  estimate,  8 

nitric  oxide,  11 

o.xy-,  7 

reduced,  7 

spectroscope  test,  9 
lucmoglobinuria,  27 
lifemolysis,  25 
Haldane  and  Lorraine   Smith's 

process,  22 
Hammarsten's  theory  of  coagula- 
tion, 17 
heated  with  sulphuric  acid,  2 
hippuric  acid  in,  20 
horse,  2,  21 

amount  of,  in,  23 

amount  of  hremoglobin  in,  8 

specific  gravity  of,  2 

azoturia  of,  10 

clotting  of,  in,  15 

relation  between,  and  body- 
weight,  22 

time  of  clotting,  1 6 
hyaline  leucocytes,  12 
hydrobilirubin  from,  11,  12 
in  disease,  26 


672     A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 


Blood,  iron  in,  21 
'  laky,'  6 
lecithin  in,  '20 

in  leucocytes,  14 
in  red  corpuscles,  5 
leech  extract  in  coagulation,  20 
lettinj^,  27 
leucocytes,  12 

cholesterin  in,  14 
composition  of,  13 
diapedesis  of,  13 
glycogen  in,  14 
lecithin  in,  14 
origin  of,  14 
proportion  of,  12 
varieties  of,  12 
basophile,  13 
eosinophile,  13 
hyaline,  12 
lymphocytes.  13 
polynuclear,  12 
liquor  sanguinis,  2 
fibrinogen  in,  3 
paraglobulin  in,  3 
serum  albumin  in,  3 
serum  globulin  in,  3 
'  living  test-tube  '  experiment,  19 
lymph   cell   distinguished   from 

white  corpuscle,  12 
lymphocytes,  13 
magnesium  phosphate  in,  21 

sulphate,  action  on  plasma,  3 
malaria  parasite,  27 
Malassez's  method  for  number  of 

corpuscles,  5 
mechanisms,    defensive,    of    the 

body,  25 
niethfemoglobin,  10,  27 
Metschnikotf,  researches  of   14 
Miiller's     estimate     of     h;emo- 

globin,  8 
muscular     work,     ett'ect     upon 

blood,  2 
Xasse,  on  time  of  coagulation,  16 
nitric  oxide  hemoglobin,  11 
nitrogen  in,  23,  25 
miclco-proteid,  3 
number  of  corpuscles  in,  Gower's 
method,  5 
Malassez's  method,  5 
odour  of,  2 
olein  in,  20 
opsonins,  26 
ox,  2,  21 

relation  between  blood  and 

body-weight,  22 
specific  gravity,  2 
time  of  clotting,  16 
oxygen  in,  23,  24 
oxy-hajmogloliin,  7,  10 


Blood,  palmitin  in,  20 

paraglobulin    in    liijuor    sangui- 
nis, 3 
peptone,  effect  on  coagulation,  20 
p)hagocytosis,  14,  26 
phosphate  of  calcium  in,  21 
physical  characters  of,  1 
physiological  salt  solution,  6 
pig,  21 

relation  between,  and  body- 
weight,  22 
specific  gravity  of,  2 
time  of  clotting,  16 
plasma,  2 

separation        of        j)roteids 
from,  3 
platelets,  7,  18 
pleuritic  elfusion,  3 
polynuclear  colourless  corpuscles, 

12 
potassium  chloride  in,  21 
pjrecipitin,  26 
process  for  calculation  of  amount 

of,  22 
proportion  of,  to  body-weight,  22 
proteids  in  ]>lasma,  3 

of  serum,  3 
pro-thrombin,  IS 
purpura,  27 

quantity  of,  in  the  body,  22 
rallies,  27 
reaction  of,  1 

red  cells,  absorbing  surface  of,  6 
production  of,  7 
seat  of  formation  of,  7 
red  corpuscles  of,  4 
reduced  haemoglobin,  7 
rinderpest,  27 
salts  of,  21 
serum,  3 

albumin,     in     liquor     san- 
guinis, 3 
precipitation  of,  3 
globulin   in   liquor    sangui- 
nis, 3 
globulicidal  action  of,  25 
proteids  of,  3 
sheep,     relation    between,    and 
body-weight,  22 
specific  gravity  of,  2 
time  of  clotting,  16 
soaps  in,  20 

sodium  bicarbonate  in,  1 
carbonate  in,  21 
chloride  in,  2,  21,  22 

action  on  plasma,  3 
phosphate  in,  1,  21 
specific  gravity  of  dog,  2 
of  horse,  2 
of  ox,  2 


INDEX 


673 


Blood,  specitic  gravity,  of  pig,  2 

of  sheep,  2 
spectroscope  test,  hemoglobin,  9 
spectrum  of,  haemoglobin,  9 

of  oxy-liperaoglobin,  9 

of  CO  hfemoglobin,  10 

of  hiiematin.  11 
splenic  artery,  leucocytes  in,  12 

vein,  leucocytes  in,  12 
staining  of  leucocytes,  13 
stearin  in,  20 
Stokes's  fluid,  9 
sugar  in,  20,  226 
supply,  foot,  545 
Sussdorf's  proportion  of,  to  body- 
weight,  22 
taste  of,  2 
temperature  of,  22 

raised  experimentally,  71 
Texas  fever,  organism  in,  27 
time  occupied  in  clotting,  16 
thrombin  in,  18 

transfusion,  solution  used  for,  6 
trypanosomes,  27 
urea  in,  20 
uric  acid  in,  20 
urobilin,  11 
venous,  20 
whipped,  16 
white  corpuscles  of,  12 
Blood-pressure.     See  Bloodvessels 
Bloodvessels,  55 

aids  to  the  circulation,  74 
air,  entrance  of,  into  veins,  65 
anacrotic  limb  of  sphygmogram, 

69 
aorta,  pressure  in,  63 
arterial  system,  55 

'  tone,'  75 
arteries,  contractility  of,  56 

elastic  properties  of,  55 

pathological  conditions  of,  83 

structure  of,  56 
artery,  anterior  mesenteric,  83 
blood-pressure,  43,  62 

brachial  vein,  64 

capillaries,  64 

crural  vein,  64 

dog,  63 

effect  of  respiratory  move- 
ments on,  63,  91 

facial  vein,  64 

horse,  63 

jugular  vein,  64 

rabbit,  63 

sheep,  63 

veins,  64 

Volkmann's  table,  63 
blood,     temperature    of,    raised 
experimentally,  71 


Bloodvessels,  blood,   velocity   of,   in 
horse,  72 
brain,  exposed,  ])ulsation  in,  81 
camel,  pulse-rate  of,  70 
capillaries,  56,  64 
carotid    artery,    horse,    velocity 

in,  72 
cat,  temperature  of,  blood  raised 

experimentally,  71 
catacrotic  limb  of  sphygmogram. 
69 
waves  of  sphygmogram,  69 
Chauveau's  observations  on  ve- 
locity in,  72 
chorda  tympani  nerves,  76,  78 
circle  of  Willis,  SI 
circulation,  aids  to,  74 
arterial  tone,  76 
duration  of  the,  73 
Bering's     observations     on 

time,  74 
influence    of    the    nervous 

system,  74 
in  the  living  tissues,  65 
mean  pressure,  60 
mechanics  of,  58 
mesentery  of  mammal,  65 
peculiarities  in,  81 
peripheral  resistance,  58 
pulse,  58 
rete  mirabile,  81 
sphygmogram,  63 
time     of,    in     dog,     horse, 

rabbit,  74 
vaso-dilator  nerves,  76 
vaso-motor  centre,  75 

subcentres,  75 
velocity  of  current,  68 
of  pulse-wave,  6S 
venous      arrangements      of 

brain,  82 
Volkmann's      estimate      of 
area,  72 
corpuscles,  white,  migration  of, 

66 
dicrotic  wave  of  sphygmogram, 

69 
dilator  nerves,  78 
dog,  division  of  spinal  cord  in,  74 
pulse-rate  of,  70 
time  of  circuit  of  circula- 
tion, 74 
vena  cava,  anterior,  negative 
pressure  in,  64 
elasticity  of  arteries,  56 
elephant,  pulse-rate  of,  70 
fi'og's  foot,  circulation  in,  65 
haemorrhage,  65 
thirst  of,  65 
horse,  blood-pressure  in,  63 

43 


G74     A  MAiNTAL  OF  VETERINARY  PHYSIOLOGY 


IMood  vessels,    horse,     pulsatiuii     in 
jugulars,  57 
pulse  ill,  57 
pulse-rate  of,  70 
spliygmograni  of,  69 
time  of  circulation  circuit, 
74  _ 
iiiflammatioii,  essential  changes 

in,  66 
jugular  vein,  velocity  of   blood 

in,  72 
mesentery,  circulation  in,  65 
metatarsal    artery,    velocity    of 

blood  in,  72 
migration  of  white  corj)uscles,  66 
negative  pressure  in  veins,  64 
nerves,  brachial,  78 

chorda  tympani,  76,  78 
dilator,  78 
governing,  77 
vaso-constrictor,  77 
nervi  erigentes,  78 
nervous    system,    influence    on 

circulation,  74 
ox,  pulse-rate  of,  70 
pathological  conditions,  83 
pig,  jiulse-rate  of,  70 
pulse,  cause  of,  66 
character  of,  83 
explanation  of,  58 
hard,  83 
large,  83 
quick,  83 
slow,  83 
small,  83 
solt,  83 
pathological  considerations, 

83 
pathologist's  '  tension,'  70 
physiologist's  'pressure,'  70 
pulse-rate    and    blood-pressure, 
relation,  71 
variations  in,  71 
tension  of,  69 
venous,  67 
wave,  67 
pulse-wave,  graphic  study  of,  68 
length  of,  67 
sphygmogram,  68 
velocity  of,  67 
rabbit,  division  of  cervical  sym- 
pathetic, 75 
time  occupied  by  circuit  of 
circulation,  74 
rete  mirabile,  81 
sciatic  nerves,  78 
sheep,  pulse-rate  of,  70 
Strongyliis  armatus,   in    horses, 

83 
use  of,  55 


]51oodvessels,  vaso-constrictor  centre, 
79 
-dilator  nerves,  76 
-motor  centre,  75 

subcentres  in  cord,  75 
veins,  57 

abdominal,  57 
capacity  of,  57 
pregnant  uterus,  57 
jiulse  in,  66 
structure  of,  57 
valves  of,  57 
venfe  cavte,  57 
venous  plexuses  of  penis,  82 
without  valves,  57 
velocity  of,  72 

Volkmann's    estimate,    area    of 
vascular  system,  72 
observations,     velocity     of 
blood,  72 
Body  temperature,  337 
Bohr  on  carbon  dioxide  in  blood,  25, 
103 
on  hiemoglobin,  9 
Bones  of  the  foot,  538 
Bouley  on  varnishing  skin,  284 
Boussingault  on  growth  of  foal,  624 

tables,  respiration,  116 
Bo  vines,  period  of  puberty,  585 
Bowel,  pendular  movements  of,  203 

strangulation  of,  215 
Bowman,  capsule  of,  287 
Brain,  circulation  in,  444 
coverings  of,  444 
exposed,  pulsation  in,  81 
movements  of,  445 
Braying  of  ass,  129 
Broad's  treatment  of  laminitis,  564 
Broken  wind,  horse,  120,  335 
Bronchitis,  horse,  120 
Brown,  H.  T. ,  on  cellulose-dissolving 
enzyme,  171 
Professor,  dentition  and  old  age, 
627 
Brunner,  glands  of,  186 
'  Brushing  '  of  horse,  512 
Buchner  on  ferments,  643 
Buck -jumping,  horse,  532 
'  Butfy  coat'  in  blood-clotting,  2,  15 
Bulb,  404,  434.     8ee  Medulla  oblon- 
gata 
functions  of,  437 
Bunge  on  bile  pigments,  220 
on  calcium,  634 
on  cellulose,  207 

digestion,  657 
on  changes  in  liver,  231 
on  iron,  666 
on  lime  salts,  664 
on  sodium  salts,  663 


INDEX 


675 


Bunge's  table  of  salts  in  blood,  "21 

analysis,  ash  of  milk,  ti^l 
Buusen,  partial  pressure  of,  98 
Burdach,  column  of,  403 
Bursa,  navicular,  5o9 
Butyric  acid,  653 

digestion,  209 

odour  of,  from  blood,  2 

in  stomach,  162 

Caecum,  the,  190 

absorption  from,  260 
Calcareous  degeneration,  liver,  241 
Calcium  carbonate,  665 
in  food,  664 
in  urine,  301 
oxalate,  283 
phosphate,  212.  219,  233,  665 

in  blood,  21 
salts,  212,  219 

effect  on  coagulation,  10 
on  milk-curdling,  20 
sulphate,  665 
Calculi,  biliary,  241 
intestinal,  210 
stomach,  210 
Calories  of  heat,  324 
Calves,  Torcy  on  growth  of,  625 
Camel,  period  of  gestation,  615 
pulse-rate  of,  70 
rutting  of,  581 
tribe,  corpuscles  of,  4 
Canter  of  horse,  526 
Cane-sugar,  657 

digestion,  187 
Capacity  of  heart,  Colin,  42 

Munk,  42 
Capillaries,  56,  64 

lymph,  243 
'  Capped  elbow,'  horse,  522 
Capric  acid,  652 
Caproic  acid,  652 
Caprylic  acid,  207,  652 
Carbamide,  647 
Carbon,  632 

dioxide  in  blood,  23,  24 
in  nutrition,  316 
monoxide,     Haldane's     experi- 
ments, 104 
poisoning  bj^  105 
Carbo-hydrates,  654 
absorption  of,  263 
oxidation  of.  323 
Carbonates  in  the  body,  665 
Carbonic  acid,  Bohr's  view,  103 
fate  of,  102 

in  cellulose  digestion,  198 
in  large  intestine,  208 
in  stomach,  178 
respiration,  95 


Carboxy-hifimoglobin,  10 
Cardiac  cycle,  35 

sounds,  40 
Cardiograph,  42 
Carnine  (muscle),  379 
Carnivora,  respiratory  quotient  in,  96 
Carotid  artery,  horse,  velocity  in,  72 
Cartilage  nictitans,  477 
Caseinogen,  619,  620 
Castration,  effects  of,  265,  583 
of,  in  fattening,  326 
of,  on  voice,  128 
Cat,  blood  of,  2 

comi)osition  of  body,  315 

effects  of  atropin,  468 
of  pilocarpin,  281 

emmetropia  in,  470 

experimental  sweating  in,  279 

generation,  518 

hair  of,  276 

hearing  of,  496 

heart,  depressor  nerve  of,  50 

iris  of,  458 

larynx  of,  129 

ovaries,  removal  of,  265 

ovary  of,  587 

period  of  gestation,  615 

section    corpora    quadrigemina, 
439 

spleen,  267 

submaxillary  ganglion,  427 

sweating,  277 

sympathetic  nervous  system,  447 

temperature  of  blood,  raised  ex- 
perimentally, 71 

vagus  nerve,  45 

voice  centre  in,  129 

vomiting  in,  180 
'Cat-hairs,'  horse,  273 
Catacrotic  limb  of  sphygmogram ,  69 

waves  of  sphygmogram,  69 
Cattle,  osteo-malacia  in,  327 

repose  of,  522 

temperature  of,  339 
Causation  of  a  muscular  contraction, 

372 
Cause  of  coagulation  of  blood,  17 

of  first  inspiration,  112 

of  heart  beat,  51 
Cells  of  Purkinje,  31 
Cellular  tissue,  absorption  from,  258 
Cellulose,  action  of  bacteria,  197 

Bunge's  observations  on,  207 

digestion  of,  194,  656 

dissolving  enzyme,  171 

fermentation,  171 
Centre,  diabetic,  230 

of  gravity,  horse  at  rest,  514 
Centres  in  the  medulla,  435 
Cerebellum,  438 

43—2 


67(5     A  MANUAL  OF  VETEIUNAKY  PHYSIOLOGY 


Ccreljelluiii,  Luciani  on,  439 

Cerebral  fluid,  445 

Cerebrum,  440 

Changes  in  active  and  resting  muscle. 

369 
Chauveaii  and  Marey,  heart  of  horse, 
38 
on  deglutition,  136 
on  heart  valves,  39 
on  larynx,  129 
on  muscle  work,  365 
on  tendon  tiexor  metatarsi,  50'.) 
on  velocity  of  blood,  72 
of  nerve  impulses,  389 
Check  ligaments,  function  of,  514 
Chemical  changes  during  contraction 
of  muscle,  370 
composition  of  muscle,  377 
constituents  of  thebody,  632 
acid,  amido-acetic,  645 
-caproic,  646 
-isethionic,  645 
-succinic,  647 
aspartic,  647 
benzoic,  650 
butyric,  653 
capric,  652 
caj^roic,  652 
caprylic,  652 
ethylidene-lactic,  653 
glycero-phosphoric,  645 
hippuric,  650 
indoxyl-sulphuric,  651 
lactic,  653 
oleic,  652 
palmitic,  652 
sarco-lactic,  653 
skatoxyl-sulphuric,  652 
stearic,  652 
tauro-cholic,  645 
acrolein,  653 
achroo-dextrin,  656 
Adamkiewicz's  reaction,  641 
albuminates,  636 
albuminoids,  638 
albumins,  derived,  636 

native,  636 
albumoses,  637 
allantoin,  649 
amides,  645 
amido-acids,  645 
-glucose,  638 
aromatic  series,  the,  650 
asparagine,  647 
Barfoed's  reagent,  658 
biuret  reaction,  641 
Buchner  on  ferments,  643 
Bunge  on  calcium,  634 

cellulose  digestion,  657 
iron,  666 


Chemical  constituents  of  the  body, 
Bunge  on  lime  salts,  664 
sodium  salts,  663 

butyric  acid,  653 

calcium  carlionate,  665 
in  food,  664 
phosphate,  665 
suljihate,  665 

cane-sugar,  657 

capric  acid,  652 

caproic  acid,  652 

caprylic  acid,  652 

cai'liamide,  647 

carbohydrates,  654 

carbon,  632 

carbonates  in  the  body,  665 

cellulose,  digestion  of,  656 

choline,  645 

chlorine,  633 

chlorophyll,  644 

cholesterin,  654 

cholesterol,  654 

chondrin,  639 

coagulated  proteids,  637 

collagen,  638 

creatine,  646 

creatinine,  646 

cresol,  651 

dextrin,  655 

dextrose,  656-7-9 

disaccharides,  657 

elastin,  639 

erythro-dextrin,  656 

ethylidene-lactic  acid,  653 

enzymes,  643 

fats,  nitrogenous,  644 

ferments,  642 

fibrins,  636 

fatty  acids,  652 

fats,  652 

Fischer,  Emil,  on   proteids, 
635 

galactose,  658 

gases  found  in  the  body,  662 

gelatin,  639 

globulins,  636 

glycerin,  653 

glycerol,  653 

glycero-phosphoric  acid,  645 

glycine,  645 

glycocoll,  645 

glycocine,  645 

glycogen,  656 

glyco-proteids,  638 

glucose,  659 

glucosamine,  638 

grape-sugar,  659 

hferaoglobin,  644 

hexosos,  660 

hippuric  acid,  650 


INDEX 


677 


Chemical  constituents  of  the  body, 
hydrogen.  632 

indican,  651 

indigo  series,  651 

indol,  651 

indoxyl-sulphuric  acid,  651 

inorganic,  662 

inosite,  660 

iron,  634 

iron  in  the  body,  665 

invert  sugar,  658 

keratin,  639 

lactose,  658 

lactic  acid,  653 

lecithin,  645 

leucine,  646 

levulose,  657,  660 

lysatiue,  646 

lysatinine,  646 

maltose,  656-8 

magnesium  salts,  665 

melanin,  644 

methyl-glycine,  645 

Micrococcus  urece,  648 

Millon's  reagent,  641 
reaction,  641 

monosaccharides,  659 

muscarine,  645 

neurine,  645 

nitrogen,  633 

nitrogenous  fats,  644 

non-nitrogenous  bodies,  652 

nucleo-proteids,  637 

oleic  acid,  652 

olein,  653 

osazone  tests,  658,  661 

oxygen,  633 

palmitic  acid,  652 

palmitin,  653 

Pasteur  on  ferments,  642 

pentoses,  661 

pepsin,  643 

peptones,  637 

phenyl -glucosazone,  659 

phenyl-maltosazone,  658 

phenyl-sulphate    of     potas- 
sium, 665 

phenol,  651,  665 

phosphates,  665 

phosphorus,  633 

pigments  of  the  body,  644 

Piotrowski's  reaction,  641 

polysaccharides,  655 

potassium  salts  in  vegetable 
food,  663 

proteid  reactions,  641 

proteids,  634 

classification,  636 

proteoses,  637 

pseudo-nuclein,  638 


Chemical  constituents  of  tlie  body, 
ptyalin,  643 
purin  series,  638 
pyrocatechin,  651 
reaction,  biuret,  641 
reactions,  proteid,  641 
Adamkiewicz's,  641 
Millon's,  641 
Piotrowski's,  841 
Xanthoproteic,  641 
reagent,  Barfoed's,  658 
saccharose,  657 
sarcolactic  acid,  653 
sarcosine.  645 
silicon,  634 
skatol,  652 

skatoxyl-sulphuric  acid,  652 
sodium    salts   in   vegetable 

food,  663 
starch,  655 
stearic  acid,  652 
stearin,  653 
sugar,  invert,  658 
tests  for,  661 

Bcittcher's,  661 
fermentation,  661 
Moore's,  661 
picric  acid,  661 
'Trommer's,  661 
sulphur,  633,  665 
taurine,  645 
tauro-cholic  acid,  645 
trypsin,  643 
tyrosine,  650 
urea,  647 

synthesis  of,  648 
uric  acid,  649 
vitellin,  638 
water,  662 
xanthine  series,  638 
xanthoproteic  reaction,  641 
zymase,  643 
zymogen,  644 
Chemical  engine,  muscle  as,  366 

excitant,  specific,  264 
Chemistry  of  respiration,  114 
'  Chestnuts  '  of  the  horse,  602 
Chloral  hydrate,  effect  on  brain,  445 
Chlorine,  633 

in  urine,  303 
Chloroform,   absorption   of,    b}'    air- 
passages,  257 
cry  of  men  and  animals,  440 
effect  on  brain,  445 
narcosis  and  heat  production,  342 
Chlorophyll,  644 

in  dandruff,  283 
Cholalic  acid.  221-2 
Cholesterin,  212,  654 
in  bile,  219,  220 


678     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Cholesteiin  in  blood,  20 

in  red  corpuscle,  5 

in  spermal  fluid,  581 

leucocytes,  14 
Cholesterol,  654 
Choline,  645 
Chondrin,  639 
Chorda  dorsalis,  600 

tympani  nerve,  76,  78,  133,  144, 
428 
Chordre  tendineaj,  32,  33 
Chorion,  602,  608 
Choroid,  455,  461 
Chromatic  aberration,  483 
Chyle,  253,  256 
Chyme,  256 

Colin's  observations,  256 

physical  characters  of,  188 
Ciliary  muscle,  461 

efl'ect  of  atropin,  468 

processes,  458 

zone,  461 
Cilio-spinal  centre  in  cord,  424 
Circle  of  Willis,  81 
Circulating  proteid  of  Voit,  319 
Circulation,  aids  to,  74 

arterial  tone,  76 

aspiration  in  thorax,  35 
in  veins,  35 

blood-pressure,  62 

circle  of  Willis,  81 

course  of,  30 

dilator  nerves,  78 

duration  of  the,  73 

effect  of  respiration  on,  91 

Hering's  observations  on  time  of, 
74 

in  mesentery  of  mammal,  65 

in  the  living  tissues,  65 

influence  of  the  nervous  system, 
74 

mean  pressure,  60 

mechanics  of,  58 

lieculiarities  of,  81 

peripheral  resistance,  58 

pulsation  in  exposed  brain,  81 

pulse,  58 

rete  mirabile,  81 

sphygmogram,  68 

time  of,  dog,  74 
horse,  74 
rabbit,  74 

vaso-dilator  nerves,  76 

vaso-motor  centre,  75 
subcentres,  75 

velocity  of  blood-current,  68 
of  pulse-wave,  68 

venous  arrangement  of  brain,  82 

Volkmann's  estimate  of  aiea  of, 
72 


Clarke,  Bracy,  on  foot  of  horse,  564 
Clarke's  column,  spinal  cord,  395 
Clipping  horses,  275 

effect  of,  on  temperature.  Siedam- 
grotsky,  349 
Coagulated  protei<ls,  637 
Coagulation  of  blood,  14 
average  time  of,  16 
cause  of,  17 

ch'cumstances  affecting,  18,  20 
effect  of  acetic  acid  (dilut.),  18 
of  ammonium  salts,  19 
of  calcium  salts,  19 
of  carbon  dioxide,  19 
of  citric  acid,  19 
of  cold,  19 
of  leech  extract,  20 
of  })eptone.  20 
of  potassium  oxalate,  19 
of  salts  of  alkalis,  19 
Hammarsten's  theory,  17 
living  test-tube  experiment,  19 
Cocain,  effect  on  iris,  459 

in  peristalsis,  203 
Cochlea,  497 
Cochlear  canal,  500 
Coilom,  600 
Colic,  213 

Colin,  figures,  chest  of  horse,  85 
observations,  absorption,  257 
antiperistalsis,  203 
blood  temperature,  338 
capacity  of  heart,  42 

of  stomach,  horse,  151 
centre  of  gravity,  horse  at 

rest,  515 
chyme,  256 
digestion  of  hay,  156 
experimental  paralysis, 

horse,  444 
experiments,  seventh  pair  of 
nerves,  113 
stomach,  177 
guttural  pouches,  128 
heat  loss,  351 
ileum,  function  of,  189 
insensible  perspiration,  277 
lymph,  246 

movement,  252 
mastication,  135 
muscle  temperature,  371 
nervous   mechanism,   heart, 

44 
pancreas,  232 
reticulum,  174 
salivary  glands,  139 
succus  entericus,  186 
Collagen,  638 
Colon,  194 

absorption  from,  199 


INDEX 


679 


Colon,  digestive  changes  in,  197 
•Colostmm,  620,  622 
Colour,  effect  of,  345 

of  blood,  1 
Colouring  matter  of  urine,  301 
Colourless  corpuscles,  12 
Columnaj  carnese,  32 
Comparison,  body  and  engine,  365 
Complemental  ail',  113 
Composition  of  atmosplieric  air,  in- 
spired, 95 
expired,  95 
of  blood,  225 
of  the  body,  314 
of  sweat  of  horse,  278 
of  urine,  292 
Compression  of  propulsion,  517 
Concussion  of  impact.  517 
Condition  in  horses,  375 
Conduction,  343 
Conductivity  of  nerves,  389 
Conjunctiva,    absorption    from,    an- 
thrax, 258 
atropin,  258 
curare,  258 
Consistence  of  urine,  306 
Contractility  of  muscle,  356 
Co-operative     antagonism,     muscles. 

506 
Co-ordinate  movement,  409 
Cornea,  455,  457 
Corona,  fracture  of,  519 
Coronary  circulation,  53 
substance  (hoof).  545 
Corpora  nigra  (horse),  460 
quadrigeniina,  439 
striata,  437 
Corpus  arantii,  34 
luteum,  590 

striatum,  heat  puncture,  342 
Corpuscles  of  blood,  red,  4 
colourless,  12 
of  camel  tribe,  4 
white,  amreboid  movements  of, 
13 
basophile  leucocytes,  13 
distinguished    from    lymph 

cell,  12 
eosinophile  cells,  13 
hyaline  leucocytes,  12 
lymphocytes,  13 
migration  of,  66 
origin  of,  14 
polynuclear,  12 
varieties  of,  12 
Corti,  organ  of,  500 
Coughing,  129 
Course  of  the  circulation,  30 
Cow,  amount  of  air  respired,  116 
analysis  of  milk,  620 


Cow,  fffices  of,  208 

generation,  578 

heart,  foreign  bodies  in,  54 

hippomanes  in,  60S 

lymph  from,  246 

cestrous  cycle,  Goodall,  579 

ovary  of,  587 

period  of  gestation,  614 

position  of  fcetus  in,  615 

refraction,  errors  of,  470 

sense  of  smell,  488 

uterine  glands  in,  614 
'  Cow-hocks,'  horse.  512 
'  Cow-kicking  '  of  horse.  532 
Cranial  nerves,  425 
Crassamentum,  15 
Creatine,  646 

in  blood,  20 

in  muscle.  379 

in  urine,  292 
Creatinine,  292,  295,  646 
Cresol,  651 

ethereal  sulphate  of,  292 

in  urine,  300 
Crico-arytenoideus  lateralis,  122 

posticus,  122 
Cui'are  and  conjunctiva,  258 

effects  on  end-plates,  354,  361 

and  heat  production,  342 
Curdling  of  milk,  effect  of  calcium 

salts,  20 
Current  of  action  (nerve),  386 

of  rest  (nerve),  386 
Cumulus  proligerus,  588 
Cutaneous  senses,  cold,  490 
pain,  490 
pressure,  490 
warmth,  490 
Cuvier  on  organ  of  Jacobson,  487 

Death,  629 

convulsions  in,  630 

rigor  mortis,  379,  631 
Decay,  628 
Decidua  reflexa,  603 

serotina,  603 

vera,  603 
Defensive  mechanisms  of  the  body,  25 
Defibrinated  blood,  16 
Deficiency  in  oxygen,  105 
Defalcation,  act  of,  211 
Degeneration  of  nerves,  389 

Wallerian.  4C0 
Deglutition,  136 
Deiters,  supporting  cells  of,  500 
Dendrites,  412 
Dentition,  horse,  626 

ox,  627 

pig,  628 

sheep,  627 


680    A  MANUAL  OF  VETEEINARY  PHYSIOLOGY 


Depressor  nerve  of  heart,  50 
Descending  tracts  (spinal  cord),  402 
Despretz  on  heat  loss,  351 
Determination  of  sex,  591 
Deutoplasni,  589 
Dentero-albunioses,  187 
Development,  593 

albumin  in  milk,  620 
allantois,  596,  606 
allantoin,  608 
amnion,  601,  605 
amphoteric  milk,  619 
ass,  analysis  of  milk,  620 

jieriod  of  gestation,  614 
Assheton,    development   of  em- 
bryo of  sheep,  594 
on  uterine  glands,  614 
blastocyst,  593 
blastoderm,  l)ilaminar,  599 
Bunge's  analysis  of  ash  of  milk. 

621 
camel,  period  of  gestation,  615 
caseinogen,  619,  620 
cat,  period  of  gestation,  615 
'  chestnuts  '  of  the  horse,  602 
chorda  dorsalis,  600 
chorion,  602,  608 
ccelom,  600 
colostrum,  620,  622 
cow,  analysis  of  milk,  620 
hippomanes  in,  608 
period  of  gestation,  614 
position  of  fcetus  in,  615 
uterine  glands  in,  614 
decidua  reflexa,  603 
serotina,  603 
vera,  603 
dog,  analysis  of  milk,  620 

period  of  gestation,  615 
ductus  arteriosus,  611 

venosus,  610 
duration  of  pregnancy,  614 
elephant,  period  of  gestation,  614 
epiblast,  structures  derived  from, 

599 
'  ergots  '  of  the  horse,  602 
Eustachian  valve,  611 
Ewart  on  development  of  embryo 

of  horse,  594 
foetal  circulation,  610 
membranes,  605 
foramen  ovalis,  611 
gestation,  periods  of,  614 
glands,  uterine,  614 
guiuea-jjig,  period  of  gestation, 

615 
hippomanes,  Goodall  on,  608 
holoblastic  ova,  597 
horse,  development  of  embryo  of, 
594 


Development,   hypoblast,  structures 
derived  from,  599 
koumiss,  621 
lactalbumiu,  620 
lactic  acid  in  allantoic  fluid,  608 
lactose,  620 
liquor  amnii,  606 
mare,  analysis  of  milk,  620 

period  of  gestation,  614 

jiosition  of  fuitus  in,  615 

uterine  glands  in,  614 
medullary  groove,  600 
meroblastic  ova,  597 
mesoblast,     structures     derived 

from,  599 
milk,  secretion  of,  617 

sugar,  620 
neural  groove,  600 
notochord,  600 
olein  in  milk,  620 
ovum,  devt'loimient  of  the,  597 
jialmitin  in  milk,  620 
parturition,  615 
pig,  period  of  gestation,  615 
placenta,  603 

cotyledonary,  604 

cumulata,  604 

discoidal,  604 

metadiscoidal,  604 

plica ta,  604 

polycotyledonary,  604 

zonary,  604 
}>regnancy,  duration  of,  614 
primitive  groove,  600 

streak,  599 
pro-amnion,  601 
rabbit,  period  of  gestation,  615 
rennin,  620 
sheep,  analysis  of  milk,  620 

fcetal  blood,  gases  in,  613 

period  of  gestation,  615 

uterine  glands  in,  614 
soraatopleure,  600,  610 
splanchnopleure.  601,  608 
stearin  in  milk,  620 
trophoblast,  601,  604 
tyrein,  620 
umbilical  cord,  608 
urachus,  607 

urea  in  allantoic  fluid,  608 
uterine  milk,  614 
Whartonian  jelly,  610 
yolk  sac,  600,  605 
zebra,  period  of  gestation,  614 
Dextrin;  187,  655 
Dextrose,  187,  656,  657,  659 
Diabetes,  226 

pancreatic,  239 
Diabetic  centre,  230 
puncture,  230 


INDEX 


681 


Dialysis,  248 

Diapedesis  of  white  corpuscles,  13 

Diaphragm,  86 

in  respiration,  89 
movements  of,  85 
rupture  of,  120 
spasm  of,  120 
Diarrha?a,  217 
Diastole  of  lieart,  35 
Dicrotic  wave  of  sphygmogram,  69 
Dieckerhoff  on  temperature,  339 
Ditl'usion  in  lymph  formation,  247 

of  gases,  98 
Digastricus  muscle,  136 
Digestion,  131 

abomasum,  175 

absorption  from  the  stomach,  176 
acetic  acid,  207,  209 
in  stomach,  162 
acid,  biliary,  212 
butyric,  209 

in  stomach,  162 
fatty,  212 
formic,  209 
hydrochloric,  161 
lactic,  209 

in  stomach,  161 
of  pig,  1 75 
malic.  209 

phenyl-proprionic,  199 
phosphoric,  210 
succinic,  209 
tannic,  210 
acids  of  stomach,  161 
amido-acids,  187,  199 
ammonio-magnesium  phosphate, 

210 
amylolytic  action,  143 
antipepsin,  178 
antiperistalsis,  203 
apomorphia,  180 
arrangement    of     food     in     the 

stomach,  157 
atropin  in  secretion  of  saliva,  146 
Bayliss    and   Starling   on   peri- 
stalsis, 203 
Bezoar  stones,  210 
bowel,  pendular  movements  of, 
203 
strangulation  of  the,  215 
Brown,  H.  T. ,  on  cellulose  dis- 
solving enzyme,  171 
Brunner,  glands  of,  186 
Bunge  on  cellulose,  207 
cascum,  the,  190 
calcium,  salts  of,  212,  219 
calculi,  intestinal,  210 

stomach,  210 
cane-sugar,  187 
caprylic  acid,  207 


Digestion,  carbonic  acid  in  celhilose 
digestion,  198 

in  large  intestine,  208 

in  stomach,  178 
cat,  vomiting  in,  180 
cellulose,  action  of  bacteria,  197 

Bunge's  observations  on,  207 

digestion  of,  194 

dissolving  enzyme,  171 

fermentation,  171 
Chauveau  on  deglutition,  136 
cholesterin,  212 

chorda  tympani  nerve,  133,  144 
chyme,   physical  characters   of, 

188 
cocaine  in  peristalsis,  203 
colic,  213 

Colin,  experiments  on  stomach, 
177 

on  antiperistalsis,  203 

on  capacity  of   stomach  in 
horse,  151 

on  hay, 156 

on  ileum,  functions  of,  189 

on  mastication,  135 

on  reticulum,  174 

on  salivary  glands,  139 

on  succus  entericus,  186 
colon,  194 

absorption  from,  199 

digestive  changes  in,  197 
cow,  ffeces  of,  208 
defffication,  act  of,  211 
deglutition,  136 
dextrin,  187 
dextrose,  187 
diarrhcea,  217 
digastricus  muscle,  136 
deutero-albumoses,  187 
drinking,  horse,  134 

ox,  134 
dog,  area  of  intestinal  tract.  201 

capacity  of  intestinal  canal, 
201 

gastric  juice,  165 

intestinal,  201 

lapping  of,  134 

mastication,  134,  135 

oesophagus  of,  138 

stomach,  149,  176 

submaxillary  gland  of,  144 

tongue  of,  133 

vomiting  of,  178 
duodenum,  153 

syphon-trap  of,  153 
Eber,  observations  of,  184 
EUenberger    on,   in   ruminants, 
173 

on  gastric  acids,  161 

on  horse-feeding,  160 


682     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Digestion,    Ellen bergcr    on    nervous 
mechanism,  ruminants,  1 86 
on  intestinal,  in  horse,  187 
on  onuiHum,  174 
on  periods  of"  stomach  diges- 
tion, 171 
on  saliva,  143 
on  the  CiEcum,  190 
enteritis,  21-1 


187 

intestinal   extract. 


enterokinase, 
enzymes 

187 
erepsin,  187 
erythrodextrin,  142 
ether,   rectal   administration  of, 

199 
fats,  in  stomach,  171 
fatty  acids,  199 

fifth  nerve,  gustatory  division  of, 
144 

lingual  branch  of,  133 
Fletcher,  experiments  on  salivary 

glands,  149 
Flourens'  experiments,  172 
faeces,  amount  of,  211 

ash  of,  210 

approximate  composition  of, 
208 

odour  of,  211 

of  cow,  208 

of  horse,  208 

of  pig,  208 

of  sheep,  208 
follicles  of  Lieberkiihn, 
formic  acid,  209 
fundus  glands,  164 
galactose,  187 
Gamgee  on  mastication, 
gases  of  the  intestines,  207 

of  the  stomach,  178 
gastric  juice,  secretion  of.  162 
gastritis,  215 
glands,  fundus,  164 

gastric,  162 

labial,  141 

molar,  141 

of  Brunner,  186 

palatine,  141 

parotid,  139,  141 

jiyloric,  164 

sublingual,  139 

submaxillary,  139 
glosso-pharyngeal  nerve,  144 

lingual  branch  of,  133 
granulose,  142 
hair-balls,  210 

in  rumen  of  cattle,  172 
hay, 154 

Heidenhain's  view   of  secretory 
nerves,  146 


186 


134 


Digestion,  herbivora,  saliva  in,  140 

teeth  in,  132 
hexone  leases,  187 
Ilofnieister  on  gastric  acids,  161 

on  periods  of  stomach,  171 
horse,   area  of  intestinal  tract, 
201 

boiled  food  for,  171 

ca})acity  of  intestinal  canal, 
201 
of  stomach,  151 

drinking  of,  134 

feeding  of,  158 

faeces  of,  208 

gastric  glands  in,  162 

intestinal  affections  in,  204 
digestion  in,  187 

mastication,  134-5 

medicines  by  mouth,  185 

cesophagus  of,  137 

periods    of    stomach   diges- 
tion, 171 

salivary  glands  of,  139 

saliva  of,  143 

stomach  of,  149 

aljsorption  from,  176 
digestion  in,  150 

sugar  formation  in,  170 

tongue  of,  133 

vomiting  of,  138,  178 

watering  of,  158 
hydrochloric  acid,  161 
hydrogen  in  large  intestine,  208 

in  stomach,  178 
hypogastric  plexus,  205 
hypoglossal  nerve,  133,  138 
ileum,  function  of,  189 
indol,  189,  199,  209 
intestinal  calculi,  210 

canal,    nervous    mechanism 
of,  205 

digestion,  186 
in  horse,  186 
in  other  animals,  201 
in  ruminants,  199 
intestines,  gases  of  the,  207 

movements  of,  201 
impaction,  217 
inverting  ferments,  187 

invertase,  187 

lactase,  187 

maltase,  187 
ipecacuanha,  180 
Kiihne's  table  of  peptones,  168 
labial  glands,  141 
lactase,  187 
lactic  acid,  161,  209 
Langley  on  gastric  glands,  164 

ol)servation  on  secretion,  147 
lapping  of  dog,  134 


INDEX 


683 


Digestion,  large  intestine,  190 
•    leiicin,  209 
leviilose,  187 

Lieberkuhn,  follicles  of,  186 
lignin  in  diet  of  herbivora,  207 
lion,  intestinal  canal  of,  201 
lips,  horse,  131 

ox.  131 

pig,  131 

sheep,  131 
llama,  stomach  of,  182 
magnesium  salts,  212 

phosphate,  219 

sulphate,  219 
masseter  muscle,  136 
Majendie's  experiment,  ISO 
malic  acid,  209 
maltase,  187 
maltose,  187 

marsh  gas  in  cellulose  digestion, 
198 

in  large  intestine,  208 

in  stomach,  178 
mastication,  Gamgee  on,  135 

nerves  of,  136 

dog,  134 

horse,  134 

ox,  134 

sheep,  134 
M'Kendrick  on  area  of  intestinal 

tract,  201 
Meade  Smith  on  saliva,  143 
mechanism  of  rumination,  183 

nervous,   of    the   intestinal 
canal,  205 

of  secretion  of  saliva,  144 
meconium,  212 
milk-sugar,  187 
molar  glands,  141 
movements  of  stomach,  184 

of  the  intestines,  201 
mucin,  140,  165 

in  stomach,  152 
Mnnk,    statistics    of    intestinal 

canal,  201 
nerve,  chorda  tympani,  133 

control,  secretion  of  saliva, 
144 

fifth,  in  mastication,  136 
lingual  branch  of,  133 

ganglia,  salivary  glands,  147 

glosso- pharyngeal,    lingual 
branch  of,  133 

hypoglossal,  133,  138 

recurrent  laryngeal,  138 

seventh  in  mastication,  136 

superior  laryngeal,  138 

supply  of  stomach,  185 

sympathetic,  144 
nerves,  deglutition,  138 


Digestion,  nerves,  hypogastric  plexus, 
205 
mastication,  136 
pharyngeal  plexus,  138 
tongue,  133 
nervous  control  of  gastric  JTiice, 
169 
of  rumination,  183 
mechanism      of     intestinal 
canal,  205 
of  secretion  of  saliva, 

144 
of    stomach    of    rumi- 
nants, 186 
nicotine  in  peristalsis,  203 

in  secretion,  147 
nitrogen  in  stomach,  178 
oats,  156 

cesophageal  groove,  180 
oesophagus,  dog,  138 
horse,  137 
ox,  138 
sheep,  138 
omasum,  174 

ox,  area  of  intestinal  tract,  201 
capacity  of  intestinal  canal, 

201 
drinking,  134 
intestinal,  201 
mastication,  134-5 
fjesophagus  of,  138 
salivary  glands  of,  139 
tongue  of.  133 
oxygen  in  stomach,  178 
palate,  grooves  in,  141 
palatine  glands,  141 
paracasein,  168 
j^aralytic  secretion,  146 
parasitic  diseases,  217 
parotid  gland,  139,  141 

secretion  in,  147 
Pawlow,  gastric  juice,  165 
pendular  movements  of  bowel, 

203 
pepsin,  165-6 
pig,  175 

secretion  of,  152 
peptones,  168,  187,  199 
periods  of  stomach  digestion,  171 
peristalsis,  203 

effects  of  cocaine,  203 

of  nicotine,  203 
Starling  and  Bayliss on,  203 
pharyngeal  plexus,  138 
phenol,  199,  209 
phenyl-acetic  acid,  199 

-proprionic  acid,  199 
pig,  area  of  intestinal  tract,  201 
capacity  of  intestinal  canal, 
201 


684     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Digestion,  pig,  feces  of,  208 

intestinal  digestion  in,  "201 
saliva  of,  143 
stomach  of,  150 

digestion  in,  175 
vomiting  of,  178 
pilocarpin  in  secretion  of  saliva, 

146 
potassium  in  faeces,  210 
sulphocyanide,  140 
prehension  of  food,  I'-il 
proprionic  acid,  207 
proteid,  167 

to   peptone,  conversion   of, 
168 
proteoses,  199 
primary,  168 
secondary,  168 
pterygoid  muscles,  135-6 
ptyalin,  140,  143 
pyloric  glands,  164 
rabbit,  saliva  of,  143 
reaction  of  contents  of  stomach, 

161 
rectum,  absoi'ption  from,  199 
recurrent  laryngeal  nerve,  138 
rennin,  165 
reticulum,  174 
Roger  on  composition  of  faeces, 

210 
rumen,  172 

ruminants,  intestinal,  199 
stomach,  172 

nervous  mechanism  of, 

186 
trouble  in,  217 
rumination,     Colin's     observa- 
tions, 180 
Flourens'  observations,  180 
mechanism  of,  182 
rupture,  217 

of  stomach,  153 
saliva,  139 

amount  of  secretion,  139 
chemical  characters,  140 
Meade  Smith  on,  143 
non-amylolytic    action    of, 

143 
physical  characters  of,  140 
salts  of,  140 
secretion  of,  144 

nerve  control  of,  144 
use  of,  142 
salivary  glands,  changes  in  cells, 
147 
classification  of,  139 
sea-sickness,  horse,  180 
secretin,  187 

secretion  of  gastric  juice,  162 
nerve  control  of,  169 


Digestion,  secretion  of  pepsin,  152 

of  saliva,  144 
secretory    nerves,   Heidenhain's 

view  of,  146 
self-,  of  the  stomach,  177 
sheep,  f;eces  of,  208 

intestinal  digestion  in,  201 

mastication,  134 

cesophagus  of,  138 
-silica  in  faces,  210 
skatol,  189,  199,209 
Smith,  Meade,  on  saliva,  143 
sodium  chloride,  212,  219 
spliincters,  212 
starch  of  plants,  142 
Starling  and  Bayliss   on  peris- 
talsis, 203 
sterno-maxillaris  muscle,  136 
stomach  acids,  161 

calculi,  210 

contents,  reaction  of,  161 

gases  of,  178 

in  horse,  150 
in  dog,  176 
in  pig,  175 
in  ruminants,  172 

movements  of,  184 

nerves  of,  185 

of  dog,  1.50 

of  horse,  150 

of  llama,  182 

of  pig,  150 

of  ox,  150 

periods  of,  171 

pouch  of  Pawlow,  166 

rupture  of,  153 

self-digestion  of,  177 
strangulation  of  the  bowels,  215 
strychnine  experiments,  177 

per  rectum,  199 
stylo-maxillaris  muscle,  136 
sublingual  gland,  139 
submaxillary  gland,  139 

of  dog,  144 
succinic  acid,  209 
suecus  entericus,  186 
sucking,  134 

suljihuretted  hydrogen  in  large 
intestine,  208 
in  stomach,  178 
superior  laryngeal  nerve,  138 
swallowing  centre,  138 
syntonin,  168 

syphon-trap  of  duodenum,  153 
Tappeiner  on  cellulose,  171 
tartar  emetic,  180 
teeth,  131 

temporal  muscle,  13G 
tiger,  intestinal  canal  of,  201 
tongue,  dog, 133 


INDEX 


685 


Digestion,  tongue,  horse,  133 
movements  of,  133 
nerves  of,  133 
ox,  133 
trypsin,  187 
trypsinogen,  187 
tympany,  217 
tyrosin,  209 

vagus,  action  on  small  intestines, 
205 
and     secretion     of    gastric 

juice,  1G9 
motor  nerve  of  stomach,  185 
vomiting,  178 
Digitalin,  action  on  heart,  53 
Dilator  nerves,  78 
Dicestrum,  577 
Dioptrics,  480 
Disaccharides,  657 
Disassociation,  process  of,  103 
Discus  proligerus,  588 
Discharge  of  urine,  311 
Disease,  blood  in,  26 
'  Dishing  '  of  horse,  512 
Distribution  of  blood  in  body,  23 
of  nerve  fibres  in  the  cord,  396 
of  the  weight  of  the  body,  515 
Disynaptic  arc  (nervous  system),  416 
Division  of  the  phrenic  nerves,  112 
of  the   seventh  pair  of  nerves, 
113 
Dog,  action  of  heart  in,  54 

amount  of  air  required,  116 

of  heat  produced,  351 
analysis  of  milk,  620 
area  of  intestinal  tract,  201 
bile,  action  of,  225 

amount  of,  per  hour,  224 
blood  of,  2,  21 

of,  time  of  clotting,  16 
brain,  localization,  441 
capacity  of  intestinal  canal,  201 
division  of  spinal  cord,  74 
efl'ect  of  atropin,  468 
emmetropia,  470 
experiments  on  kidney  of,  285, 

290 
gastric  juice,  165 
generation,  578 
hair  of,  276 
heart,  pressure  in,  37 

valves,  vegetations  on,  54 
intestinal  digestion  in,  20] 
intracardiac  pressure,  42 
iris  of,  458 
kidney,  amount  of  blood  through, 

289 
larynx  of,  129 
lapping  of,  134 
loss  of  heat,  351 


Dog,  lymph  from,  247 
mastication.  134,  135 
number  of  respirations,  90 
cesophagus  of,  138 
ovaries,  removal  of,  265 
ovary  of,  587 
pancreatic  fluid  of,  232 
period  of  gestation,  615 

of  puberty,  585 
pilocarpin  in,  281 
pressure  in  pleural  cavity,  90 
psychical  power's,  449 
pulse-rate  of,  70 
reflex  action  in,  409 
relation  between  blood  and  body- 
weight,  22 
respiratory  curves  of,  92 
sense  of  smell,  488 
spleen,  267 
stomach  of,  149 

absorption  in,  259 
digestion  in,  176 
submaxillary  ganglion,  427 

gland  of,  144 
sweating,  277 

sympathetic  nervous  system,  4^7 
temperature  of,  339 
tendon  reflexes  in,  423 
time  of  circuit  of  circulation,  74 
tongue  of,  133 

urine,  Bischoff  and  Voit  on,  310 
vagus  nerve,  45 
vena     cava     anterior,    negative 

pressure  in,  64 
voice  centre  in  cerebral  cortex, 

128 
vomiting  of,  178 
Donkey,  sweating  of,  277 
Dormouse,  hibernation  of,  350 
Draught,  534 

I3runel  on,  535 
Drinking,  horse,  134 

ox, 134 
Ductless  glands,  264 

acromegaly  in  man,  270 
Addison's   disease   in  man, 

269 
adrenals,  269 
adrenalin,  270 
castration,  eft'ects  of,  265 
cat,  ovaries,  removal  of,  265 

spleen,  267 
chemical  excitant,  specific, 

264 
dog  spleen,  267 

ovaries,  removal  of,  265 
Edkins  on  gastric  juice,  264 
hormone,  264 
internal  secretion,  264 
iodothyrin,  269 


686     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Ductless   glands,    Leeney's   observa- 
tions, ovarian  tissue;,  265 
ovary,  influence  of,  265 
parathyroid,  268 
phagocytosis  in  spleen,  267 
pineal  l)ody,  270 
pituitary  body,  270 
secretin    of    Starling     and 

Bayliss,  233.  264 
spleen,  266 

enzyme  in,  267 
use  of,  267 
thymus,  269 

influence  of  castration 
on,  266 
thyroid  gland,  268 
Ductus  artei'iosus,  611 

veuos  s,  610 
Duodenum,  153 

syphon-trap  of,  153 
Duration  of  the  circulation,  73 

of  pregnancy,  614 
Durham,  researches  on  hair  pigment, 

274 
Dyspnoea,  106 

Ear,  auditory  sensations,  501 
cat,  hearing  of,  496 
cochlea,  497 
cochlear  canal,  500 
Corti,  organ  of,  500 
Deiters,  supporting  cells  of,  500 
endolymph,  499 
Eustachian  tube,  497 
external,  496 

movements  of,  496 
fenestra  ovalis,  498 

rotunda,  498 
Galton  on  sounds,  496 
guttural  pouches,  497 
harmonics,  495 
hearing,  495 
helicotrema,  500 
Hensen's  cells,  501 
incus,  497 
internal,  497 
labyi-inth,  497,  502 
lamina  spiralis,  499 
macula  acustica,  502 
malleus,  497 
membrana  basilaris,  499 
membrane  of  Reissner,  500 
middle,  497 
musical  sounds,  495 
noise,  496 
perilymph,  498 
pillars  of  Corti,  499 
otoliths,  499 

calcium  carbonate,  504 
saccule,  499 


Ear,  scala  tympani,  499 
vestibuli,  499 

semicircular  canals,  497,  502 

Sherrington   on    the    labyrinth, 
503 

sound,  nature  of,  495 

stapes,  497 

tympanum,  497 

utricle,  499 

vestibule,  497 
Eber,  observations  of,  on  rumen,  184 
Edkins  on  gastric  juice,  264 
Efl'ect  of  respiration  on  circulation, 

91 
Ett'ector  organ,  412 
Efferent  nerves,  382 

paths  in  the  cord,  405 
Eighth  pair  cranial  nerves,  429 
Elastic  tension,  nmscle,  364 
Elasticity  of  arteries,  56 

of  muscle,  363 

provisions  for,  in  the  foot,  559 
Elastin,  639 
Electric  phenomena  of  muscle,  369 

of  nerves,  385 
Electrotonus  (nerve),  387 
Elephant,  jieriod  of  gestation,  614 

pulse-rate  of,  70 

psychical  powers  of,  449 

rutting  of,  581 
Eleventh  pair  cranial  nerves,  434 
Ellenberger  on  bile,  219 

on  ciecupi,  the,  190 

calculation   of    number    of    red 
cells,  6 

on  digestion  in  ruminants,  173 

estimate    of     htemoglobin     per- 
centage, 8 

on  gastric  acids,  161 

on  horse-feeding,  160 

on  intestinal  digestion  in  horse, 
187 

on  nervous  mechanism,  stomach 
of  ruminants,  186 

on  omasum,  174 

on  periods  of  stomach  digestion, 
171 

on  saliva,  143 

on  varnishing  the  skin,  284 
Emmetropic  eyes,  468 
Enmlsitication  of  fat,  261 
Endolymph,  499 
End-plate  in  muscle,  354 
Enlargements  of  liver,  241 
Enteritis,  214 
Enterokinase,  187,  234 
Enzymes,  643 

diastatic,  234 

in  intestinal  extract,  187 

intracellular,  336 


INDEX 


687 


Enzymes,  lipolytic,  234 

proteolytic,  234 
Eosinophile  cells,  13 
Epiblast,  structures  derived  from,  599 
Epiglottis,  125 
Ercolani  on  sweat-gland   in  foot  of 

horse,  557 
Erection,  act  of,  585 

centre  in  cord,  424 
Erepsin,  187,  236 
'  Ergots  '  of  the  horse,  602 
Erythro-dextrin,  142,  656 
Eserin,  effect  on  iris,  459 
Ether,  absorption  of,  by  air-passages, 
257 

effect  on  brain,  445 

per  rectum,  199,  261 
Ethylidene-lactic  acid,  653 
Evaporation,  343 
Eversbusch  on  iris  of  horse,  459 
Eustachian  tube,  497 

valve,  611 
Ewart  on  development  of  embryo  of 
horse,  594 

on  ovulation  in  mare,  591 
Excitability  of  nerves,  385 
Excretion,  detinition  of,  285 
Expansion  of  foot,  569 

Lungwitz  on,  571 
Expenditure  and  income  of  body,  315 
Experimental  paralysis,  horse,  444 
Expiration,  88 

muscles  of,  89 
Expired  air.  composition  of,  95 
Extensibility  of  muscle,  363 
External  ear,  496 

intercostal    muscles   in   respira- 
tion, 89 

respiration,  99 
Extractives  of  blood,  20 

of  muscle,  379 
Eye,  454.     See  Sight 

schematic,  478 

structure  of  the,  454 
Eyeball,  movements  of  the,  470 

muscles  of,  471 
Eyelashes  of  horse,  477 

Facial  nerve,  428 

sinuses,  94 
False  nostril,  92 
Fatigue  fever,  341 

nuiscle,  372 

Wedenski  on,  375 
Fat,  absorption  of,  261 

emulsification  of,  261 
Fatty  acids,  199,  652 

liver,  241 
Fats,  description  of,  652 

in  blood,  20 


Fats,  in  stomach,  171 

nitrogenous,  644 
Fenestra  ovalis,  498 

rotunda,  498 
Ferment,  liver,  229 
Ferments,  642 
Fetlock  joint,  513 
Fever,  effect  on  nutrition,  335 
Fibrin,  4,  17 

ferment,  3,  18 
Fibrinogen  in  liquor  sanguinis,  3 

in  lymph,  245 

precipitation  of,  3 
Fibriuo -globulin,  3 
Fibrins,  636 

Fick  on  muscle-work,  365 
Fifth  nerve,  gustatory  division  of,  144 
lingual  branch  of,  133 

pair,  cranial  nerves,  425 
inferior  maxillary,  426 
ophthalmic,  426 
superior  maxillary,  426 
Filtration  in  lymph  formation,  247 
Fischer,  Emil.  on  proteids,  635 

on  purin,  296 
Fishes,  sight  of,  468 
Fleming  on  age-limit  of  procreation, 

585 
Fletcher,    experiments    on    salivary 

glands,  149 
Flourens,  experiments   on  digestion 

in  ruminants,  172 
Foal,  bones  of,  624 

growth  of,  Boussingault,  624 
Focus,  lenses,  482 
Fsjces.  amount  of,  211 

ash  of,  210 

approximate  composition  of,  208 

cow,  208 

horse,  208 

odour  of,  211 

pig,  208 

sheep,  208 
Foetal  circulation,  610 

lung,  88 

membranes,  605 

inspiration,  first,  112 
Follicles  of  Lieberkiihn,  186 
Food,  amount  of,  required,  333 

inorganic,  327 
Foot-and-mouth  disease,  27 
Foot,  the,  537 

anti-concussion,    mechanism   of, 
568 

bars  (hoof),  548 
use  of,  566 

blood-supply,  545 

bones  of  the,  538 

Broad,  laminitis,  treatment  of, 
564 


688     A  MANUAL  OF  VETEllINARY  PHYSIOLOGY 


Foot,  bursa,  navicular,  539 

Clarke,  Bracy,  on  foot  of  horse, 

coronary  substance,  545 
elasticity,  provisions  for,  in  foot, 

559 
Ercolani  on  sweat-gland  in  foot 

of  horse,  557 
expansion,  569 

Lungwitz  on,  571 
Franck  on  sweat-glands  in  foot 

of  horse,  557 
'  frog,'  551 
hoof,  546 

horn,  chemistry  of,  557 
growth  of,  559 
laminse,  549 

origin  of,  564 
salts  of,  558 
structure  of,  552 
use  of  the  moisture  in,  556 
horse,  bones  of,  538 

laminal  tissue,  538 
how  it  carries  the  weight,  561 
joint,  538 
keratin,  558 
laminaj,  area  of,  564 
Clarke,  Bracy,  564 
Gader,  564 
Moeller,  564 
laminal  tissue,  542 
laminitis,  564 

Broad's  treatment,  564 
lateral  cartilages,  541,  567 
Lupton,   J.    Irvine,   on   foot    of 

horse,  559 
Lungwitz  on  expansion  of,  571 
Macdonald  on  horse's,  572 
Mechanism  of  foot,  anti-concus- 
sion. 568 
Moeller    on    sublaminal    tissue, 
543 
laminal  tissue,  565 
navicular  bone,  538 
pad  (frog),  551 
use  of,  566 
pathological,  576 
pedal  bone,  538 

descent  of,  572 
periople,  546 

jihysiological  shoeing,  575 
l^lantar  cushion,  542 
'  side-bone,'  cause  of  lameness  in, 

563 
sole,  550 

use  of,  566 
Storch,  venous  system,   horse's, 

573 
stratum,  periostale  of  sublaminal 
tissue,  543 


Foot,  stratum  vasculosum,  543 

toughness,  provision  for,  in  foot, 

559 
vascular  mechanism  of,  573 
sole,  515 
wall,  542 
wall  -  secreting      substance      of, 

545 
weight,  how  carried  by,  561 
Forarnen  ovale,  611,  613 
Foreign  bodies  in  heart,  54 
Formic  acid,  209 
Foster  on  levers,  506 
Fourth  pair  cranial  nerves,  425 
Franck   on   sweat-glands   in   foot  of 

horse,  557 
'  Frog,'  551 

Frog's  foot,  circulation  in,  65 
Frog,  reflex  action  in.  409 
Functions  of  blood,  1 
of  sjjinal  cord,  425 
nerves,  399 
Fundus  glands,  1 64 
Furfurol,  222 

Galactose,  187,  658 
Gall-bladder,  223,  224 

animals  without,  224 
Gallop  of  horse,  528 
Galton  on  sounds,  496 
Gamgee  on  mastication,  134 
Ganglia,  collateral,  447 

terminal,  447 

vertebral,  446 

on  nerves,  384 
Gases,    absorption    of,    in     liquids, 
97 

of  blood,  23 

Dalton  and  Henry's  law,  97 

diffusion  of,  98 

found  in  the  body,  662 

of  the  intestines,  207 
stomach,  178 

partial  pressure  of  Bunsen,  98 
Gaskell  on  the  heart,  49 

sympathetic  nerves,  446 
Gasserian  ganglion,  426 
Gastric  juice,  secretion  of,  162 
Gastritis,  215 
Gastrocnemius  of  frog,  357 
Gelatin,  639 
Generation,  577 

anijistrous  jjeriod,  577 

ancestrum,  577 

artificial  insemination,  591 

Ascaris  nxegaloce-phala,  589 

Assheton    on    impregnation     in 
sheep,  593 

Balfour  on  polar  bodies,  590 

bear,  578 


INDEX 


689 


Generation,  Beneden,  Van,  on  polar 
bodies,  589 
bovines,  puberty,  period  of,  585 
camel,  rutting  of,  581 
castration,  effects  of,  583 
cat,  518 

ovary  of,  587 
cholesterin  in  spermal  fluid,  584 
corpus  luteum,  590 
cow,  578 

cestrous  cycle,  Goodall,  579 

ovary  of,  587 
cumulus  proligerus,  588 
deutoplasm,  589 
determination  of  sex,  591 
dicestrum,  577 
discus  proligerus,  588 
dog,  578 

ovary,  587 

period  of  puberty  of,  585 
elephant,  rutting  of,  581 
erection,  act  of,  585 
Ewart  on  ovulation  in  mare,  591 
Fleming  on  age  limit  of  procrea- 
tion, 685 
germinal  epithelium,  587 

spot,  589 

vesicle,  589 
Graafian  follicle,  588 
Heape  on  ovary  of  rabbit,  592 

sex  determination,  592 

sexual  season  of  mammals, 
577 
Heuson  on  Graafian  follicle,  589 
horse,  period  of  puberty,  585 

spermatozoa,  584 
impregnation,  593 
inosit  in  spermatic  fluid,  584 
kreatin  in  spermatic  fluid,  584 
lecithin  in  spermatic  fluid,  584 
Leency  on  stag  (rutting),  581 
leucin  in  spermatic  fluid,  584 
lioness,  578 
liquor  folliculi,  588 
mare,  578 

ovary,  587 
Marshall  and  Jolly,  cestrous  cycle 

in  dog,  578 
mechanism  of  ejaculation,  587 
membrana  granulosa,  588 
metcestrous  jJeriod,  577 
metiestrum,  577 
Minot  on  polar  bodies,  590 
raomestrous  mammals,  577 
monkey,        direstrous         cycles 

(Heape),  580 
nuclein  in  spermatic  fluid,  584 
cestrus,  577 

ostium  abdominale,  589 
ostrich,  rutting  of,  581 


Generation,  otter,  578 
ovaries,  587 

effect  of  removal  of,  582 
ovariotomy,  effects  of,  583 
ovulation,  ass,  591 

cat,  591 

cow,  591 

deer,  591 

dog,  591 

elephant,  591 

ferret,  591 

mare,  591 

monkey,  591 

pig,  591 

rabbit,  591 

sheep,  591 

wolf,  591 
ovum,  589 

parthenogenesis,  586 
pig,  578 

cestrous  cycle,  580 

ovary  of,  587 

puberty,  period  of,  585 
poly  cestrous  mammals,  577 
procestrous  period,  577 
proLiestrum,  577 

external  signs  of,  580 
prostate,  secretion  of,  584 
prototheria,  589 
puberty,  period  of,  585 
rabbit,  ovary  of,  587 
seminal  vesicles,  secretion  of,  584 
seminiferous  tubules,  583 
sexual  ova,  592 

intercourse,  586 

season  of  animals,  577 

spermatozoa,  592 
sheep,  578 

impregnation  in,  593 

cestrous  cycle  (Goodall),  479 

ovary  of,  587 

puberty,  periocl  of,  585 
spermatic  fluid,  584 
spermatoblasts,  583 
spermatogen,  583 
spermatozoa,  583 
stag,  rutting  of,  581 
sustentacular  cells,  5S3 
testicles,  583 

effect  of  removal,  582 
tunica  albuginia,  583 

fibrosa,  588 
tyrosine  in  spermatic  fluid,  584 
uterus,  changes  in,  during  pro- 
oestrum,  581 
Van   Beneden  on   polar  bodies, 

589 
vitelline  membrane,  589 
Weissmann  on  polar  bodies,  590 
wolf,  578 

44 


(iOO    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Generation,  zona  radiata,  589 
Oenito-spinal  centre  in  cord,  424 
Germinal  epithelium,  587 

spot,  589 

vesicle,  589 
Gestation,  periods  of,  614 
Glands,  fundus,  164 

gastric,  162 

Harderian,  477 

lachrymal,  477 

labial,  141 

lymphatic,  244 

Meibomian,  477 

molar,  141 

of  Brunner,  186 

palatine,  141 

parotid,  139,  141 

pyloric,  164 

sublingual,  139 

submaxillary,  139 

uterine,  614 
Glia  cells,  cord,  395 
Globin,  11 

Globulicidal  action  of  serum,  25 
Globulins,  636 
Glosso-pharyngeal  nerve,  144,  430 

lingual  branch  of,  133 
Glottis,  94,  124 
Glucosamine,  638 
Glucose,  659 
Glutaminic  acid,  236 
Glycerin,  653 
Glycerol,  653 

Glycero-phosphoric  acid,  645 
Glycine,  221,  222,  292,  645 
Glycocholate  of  soda,  221 
Glycocholic  acid,  222 
Glycocine,  645 
Glycocoll,  222,  293,  645 
Glycogen,  description,  656 

in  leucocytes,  14 

in  liver,  225 

in  mirscle,  227 

sources  of,  228 

use  of,  227 
Glyco-proteids,  638 
Glycosuria,  228,  239 
Goat,  larynx  of,  129 

number  of  respirations,  90 
Golgi,  organ  of,  391 

tendon,  organs  of,  353 
Goll,  column  of,  403 
Goodall  on  hippomanes,  60S 
Gowers'  method  for  number  of  cor- 
puscles in  blood,  5 
Graafian  follicle,  568 
Grandeau  on  amount  of  sweat,  277 

and  Leclerc  on  diet,  333 
Granulose,  142 
Grape-sugar,  659 


Grape-sugar  in  blood,  20 

Grey  horses  and  loss  of  heat,  346 

Growth,  623 

Guinea-pig,  period  of  gestation,  615 

Guttural  pouches,  128,  497 

Haematin,  11 

spectrum  of,  11 
Hsematogen,  8 
Ha?niin,  11 

H;ematopor})hyrin,  11 
Hajmatoidin,  11 
Hfemochromogen,  11 
Haemoglobin,  1,8,  644 

absorption  bands  of,  9 

amount  of,  in  horse's  body,  8 

carboxy-,  10 

compounds  of,  10 

decomposition  of,  11 

Ellenberger's  estimate,  8 

in  blood-corpuscles,  5 

met-,  10 

Midler's  estimate,  8 

nitric  oxide,  11 

oxy-,  7 

reduced,  7 

spectroscope  test,  9 
Hemoglobinuria,  27 

in  horse,  335 
Hcemolysis,  25 
Haemorrhage,  65 

thirst  in,  65 
Hair,  272 

eat,  276 

dog,  276 

horse,  272 

clipping  of,  275 

permanent,  273 

pigment  in,  274 
'  Hair-balls  '  in  rumen  of  cattle,  172, 

210 
Haldane    and    Smith's    process    for 
amount  of  blood,  22 
results,  respiration,  119 

and  Priestley's  experiments,  res- 
piration, 117 
Hammarsten  on  bilirubin,  221 

theory  of  coagulation,  17 
Harderian  gland,  477 
Harmonics,  495 
Hay,  digestion  of,  154 
Heape  on  ovary  of  rabbit,  592 

on  sex  determination,  592 

on  sexual  season  of  mammals,  577 
Hearing,  495.     See  Ear 
Heat,  calorie  of,  324 

kilocalorie  of,  324 

loss,  343 

production,  340 

puncture,  342 


INDEX 


691 


Heat  regulation,  343 
Heart,  28 

accelerator  centre  in  medulla,  49 
action  of  aconitin,  53 

of  adrenalin,  53 

of  atropin,  53 

of  calcium  salts,  52 

of  digitalin,  53 

of  drugs,  53 

of  muscarin,  48,  53 

of  nicotin,  53 

of  physostigmin,  48,  53 

of  pilocarpin,  53 

of  potassium  salts,  52 

of  sodium  salts,  52 

of  valves  of  heart,  39 
aortic  valve,  29 
apex-beat,  non-existent,  39 
auriculo-ventricular  valves,  33 
beat,  cause  of,  51 

of  the,  31 

ratio  of,  to  respiration,  91 
Bicuspid  valve,  33 
blood-pressure,  43 
capacity  of,  42 

Colin,  42 

Munk,  42 
cardiac  cycle,  35 

sounds,  40 
cardiograph,  42 
cat,  depressor  nerve,  50 

vagus  nerve,  45 
cause  of  heart-beat,  51 
cells  of  Purkinje,  31 
Chauveau   and    Marey's   experi- 
ments, 38 

on  valves,  39 
chordae  tendinece,  32,  33 
circulation,  aspiration  in  thorax, 
35 
in  veins,  35 
Colin  on  capacity  of,  42 

on  nervous  mechanism  of,  44 
columnse  carne<e,  32 
coronary  circulation,  53 
corpus  Arantii,  34 
course  of  circulation,  30 
cow,  foreign  bodies  in,  54 
depressor  nerve  of,  50 
diastole  of,  35 
digitalin,  action  on,  53 
dog,  action  of,  54 

intracardiac  pressure,  42 

pressure  in,  37 

vagus  nerve,  45 

valves,  vegetations  on,  54 
fibrous  rings  in,  32 
foreign  bodies  in,  54 
Gaskell's  observations,  49 
horse,  aortic  ring,  32 


Heart,  horse,  depressor  nerve,  50 
endocardium  of,  32 
pleurisy  in,  54 
time  of  cycle,  38 
valvular  disease  in,  54 
horse-power  of,  43 
impulse  of  the,  31,  38 
internal  pressure,  40 

dog,  42 
intracardiac  pressure,  40 
Marey's  law,  48 
moderator  bands,  34 
movements  of,  35 
ilunk  on  horse-power  of,  43 
muscle,  31,  356 
musculi  papillares,  32,  33 
negative  pressure  in,  37 
nerve-centre  for,  48 
nervous  mechanism  of,  44 
mitral  valve,  29,  33 
ox,  aortic  ring  of,  32 
pathological  conditions,  53 
pericarditis  in  horse,  54 
pericardium,  use  of,  39 
pleurisy  in  horse,  54 
pig,  vegetations  on  valves,  54 
pneumogastric  nerves,  45 
position  of  the,  31 
pulmonary  valve,  29 
pulmonic  circulation,  29 
rabbit,  depressor  nerve,  50 
refractory  j^eriod,  52 
relations  of,  31 
rotation  of,  36 
rupture  of,  54 
semilunar  valves,  29,  34 
sigmoid  valves,  34 
sounds  of,  36,  40 
sympathetic,  45,  49 
systemic  circulation,  29 
systole  of,  35 
tricuspid  valve,  29 
vagus  in  neck,  cat,  45 

dog,  45 
valves  of,  29 

action  of,  39 

aortic,  29 

auriculo-ventricular,  33 
right,  29 

bicuspid,  33 

Chauveau  on,  39 

mitral,  29,  33 

pulmonary,  29 

semilunar,  29,  34 

sigmoid,  34 

tricuspid,  29 

use  of  the,  30 
valvular  disease,  horse,  54 
work  of,  43 
Helicotrema,  500 

44—2 


(592     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Ilcidenhain,       lymph       production 
theory,  250 

on  secretory  nerves,  146 
Helmholtz  on  accommodation,  467 
Henle,  ascending  limb  of,  289 

loop  of  (kidney),  289 
Hansen  on  Graafian  follicle,  589 
Henscn's  cells,  501 
Herbivora,  respiratory  quotient  in,  96 

saliva  in,  140 

stomach  absorption  in,  259 

teeth  in,  132 
Hexone  bases,  187 
Hexoses,  660 
Hibernating  animals,  338 
Hibernation,  349 
Hiccough,  130 
Hip-joint,  512 
Hippomanes,  608 

Goodall  on,  608 
Hippuric  acid,  650 

in  blood,  20 

in  urine,  222 
Histidine,  236 
Hock-joint,  508 
Hofmeister  on  bile,  225 

on  gastric  acids,  161 

on  periods  of  stomach  digestion, 
171 
Holoblastic  ova,  597 
Horaoithermal  animals,  337 
Hoof,  546 
Hoppe  Seyler,  analysis  of  pancreatic 

juice,  232 
Hormone,  264 
Horn,  272 

chemistry  of,  557 

growth  of,  559 

laminse,  549 

origin  of,  564 

salts  of,  558 

structure  of,  552 

use  of  the  moisture  in,  556 
Horse,  absorption  in,  257 

act  of  standing,  521 

amble  of,  526 

amount  of  air  required,  113,  116 
of  blood  in,  23 
of  hfemoglobin  in,  8 
of  heat  produced,  351 

aortic  ring,  32 

apopjlexy  of  lungs,  120 

area  of  acute  vision,  464 
of  intestinal  tract,  201 

astigmatism  of,  469 

azoturia  in,  10 

Bell's  experiment,  427 

bile,  action  of,  225 

amount  of,  per  hour,  224 
specific  gravity  of,  21 8 


Horse,  lilood  of,  2,  21 

time  of  clotting,  16 

pressure  in,  63 
boiled  food  for,  171 
broken  wind,  120 
bronchitis,  120 
'  brushing,'  512 
buck -jumping,  532 
canter  of,  526 
capacity  of  intestinal  canal,  201 

of  stomach,  151 
'  cat-hairs,'  273 
chest,  Colin 's  figures,  85 
ciliary  muscle,  461 

and  atropin,  468 
clotting  of  blood  in,  15 
corpora  nigra,  460 
'cow-kicking,'  512,  532 
dandrutt',  283 

development  of  embryo  of,  594 
diaphragm,  86 
'dishing,' 512 
division   of  phrenic  nerves   in, 

112 
draught,  534 
drinking  of,  134 
endocardium,  32 
experimental  paralysis,  444 
eyelashes,  477 
feeding  of,  158 
faces  of,  208 

foot,  bones  of,  538.     See  Foot 
fundus  oculi,  465 
gallop  of,  528 
gastric  glands  in,  162 
growth  of,  Percival  on,  625 
hair  of,  273 
heart,  depressor  nerve  of,  50 

experiments    by   Chauveau 
and  Marey,  38 
intestinal  affections  in,  204 

digestion  in,  187 
iris  of,  458 

Eversbusch  on,  459 
'jibbing,'  450 
jump  of,  530 
kicking  of,  532 
laryngitis,  121 
larynx,  121,  122 
lying  down,  521 
lymph  from,  247 
lymphangitis  in,  335 
mastication,  134 
maximum  muscular  effort,  536 
medicines  by  the  mouth,  185 
muscles  of  eyeball,  471 
myopia  in,  468,  481 
neighing  of,  129 
normal  daily  work  of,  532 
nostril  of,  93 


INDEX 


693 


Horse,  number  of  respirations,  90 
redema  of  legs,  251 
resophagus  of,  137 
old  age  in,  629 
osteoporosis  in,  335 
pancreatic  fluid  of,  232 
paralysis,  larynx,  432 
pericarditis,  54 
period  of  puberty,  585 
periods  of  stomach  digestion,  171 
pilocarpin  in,  281 
plantar  neurectomy  in,  317 
pleural  cavities  of,  84 
pleurisy  in,  54,  120 
pneumonia  in,  120 
pressure,    negative,    in    pleui'al 

cavity,  90 
pulsation  in  jugulars,  57 
pulse-rate  of,  70 
psychical  powers,  affection,  449 
intelligence,  449 
memory,  449 
Rankine  on  normal  daily  work, 

532 
rearing  of,  532 

Redtenbacher  on  daily  work,  533 
refraction,  errors  of,  470 
relation     between     blood-     and 

body- weight,  22 
retina  of,  465 
rising,  522 

roaring,  120,  126,  431 
rupture  of  the  diaphragm,  120 
saliva  of,  143 
salivary  glands  of,  139 
sense  of  smell,  488 
spasm  of  the  diaphragm,  120 
spermatozoa,  584 
sphyguiogram  of,  69 
'speedy -cutting,'  512 
'  staleness,'  375 
Stillman  on  motion,  506 
stomach  of,  149 

absorption  from,  176 

digestion  in,  150 
sugar  formation  in,  170 
sweat,  composition  of,  278 
sweating  of,  276 
temperature  of,  338 
tendon  reflex  in,  423 
thirst  in,  494 
thrombosis  of  iliac  arteries  in, 

281 
time  of  circulation  circuit,  74 
tongue  of,  133 
urine  of,  colour,  306 

odour,  306 

quantity,  305 

salts  in,  801 

solids  in,  306 


Horse,  urine  of,  specific  gravity,  306 

valvular  disease,  54 

velocity  of  gallop,  533 
of  trot,  533 

voice  production  of,  127 

vomiting  of,  138,  178 

walk  of,  523 

'  wall-eyed,'  458 

watering  of,  158 

weight  he  can  carry,  534 

Zuntz    and   Lehmann's    experi- 
ments, 105 
Horse-power,  365 

Watt  on,  535 
Hunger,  494 
Hyaline  leucocytes,  12 
Hydrobilirubin,  11,  12 
Hydrochloric  acid,  161 
Hydrogen,  632 

in  expired  air,  97 

in  large  intestine,  208 

in  nutrition,  315 

in  stomach,  178 
Hyoglycocholic  acid,  221 
Hyotaurocholic  acid,  221 
Hypermetropia,  469 
Hyperpncea,  105 
Hypoblast,  structures  derived  from, 

599 
Hypogastric  plexus,  205 
Hypoglossal  nerve,  133,  138 
Hypoxanthine,  296,  379 

Hiac  arteries,   thrombosis   in  horse, 

281 
Heum,  function  of,  189 
Impaction,  217 
Impregnation,  593 
Impulse  of  the  heart,  31 

how  given,  38 
Inco-ordinate  movements,  409 
Incus,  497 
Indican,  300,  651 
Indigo  series,  651 
Indol,  236,  651 

in  digestion,  189,  199,  209 

in  urine,  300 
Indoxylsulphuric  acid,  651 
Inferior  laryngeal  nerve,  431 
Inflammation,  essential  changes  in,  66 
Influence  of  heat  and  cold,  346 

of  nervous  system  on  heat  pro- 
duction, 341 

of  vagus  on  respiration,  110 

of  work  on  respiration,  117 
Inogen,  372 

Inorganic  constituents  of  the  body, 
662 

substances  in  urine,  301 
Inosit,  660 


694  A  MANUAL  OF  YETEPJNAKY  PHYSIOLOGY 


Inosit  in  spermatic  liuid,  584 
Insensible  perspiration,  277 
Inspiration,  84 

cause  of  the  first,  112 

muscles  of,  89 
Inspiratory  centre,  108 

tetanus,  111 
Inspired  air,  composition  of,  95 
Instinct  in  animals,  452 
Intelligence  in  animals,  452 
Internal  intercostals  in  respiration,  89 

respiration,  99 

secretions,  264 
Intestinal  absorption,  259 

calculi,  210 

canal,  nervous  mechanism  of,  205 

digestion,  186 
in  horse,  187 
in  other  animals,  201 
in  ruminants,  199 
Intestines,  gases  of  the,  207 

movements  of,  201 
Intracardiac  pressure,  40 
Intracellular  enzymes,  336 
Invert  sugar,  658 
Inverting  ferments,  invertase,  187 
lactase,  187 
maltase,  187 
Involuntary  muscle,  355 
lodothyrin,  269 
Ipecacuanha,  180 
Iris,  457,  458 

Langley  and  Anderson  on,  458 
Iron,  634 

in  blood,  21 

in  the  body,  665 

in  red  cells,  327 

phosphate  of,  in  bile,  219 
Irradiation  in  reflex  action,  408 
Irritability  of  muscle,  356 
Islands  of  Langerhans,  240 

Jacobson,  organ  of,  487 
Jaundice,  240 
Joints,  507 

elbow,  512 

fetlock,  513 

foot,  538 

hip,  512 

hock,  508 

knee,  512 

shoulder,  512 

stifle,  511 
Jugular  vein,  horse,  velocity  of  blood 
in,  72 
pulse  in,  57 
Jump  of  horse,  530 

Katabolism,  316 
Kathelectrotonus,  387 


Katoptric  test,  467 
Keratin,  558,  639 
Kicking  of  horse,  532 
Kidney.     See  Urine 

amount  of  blood  through,  289 

Malpighian  tufts,  286 

movements  of,  286 

oncometer  of  Roy,  286 

pathological,  313 

structure  of,  286 

uriniferous  tubules,  286 
Kinase,  234 
Knee-jerk,  423 

-joints,  512 
Koumiss,  621 
Krause,  end-bulbs  of,  391 
Kreatin,  in  spermatic  fluid,  584 
Krypton,  95 
Kiihne,  pancreas  of  rabbit,  238 

table  of  peptones,  168 

Labial  glands,  141 
Labyrinth,  497,  502 

and  muscle  tonus,  379 
Lachrymal  gland,  477 
Lactalbumin,  620 
Lactase,  187 
Lacteal  vessel,  254 
Lactic  acid,  653 

in  allantoic  fluid,  608 

(digestion),  161,  209 
Lactose,  620,  658 
'  Laky  '  blood,  6 
Lameness,  production  of,  516 
Lamina  spiralis,  499 
Laminpe,  area  of,  Bracy  Clarke,  564 

Moeller,  564 

Gader,  564 
Laminal  tissue,  542 
Laminitis,  Broad's  treatment,  564 
Lang  and  Barrett  on  ciliary  muscle, 
468 

on  errors  of  refraction,  470 
Langerhans,  islands  of,  240 
Langley  and  Anderson  on  iris,  458 

on  gastric  glands,  164 

on  nerve  fibres,  446 

on  secretion,  147 
Lanolin  in  dandruff,  283 
Lapping  of  dog,  134 
Large  intestine,  190 
Laryngitis,  horse,  121 
Larynx,  the,  122 

cat,  129 

dog. 129 

goat,  129 

horse,  muscles  of,  121 

nervous  mechanism  of,  125 

ox,  129 

sheep,  129 


INDEX 


695 


Larynx,  the,  ventricles  of,  129 
Lateral  cartilages,  541,  567 
Lathyrus  sativus,  poisoning  by,  432 
Latissimns  dorsi  in  respiration,  89 
Law  of  Dal  ton  and  Henry,  97 
Lawes  and  Gilbert,   on  composition 
of  body,  314 
on  storage,  328 
Lea,  Sheridan,  on  pancreas,  238 
Lecithin,  220,  645 

in  blood,  20 

in  leucocytes,  14 

in  bile,  219,  220 

in  red  corpuscle,  5 

in  spermatic  fluid,  584 
Leech  extract,  in  coagulation,  20 
Leeney  on  rutting  of  stag,  581 

on  ovarian  tissue,  265 
Lens,  457,  458 

passage  of  light  through,  481 
Letting  blood,  effects  of,  27 
Leucin,  209,  236,  293,  318,  584,  646 
Leucocytes,  12 

cholesterin  in,  14 

composition  of,  13 

diapedesis  of,  18 

glycogen  in,  14 

lecithin  in,  14 

origin  of,  14 

proportion  of,  12 

varieties  of,  12 
basophile,  13 
eosinophile,  13 
hyaline,  12 
lymphocytes,  13 
polynuclear,  12 
Leucocythsemia,  296 
Levatores  costarum  in  respiration,  89 
Levers,  505 

Foster  on,  506 
Levulose,  187,  657,  660 
Lieberkiihn,  follicles  of,  186 
Ligamentum  inhibitorium,  iris,  460 

pectinatum,  456,  460 
Lignin  in  diet  of  herbivora,  207 
Limbs,    function   of,    in   relation   to 
lameness,  516 

structvire  of,  in  relation  to  lame- 
ness, 516 
Lingual  nerve,  434 
Lion,  intestinal  canal,  201 
Lioness,  generation,  578 
Lipase,  234,  236,  325 
Lips,  horse,  131 

ox,  131 

pig,  131 

sheep,  131 
Liquids,  absorption  of  gases  in,   97 
Liquor  amnii,  606 

folliculi,  588 


Liquor  sanguinis,  2 

fibrinogen  in,  3 

paraglobulin  in,  3 

serum  albumin  in,  3 
globulin  in,  3 
Liver,  218 

abscess  of,  241 
acid,  benzoic,  222 

cholalic,  221 

glycocholic,  222 

hippuric,  222 

hyoglycocholic,  221 

hyotaurocholic,  221 

sulphuric,  231 
Bernard,    Claude,   on  glycogen, 

225 
bile  acids,  origin  of,  222 

amount  of,  secreted,  223 

Ellenberger  on,  219 

Gmelin's  test,  220 

Hofmeister  on,  225 

of  horse,  218 

of  ox,  218 

of  sheep,  218 

percentage  composition,  219 

Pettenkofer's  test,  222 

pigments,  220 

salts,  221 

use  of,  224 

Voit's  experiments,  225 
biliary  calculi,  241 
bilirubin,  220 

Hammarsten  on,  221 
biliverdin,  220 
blood,  sugar  in,  226 
blood-supply  of,  218 
Bunge  on  bile  pigments,  220 

on  changes  in,  231 
calcareous  degeneration  of,  241 
calculi,  biliary,  241 
centre,  diabetic,  230 
cholalic  acid,  221,  222 
cholesterin,  219,  220 
diabetes,  226 

centre,  230 

puncture,  230 
disorders  of,  in  tropics,  321 
dog,  bile,  action  of,  225 

amount   of,    per   hour, 
224 
Ellenberger  on  bile,  219 
enlargements  of,  241 
fatty.  241 
ferment,  229 
furfurol,  222 
gall-bladder,  223,  224 

animals  without,  224 
glycine,  221,  222 
glycocoU,  222 
glycogen,  225 


696     A  MANUAL  OF  VETEKINARY  PHYSIOLOGY 


Liver,  glycogen  in  muscle,  '227 
sources  of,  228 
use  of,  227 
glycocliolate  of  soda,  221 
glycocholic  acid,  222 
glycosuria,  228 
Gmelin's  test  for  bile,  220 
Hammarsten  on  bilirubin,  221 
hippuric  acid,  222 
histidine,  230 
Hofnieister  on  bile,  225 
horse,  bile,  action  of,  225 

amount   of,    per   hour. 

224 
specific  gravity  of,  218 
hyoglycocholic  acid,  221 
hyotaurocholic  acid,  221 
iron,  phospliate  of,  in  bile,  219 
jaundice,  240 
lecithin,  219,  220 
nucleo-alburain  in  bile,  219 
ox,  bile,  action  of,  225 

amount   of,   per   hour, 

224 
specific  gravity,  218 
sulphur  in,  219 
parasitic  disease  of,  241 
pathological  conditions,  240 
rupture  of,  241 
Pettenkofer's  test  for  bile  acids, 

222 
phloridzin,  228 
pig,  bile,  action  of,  225 

amount   of,    per   hour, 

224 
sulphur  in,  219 
puncture,  diabetic,  230 
secretin,  223 
sheep,  bile,  action  of,  225 

amount   of,   per   hour, 

224 
specific  gravity  of,  218 
sulphur  in,  219 
soda,  glycocliolate,  221 

taurocholate,  221 
stercobilin,  210,  221 
sugars,  conversion  of,  228 
in  the  blood,  226 
supply,  how  regulated,  229 
sulphuric  acid,  231 
sulphur  in  bile,  219 
taurine,  222 

taurocholate  of  soda,  22] 
urea  in,  231 

Voit's  experiments,  liile,  225 
Living  test-tube  experiment,  19 
Llama,  stomach  of,  182 
Locomotion,  522 
Locomotor  apparatus,  505 

amble  of  horse,  526 


Locomotor  apparatus,  anti-concussion 

mechanisms,  518 
astragalus,  screw  action  of, 

508 
'  brushing  '  of  horse,  512 
buck -jumping  of  horse,  532 
canter  of  horse,  526 
'  capped  elbow,'  horse,  522 
cattle,  repose  of,  522 
centre  of  gravity,  horse  at 

rest,  514 
Chauveau  on  tendon   flexor 

metatarsi,  509 
check    ligaments,    function 

of,  514 
Colin  on  centre  of  gravity, 

horse,  515 
compression  of  propulsion, 

517 
concussion  of  impact,  517 
CO  -  operative     antagonism, 

muscles,  506 
corona,  fracture  of,  519 
'cow-hocks,'  horse,  512 
'  cow-kicking '  of  horse,  532 
'  dishing  '  of  horse,  512 
distribution  of  the  weight 

of  the  body,  515 
draught,  534 

Brunei  on,  535 
fetlock -joint,  513 
Foster  on  levers,  506 
gallop  of  horse,  528 
hip-joint,  512 
hock -joint,  508 
'horse-power,'  Watt  on,  535 
jump  of  horse,  530 
kicking  of  horse,  532 
knee-joint,  512 
lameness,  production  of,  516 
lying  down,  horse,  521 
levers,  505 

limbs,  function  and   struc- 
ture   of,    in    relation    to 

lameness,  516 
locomotion,  522 
Lupton  on   paces  of  horse, 

530 
Marey    on     locomotion    in 

horse,  522 
maximum    muscular    effort 

of  horse,  535 
mechanisms,     anti  -  concus- 
sion, 518 
Muybridge    on    locomotion 

in  horse,  522 
normal  daily  work  of  liorse, 

532 
pastern,  fracture  of,  519 
pathological,  536 


INDEX 


697 


Locomotor   apparatus,    Rankine    on 
daily  work,  horse,  533 
rearing  of  horse,  532 
Redtenbacher  on  daily  work, 

horse,  533 
rising  of  horse,  522 
spavin,  position  of,  510 
'speedy-cutting,'  horse,  512 
shoulder-joint,  512 
standing,  act  of,  521 
Stanford   on  locomotion  in 

horse,  522 
stifle-joint,    discussion    of, 
508 
description  of,  511 
Stillman    on     function     of 
suspensory  ligament, 
513 
on  locomotion  in  horse, 

522 
on   muscles  of  propul- 
sion, 506 
sutfraginis,  fracture  of,  519 
suspensory  ligament,   func- 
tion of,  513 
synovia,  507 
trot  of  horse,  524 
velocity  of  gallop,  533 

of  trot,  533 
walk  of  horse,  523 
Waller's    '  co-operative   an- 
tagonism,' 506 
weight  of  the  body,  distri- 
bution of,  515 
which  a  horse  can  carry, 
534 
Luciani  on  cerebellum,  438 
Lungs,  84.     See  Respiration 

apoplexy  of,  in  horse,  120 
Lungwitz  on  expansion  of  foot,  571 
Lupton,  J.  L ,  on  foot  of  horse,  530, 

559 
'  Luxus    consumption  '   of  nitrogen, 

321 
Lying  down,  horse,  521 
Lymph,  242,  245 
capillary,  243 
cell    distinguished    from    white 

corpuscle,  12 
Colin  on,  246 
formation  of,  247 
movement  of,  251 
Colin  on,  252 
"Weiss  on,  253 
plasma,  246 

production,  Heidenhain  on,  250 
physical  theory,  247 
secretory  theory,  250 
Starling  on,  249 
quantity  of,  246 


Lymph  spaces,  242 

sinus,  245 
Lymphagogues,  250 
Lymphangitis,  321 
Lymphatic  glands,  244 

vessels,  243 
Lymphocytes,  13 
Lysatinine,  646 
Lysine,  236 

Macdonald  on  foot  of  horse,  572 
Macula  acustica,  502 
Magnesium  in  urine,  301,  303 
phosphate,  21,  212,  219.  233 
salts,  212,  219,  665 
sulphate,  action  on  plasma,  3 
Majendie's    experiment    (vomiting), 

180 
Malaria  parasite,  27 
Malassez's    metliod    for    number   of 

corpuscles,  5 
Malic  acid,  209 
Malleus,  497 
Maltase,  187 
Maltose,  187,  656,  658 
Mare,  analysis  of  milk,  620 
generation,  578 
ovary  of,  587 
period  of  gestation,  614 
position  of  fcetus  in,  615 
uterine  glands  in,  614 
Marey's  law,  circulation,  48 

observations,      locomotion       in 
horse,  522 
Marmot,  hibernation  in,  350 
Marsh  gas  in  cellulose  digestion,  198 
in  expired  air,  97 
in  large  intestine,  208 
in  stomach,  178 
JIarshall  and  Jolly,  cestrous  cycle  in 

dog.  578 
Masseter  muscle,  136 
Mastication,  dog,  134 
Gamgee  on,  135 
horse,  134 
nerves  of,  136 
ox,  134 
sheep,  134 
Maximum  muscular  effort,  horse,  535 
Medulla  oblongata,  434 

centres,  cardiac,  48,  49 
diabetic,  230 
mastication,  436 
respiratory,  108 
saliva,  436 
swallowing,  436 
sweat,  280 
vaso-motor,  75 
vomiting,  436 
Medullary  groove,  600 


698     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Medullary  sheath,  nerves,  383 
Mechanism,  anti-concussion,  518 
of  foot,  568 

defensive,  of  the  body,  25 

of  ejaculation,  587 

nervous,  of  the  intestinal  canal, 
205 
of  the  larynx,  125 
of  sweating,  279 

pancreatic  secretion,  233 

rumination,  183 

secretion  of  saliva,  144 
Meconium,  212 
Meibomian  glands,  477 
Melanin,  274,  644 
Merabrana  basilaris,  499 

granulosa,  58S 

nictitans,  454 
Membrane  of  Reissner,  500 
Mendel's  theories  of  heredity,  274 
Meroblastic  ova,  597 
Mesentery,  circulation  in,  65 
Mesoblast,  structures  derived  from, 

599 
Metabolism,  316 

views  of  Pfluger,  318 
of  Voit,  318 
Metatarsal  artery,  horse,  velocity  of 

blood  in,  72 
Methpemoglobin,  10,  27 
Methylene  blue  experiment,  102 

absorption    of,     from    pleura, 
259 
Methyl-glycine,  645 
Metoestrous  period,  577 
Metcestrura,  577 
Metschnikotf,  researches  of,  14 
M'Kendrick   on    area    of    intestinal 
tract,  201 

on  taste  goblets,  489 
Micrococcus  urcce,  648 
Micturition,  act  of,  313 
Mid-brain,  439 
Middle  ear,  497 

JMigration  of  white  corpuscles,  66 
Milk,  secretion  of,  617 

sugar,  187,  620 
Millon's  reagent,  641 
Minot  on  polar  bodies,  590 
Mitral  valve,  29,  33 
Moderator  bands  (heart),  34 
Moeller  on  laminse,  565 

on  sublamiual  tissue,  543 

on  urine  of  calves,  309 
Moisture  iu  air,  95 
Molar  glands,  141 
Monojstrous  mammals,  577 
Monkey,  diojstrous  cycle  of,  580 
Monocular  vision,  473 
Monosaccharides,  659 


Morgan,  on  intelligence,  instinct,  and 

reason,  452 
Morphia,  absorption  of,   by  air-pas- 
sages, 257 

effect  on  iris,  459 
Motor  areas  of  brain,  441 

oculi  nerves,  425 
Movements  of  diaphragm,  85 

of  eyeball,  470 

of  heart,  35 

of  intestines,  201 

of  stomach,  184 
Mucin,  140,  152,  165 
Mule,  psychical  powers  of,  449 

subepiglottic  sinus  of,  129 

sweating,  277 
Miiller's  estimate  of  htemoglobin,  8 
Munk  on  capacity  of  heart,  42 

on  horse-power  of  heart,  43 

on  ox  urine,  308 

on  phosphates  in  urine,  304 

statistics  of  intestinal  canal,  201 
Murmur,  respiratory,  119 

vesicular,  119 
Muscarin,  645 

action  on  heart,  48,  53 
Muscle  antagonism,  354 

currents,  366 

curve,  360 

nerve  preparation,  357 

plasma,  377 

sense,  353,  492 

wave,  361 
Muscles  of  eyeball,  471 

of  respiration,  89 
Muscular  system,  352 

acid,  sarco-lactic,  374,  378,  379 
uric  (muscle),  379 

active  muscles,  changes  in,  370 

ash,  composition  of  (muscle),  379 

carnine,  379 

causation    of    a    muscular    con- 
traction, 372 

changes   in   active   and    resting 
muscles,  369 

Chauveau  on  muscle  work,  365 

chemical    changes   during    con- 
traction of  muscle,  370 
composition  of  muscle,  377 
engine  (muscle  as),  366 

Colin    on    muscle   tempei'ature, 
371 

comparisons,  body   and   engine, 
365 

condition  in  horses,  375 

contractility  of  muscle,  356 

creatine,  379 

curare,  effect  on  end-plates,  354, 
361 

elasticity,  muscle,  363 


INDEX 


699 


JM.uscular     system,     elastic     tension 
muscles.  364 
electric  phenomena  of  muscles. 

366-369 
effect     of    muscular     work     on 

blood,  2 
end-plate  in  muscle,  354 
extensibility  of  muscle,  363 
extractives  (muscle),  379 
fatigue  (muscle),  372 

Wedenski  on,  375 
Fick  on  muscle  work,  365 
gastrocueraius  of  frog,  357 
Golgi,  tendon  organs  of,  351 
heart  muscle,  356 
horsd-jjower,  365 

'  staleness,'  375 
hypoxanthine,  379 
inogen,  372 

involuntary  muscle.  355 
irritability  of  muscle,  356 
labyrinth  and  muscle  tonus,  379 
muscle  antagonism,  354 

currents,  366 

ciu-ve,  360 

-nerve  preparation,  357 

plasma,  377 

sense,  353 

wave,  361 
myosin,  378 
myosinogen,  378 
neuro-muscular  spindles,  353 
pale  muscle,  355 

phenomena    of    cf>ntraction 
of,  380 
phosphoric  acid  (muscle),  379 
potassium  salts,  379 
resting  muscle,  changes  in,  379 
rigor  mortis,  379 
sarcolemma,  352 
sarcomere,  353 
sarcoplasm,  353 
sarcostyles,  353 
sarco-lactic  acid,  374,  378,  379 
Sch;ifer's  views  on  muscle,  353 
smooth    muscle,    phenomena    of 

contraction,  380 
structure  of  muscle,  352 
summation  of  contractions,  361, 

380 
taurine,  379 
tetanus,  362 
'  tone,'  364 
'  training,'  377 
urea  (muscle),  379 
voluntary  muscle,  352 
xanthine,  379 
Zunt;^  on  muscle  work,  365 
Musculi  papillares,  32,  33 
Musical  sounds,  495 


Muybridge  on  locomotion  in  horse, 

522 
Myopia,  468 
Myosin,  378 
Myosinogen,  378 

Nasal  chamber,  olfactory  portion,  93 

respiratory  portion,  93 
Nasse  on  time  of  coagulation,  16 
Nature  of  nervous  impulses,  389 
Navicular  bone,  538 
Negative  pressure  in  heart,  37 

in  pleural  cavity,  dog,  90 
horse,  90 
sheep,  90 

in  respiration,  90 
Negative  pressure  in  veins,  64 

variation  (nerve),  386 
Negro's  skin,  346 
Neighing  of  horse,  129 
Neural  groove,  600 
Neurilemma,  383 
Neurine,  645 
Neuroglia,  395 

Neuro-muscular  spindles,  353,  492 
Neurone,  412 
Neutral  point  (nerve),  387 
Nerve  centre  for  heart,  48 

brachial,  78 

chorda  tympani,  76,  78,  133 

facial,  108 

fifth,  in  mastication,  136 

lingual  branch  of,  133 

ganglia.     See  Ganglia 
salivary  glands,  147 

hypoglossal,  133.  138 

supply  of  stomach,  185 
Nerves,  afferent,  382 

deglutition  of,  138 

dilator,  78 

dorso-lumbar,  108 

glosso-pharyngeal,  110 

lingual  branch  of,  133 

hypogastric  plexus,  205 

mastication,  136 

motor,  of  respiration,  108 

nasal  of  fifth,  110 

ocular  muscles,  472 

pharyngeal  plexus,  138 

phrenic,  108 

division  of.  112 

recurrent  laryngeal,  125,  138 

sciatic,  78 

seventh  pair,  division  of,  113 
in  mastication,  136 

superior  laryngeal,  110,  125,  138 

vaso-constrictor,  77 

sympathetic,  144 

tongue,  133 
Nervi  ei'igeutes,  78 


700     A  MANUAL  OF  VETEKINARY  PHYSIOLOGY 


Nervi  nervorum,  385 
Nervous  control  of  bloodvessels,  77 
of  gastric  juice,  169 
of  pancreatic  secretion,  233 
of  rumination,  183 
of  secretion  of  saliva,  144 
of  heat  jiroduction,  341 
Nervous  mechanism  of  heart,  44 
of  the  intestinal  canal,  205 
of  the  larynx,  125 
of  stomach  of  ruminants,  186 
of  respiration,  108 
of  sweating,  279 
Nervous  system,  382 

abducens  cranial  nerve,  428 
afferent  nerves,  382 

paths  in  the  cord,  405 
anelectrotonus,  387 
ano-spinal  centre  in  cord,  424 
arrangement  of  the  cord  (spinal), 

392 
Arloing  on  sympathetic  nerves, 

448 
ass,  psychical  powers  of,  449 
ascending  tracts   (spinal   cord), 

402 
automatic  action,  423 
axis-cylinder,  nerves,  383 
axone,  412 

Bell's  experiment,  horse,  427 
Bernard  on  division  of  facial,  429 
brain,  circulation  in,  444 

coverings  of,  444 
brain,  movements  of,  445 
bulb,  functions  of,  434,  437 

used  for  medulla  oblongata, 
404 
Burdach,  column  of,  403 
cat,   section  of  corpora  quadri- 
gemina,  439 
submaxillary  ganglion,  427 
sympathetic,  447 
centres  in  the  medulla,  435 
cerebellum,  438 

Luciani  on,  439 
cerebral  fluid,  445 
cerebrum,  440 
Chauveau  on  velocity  of  nerve 

impulses,  389 
chloral  hydrate,  effect  on  brain, 

445 
chloroform,  effect  on  brain,  445 
cry  of  men  and  animals,  440 
chorda  tympani  of  facial,  428 
cilio-spinal  centre  in  cord,  424 
Clarke's    column   (spinal    cord), 

395 
Colin  on  experimental  paralysis, 

horse,  444 
conductivity  of  nerves,  389 


Nervous   system,  co-ordinate   move- 
ment, 409 
corpora  quadrigemina,  439 
corpora  striata,  437 
cranial  nerves,  425 

eighth,  429 

eleventh,  434 

fifth,  425 

fourth,  425 

ninth,  430 

seventh,  428 

sixth,  428 

tenth,  430 

third,  425 

twelfth,  434 
current  of  action,  386 

of  rest,  386 
degeneration  of  nerves,  389 

Wallerian,  400 
dendrites,  412 
descending  tracts  (spinal  cord), 

402 
distribution  of    nerve   fibres   in 

the  cord,  396 
disynaptic  arc,  416 
dog,  brain,  localization,  441 

psychical  powers,  449 

reflex  action  in,  409 

submaxillary  ganglion,  427 

sympathetic,  447 

tendon  reflexes  in,  423 
eflerent  nerves,  382 

paths  in  the  cord,  405 
effector  organ,  412 
eighth  pair  cranial  nerves,  429 
electric    phenomena  of    nei'ves, 

385 
electrotonus,  387 
elephant,  psychical  powers,  449 
ether,  effect  on  brain,  445 
eleventh  pair  cranial  nerves,  434 
erection  centre  in  cord,  424 
excitability  of  nerves,  385 
experimental     paralysis,    horse, 

444 
facial  nerve,  428 
fifth  pair  cranial  nerves,  425 

inferior  maxillary,  426 

ophthalmic,  426 

superior  maxillary,  426 
fourth  pair  cranial  nerves,  425 
frog,  reflex  action  in,  409 
functions  of  spinal  cord,  425 

nerves,  399 
ganglia,  collateral,  447 

terminal,  447 
ganglia  on  nerves,  384 
Gaskell  on  sympathetic  nerves, 

446 
Gasserian  ganglion,  426 


INDEX 


701 


Nprvous  system,  genito-spinal  centre 
in  cord,  424 
glia  cells,  cordj  395 
glosso-pharyngeal  nerve,  430 
GoU,  column  of,  403 
Golgi,  oi'gan  of,  391 
liorse,  Bell's  experiment,  427 

experimental  paralysis,  444 

'jibbing,'  450 

paralysis  of  larynx,  432 

psychical  powers,  444 

memory,  444 

affection,  444 

intelligence,  444 

roaring  in,  4,  31,  126 

tendon  reflex  in,  423 
inco-ordinate  movements,  409 
inferior  laryngeal  nerve,  431 
influence  on  cii'culation,  74 
instinct  in  animals,  452 

V.  reason,  452 
intelligence  in  animals,  452 
irradiation  in  reflex  action,  408 
kathelectrotonus,  387 
knee-jerk,  423 
Krause's  end-bulbs,  391 
Langley  on  nerve  fibres,  446 
larynx,  paralysis  in  horses,  432 
Latki/na  satirus,  poisoning,  432 
lingual  nerve,  434 
Luciani  on  cerebellum,  438 
medulla  oblongata  centres,  434 

cardiac,  48.  49 

diabetic,  230 

masticatory,  436 

respiratory,  108 

saliva,  436 

swallowing,  436 

vaso-motor,  75 

vomiting,  436 
medullary  sheath,  383 
mid-brain,  439 
Morgan  on  intelligence,  instinct, 

and  reason,  452 
motor  ai'cas,  brain,  441 

oculi  nerves,  425 
mule,  psychical  powers,  449 
nature  of  nervous  imimlses,  389 
negative  variation  (nerve),  386 
nerves,  afl'erent,  382 
nervi  nervorum,  385 
nervus  intermedins  of  facial,  428 
neurilemma,  383 
neuroglia,  395 
neurone,  412 
neutral  point  (nerve),  387 
nicotin,  action  on  nerve-cells,  446 
ninth  pair  cranial  nerves,  430 
nociceptive  arc,  417 
ox,  sympathetic,  448 


Nervous  system,  parturition  centre  in 

cord,  424 
pathetic  nerve,  425 
pars  trigemini  nerve,  425 
perikaryon,  412 
pharyngeal  nerve,  431 
pneumogastric  nerve,  430 
pons  Varolii,  437 
portio  dura,  428 
mollis,  429 
principle  of  the  common  path, 

411 
psychical    powers    of    animals, 

448-9 
rabbit,  sympathetic,  447 
reason  in  animals,  452 

;'.  instinct,  452 
receptor  organ,  412 
recurrent  laryngeal  nerve,  431 

sensibility,  399 
reflex    act,    time    occupied    by, 
422 
action,  409 
arc,  411 
roaring  in  horses,  126,  431 
Romanes  on  instinct  and  reason, 

452 
scratch  reflex,  414 
sensory  areas,  brain,  441 
seventh  pair  cranial  nei'ves,  428 
sixth  pair  cranial  nerves,  428 
Smith,  Sydney,  on  instinct  and 

reason,  452 
special  centres  in  the  spinal  cord, 

424 
spinal  accessory  nerves,  434 
cord,  391 
nerves,  392 

function  of,  399 
stepping  reflex,  410 
structure  of  nerves,  383 
strychnine,  eflect  on  brain,  445 
submaxillary  ganglion,  cat  and 

dog,  427 
superior  laryngeal  nerve,  431 
sympathetic,  446 
tactile  cells,  391 
sweat  centres  in  cord,  424 
synapses,  412 
tendon  reflexes,  423 
tenth  pair  cranial  nerves,  430 
termination  of  nerves,  391 
thalami  optici,  437 
third  pair  cranial  nerves,  425 
tracts  in  spinal  cord,  401,  403 
trophic  centres  in  cord,  424 
Tiirck,  column  of,  403 
twelfth  pair  cranial  nerves,  434 
vaso-motor  centres  in  cord,  424 
velocity  of  nerve  itnpulses,  389 


702    A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Nervous  system,  vesico-spinal  centre 
in  cord,  424 
Wallerian    degeneration    (spinal 
nerve),  400 
Nervus  intermedins  of  facial,  428 
Newaom,  calculation,  hair  of  horse, 

273 
Nicotin,  action  on  heart,  53 
on  nerve  cells,  446 
in  peristalsis,  203 
in  secretion,  147 
Ninth  pair  cranial  nerves,  430 
Nitre  in  veterinary  practice,  282 
Nitric  oxide  hsmoglobin,  11 
Nitrogen,  95,  633 
in  blood,  23,  25 
in  nutrition,  315 
in  stomach,  178 
Niti'ogenous  equilibrium,  319,  320 
food,  318 
fats,  644 

substances  in  urine,  292 
Nociceptive  arc,  417 
Noise,  496 
Non-nitrogenous  bodies,  652 

food,  322 
Normal  temperature  of  animals,  338 
Nostrils,  92 
false,  92 
Notochord,  600 

Nuclein  in  spermatic  fluid,  584 
Nucleo-albumin  in  bile,  219 

-proteid,  3,  637 
Number  of  respirations,  90 
Nutrition,  314 

amount  of  food  required,  332 

anabolism,  316 

arginine,  318 

azoturia,  321 

broken  wind,  120,  335 

calories  of  heat,  324 

carbohydrates,  oxidation  of,  323 

carbon  in,  316 

castration,  effect  of,  on  fattening, 

326 
cat,  composition  of  body,  315 
cattle,  osteomalacia  in,  327 
circulating  proteid  of  Voit,  319 
composition  of  the  body,  314 
expenditure  and  income  of  the 

body,  315 
fever,  effect  on  nutrition,  335 
food,  amount  of,  required,  333 

inorganic,  327 
Grandeau  and  Leclerc  on  diet,  333 
hsemoglobinuria  in  horse,  335 
heat,  calorie  of,  324 

kilocalorie  of,  324 
horse,  lymphangitis  in,  335 
osteoporosis  in,  335 


Nutrition,  horse,  plan  tar  neurectomy, 
317 
hydrogen  in,  315 
iron  in  red  cells,  327 
katabolism,  316 

Lawes  and  Gilbert  on  composi- 
tion of  body,  314 
on  storage,  328 
leucine,  318 
lipase,  325 
liver      disorders      of       tropical 

climates,  321 
'  luxus  consumption '  of  nitrogen, 

3-21 
lymphangitis,  321 
metabolism,  316 
Pfliiger  on,  318 
Voit  on,  318 
nitrogen  in,  315 

nitrogenous     equilibrium,     319, 
320 
food,  318 
non-nitrogenous  food,  322 
pig,  composition  of  body,  314 
obesity  in  show  cattle,  326 
ox,  composition  of  body,  314 
pathological  disorders  of  nutri- 
tion, 335 
potassium  in  red  cells,  327 

in  sweat,  327 
Rubner's  experiments,  332 
salts  in  nutrition,  316 
sheep,  composition  of  body,  314 
sodium  in  blood  plasma,  327 
starch,     proteid-sparing     action 

of,  320 
starvation,  329 
storage  of  tissue,  328 
subsistence  diet,  333 
sulphur  in  hair,  327 
in  nutrition,  316 
tissue  proteid  of  Voit,  319 

storage  of,  328 
tyrosine,  318 

Voit's  theory,  metabolism,  318 
water  in  tissues,  328 
Wolff  on  diet,  334 
Nux  vomica,   absorption  of,   by  air- 
passages,  257 
in  csecum,  260 

Oats,  digestion  of,  156 
Obesity  in  show  cattle,  326 
Odour  of  blood,  2 
Gidema,  production  of,  251 
Esophageal  groove,  180 
Oesophagus  of  horse,  137 

of  dog,  138 

of  ox,  138 

of  sheep,  138 


INDEX 


703 


Qistrus,  577 
Oleic  acid,  652 
Olein,  653 

in  blood,  20 

ill  milk,  620 
Omasum,  174 

Omnivora,  respiratory  quotient  in,  96 
Oncometer  of  Roy,  286 
Ophthalmia,  sympathetic,  455 
Ophthalmoscope,  464 
Opsonins,  26 
Optic  disc,  464 

nerve,  454 

decussation  of,  455 
Optics,  physiological,  478 
Osazone  tests,  658,  661 
Osmosis  in  lynipli  formation,  247 
Osmotic  pressure,  248 
Ostrich,  rutting  of,  581 
Ostium  abdominale,  589 
Otoliths,  499,  504 
Otter,  generation  of,  578 
Ovaries,  587 

effect  of  removal  of,  582,  583 

influence  of,  265 
Overtones,  495 

Ovulation  :  in   ass,   cat,   cow,    deer, 
dog,  elephant,  ferret,  mare,  mon- 
key, pig,  rabbit,  sheep,  wolf,  591 
Ovum,  589 

development  of  the,  597 
Ox,  aortic  ring  of,  32 

area  of  intestinal  tract,  201 

bellowing  of,  129 

bile,  action  of,  225 

amount  of,  per  hour,  224 
specific  gravity  of,  218 
sulphur  in,  219 

blood  of,  2,  21 

time  of  clotting,  1 6 

capacity  of  intestinal  canal,  201 

composition  of  body,  314 

drinking  of,  134 

heat  lost  by,  351 

intestinal  digestion  in,  201 

iris  of,  458 

larynx  of,  129 

lymph  from,  247 

mastication,  134 

number  of  respirations,  90 

cesophagus  of,  138 

pulse-rate  of,  70 

relation     l)etween     blood-    and 
body-weight,  22 

salivary  glands  of,  139 

sweating,  277 

sympathetic  nervous  system,  448 

tongue  of,  133 
Oxidases,  337 
Oxidations  in  animal  heat,  336 


Oxygen,  95,  633 

in  blood,  23,  24 

deficiency  in,  105 

excess  of,  106 

fate  of,  in  the  tissues,  100 

inhalation  of,  in  disease,  107 

intramolecular,  101 

in  stomach,  178 
Oxyhiemoglobin,  7 

Palate,  grooves  in,  133 
Palatine  glands,  141 
Pale  muscle,  355 

phenomena   of    contraction 
of,  380 
Palmitic  acid,  652 
Palmitin,  653 
in  blood,  20 
in  milk,  620 
Pancreas,  232 

acid,  aspartic,  236 

glutaminic,  236 
albumose,  236 
amido-acids,  236 
aniylopsin,  234,  236 
arginine,  236 
calcium  phosphate,  233 
Colin  on,  232 
enterokinase,  234 
enzyme,  diastatic,  234 

lipolytic,  234 

proteolytic,  234 
erepsiii,  236 
kinase,  234 
Kiihiie  on  rabbit,  238 
Langerhans,  islands  of,  240 
Lea,  Sheridan  on,  238 
leucin,  236 
lipase,  234,  236 
lysin,  236 

magnesium  phosphate,  233 
pathological  disturbances,  241 
Pancreatic  cells,  changes  in,  237 
diabetes,  239 
fluid,  dog,  232 

Hoppe-Seyler  on,  232 

horse,  232 

Pawlow  on,  237 
secretion,  amount  of,  238 

Bayliss  and  Starling  on,  233 

mechanism  of,  233 

nerve  control  of,  233 

peptones,  236 

phenol,  236 

potassium  chloride,  233 

Schmidt's  analysis,  232 

secretin,  233 

skatol,  236 

sodium  carbonate,  233 
chloride,  233 


704     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Pancreatic   secretion,    sodium    plios- 
phate,  233 

Starling  on,  235 

steapsin,  234,  236 

trypophan,  236 

trypsin,  234 

tryjisinogen,  234 

tyrosine,  236 

uses  of,  233 
Papilla  (retina),  464 
Papillas  of  tongue,  circumvallate,  489 

filiform,  489 

fungiform,  489 
Paracasein,  168 
Paraglobulin,  3,  245 
Paralytic  secretion,  146 
Parasitic    diseases,    digestive    canal, 
217 
of  liver,  241 
Parathyroid,  268 
Parotid  gland,  139,  141 

secretion,  147 
Pars  trigemini,  nerve,  425 
Parthenogenesis,  586 
Partial  pressure  of  Bunsen,  98 
Parturition,  615 

centre  in  cord,  424 
Pastern,  fracture  of,  519 
Pasteur  on  ferments,  642 
Pathetic  nerve,  425 
Pathological  conditions,  arteries,  83 

foot,  576 

heart,  53 

liver,  240 

locomotor  apparatus,  536 

nutrition,  335 

pulse,  83 

respiration,  120 

'  roaring,'  126 
Pawlow  on  gastric  juice,  165 
on  pancreatic  juice,  237 
Pedal  bone,  538 

descent  of,  572 
Pendular  movements  of  bowel,  203 
Pentoses,  661 

Pepsin,  152,  165-6,  175,  643 
Peptones,  168,  187,  199,  236.  637 

effect  on  coagulation,  20 
Peptonuria,  263 

Percival  on  gi'owth  of  horse,  625 
Pericarditis  in  horse,  54 
Pericardium,  use  of,  39 
Perikaryon,  412 
Perilymph,  498 

Periods  of  stomach  digestion,  171 
Periople,  546 
Peristalsis,  203 

effects  of  cocain,  203 

of  nicotin,  203 
Starling  and  Bayliss  on,  203 


Peritoneal  cavity,   absorption  from, 

258 
Peroxidases,  337 

Perspiration,  insensible,  Colin,  277 
Pettenkofer's  test  for  bile  acids,  222 
Peyer's  patches,  255 
Phagocytosis,  14,  26 

in  spleen,  267 
Pharyngeal  plexus,  138 

nerve,  431 
Phenol,  651,  665 

digestion,  199,  209 

jjancreas,  236 

urine,  300 
Phenol,  ethereal  sulphate  of,  292 
Phenyl-acetic  acids,  199 

-glucosazone,  659 

-maltosazone,  658 

-proprionic  acid,  199 

-sulphate  of  potassium,  665 
Phloridzin,  228 
Phonation,  127 
Phosphates,  21,  665 
Phosphoric  acid,  379 
Phosphorus,  633 
Phrenic  nerves,  division  of,  112 
Physical  characters  of  blood,  1 

of  chyme,  188 
Physiological  optics,  478 

salt  solution,  6 

shoeing,  575 
Physostigmin,    absorption     by     air- 
passages,  257 

action  on  heart,  53 
Pig,  area  of  intestinal  tract,  201 

amount  of  air  respired,  116 

bile,  action  of,  225 

amount  of,  per  hour,  224 
sulphur  in,  219 

blood  of,  21 

time  of  clotting,  16 

capacity  of  intestinal  canal,  201 

composition  of  body,  314 

generation,  578 

fieces  of,  208 

growth  of,  625 

heart  valves,  vegetations  on,  54 

heat  lost  by,  351 

intestinal  digestion  in,  201 

cestrous  cycle,  580 

ovary  of,  587 

period  of  gestation,  615 
puberty,  585 

pulse-rate  of,  70 

relation     between     blood-    and 
body-weight,  22 

number  of  respirations,  90 

saliva  of,  143,  150 

stomach,  digestion  in,  175 

sweating,  277 


INDEX 


705 


Pig,  urine  of,  305,  309 

.    vomiting  of,  178 
Pigment  in  hair,  274 
Pigments  of  the  body,  644 
Pillars  of  Corti,  499 
Pilocarpin,    absorption    by    air-pas- 
sages, 2iU 

action  on  heart,  53 

in  cat,  281 

in  dog,  281 

in  horse,  281 

in  man,  281 

in  secretion  of  saliva,  14tj 

in  sweating,  280 
Pineal  body,  270 
Piotrowski's  reaction,  641 
Pituitary  body,  270 
Placenta,  603 

cotyledonary,  604 

cumulata,  604 

diseoidal,  604 

metadiscoida],  604 

plicata,  604 

polycotyledouous,  604 

zonary,  604 
Plantar  cushion,  542 
Plasma,  blood,  2 

separation  of  proteids  from,  3 
Platelets,  lilood,  7,  18 
Pleural  cavity,  absorption  from,  258 

methylene  blue,  259 
Pleurisy,  effusion  in,  3 

in  horse,  54,  120 
Pneumonia,  horse,  120 
Pneimiogastric  nerve,  45,  430, 
Poikilothermal  animal,  337 
Polynuclear  colourless  corpuscles,  12 
Polyo:'strous  mammals,  577 
Polysaccharides.  655 
Pons  Varolii,  437 
Portia  dura,  seventh  nerve,  428 

mollis,  eighth  nerve,  429 
Position  of  the  heart,  31 
Post-mortem    rises    of    temperature, 

351 
Potassium  chloride  in  blood,  21 
in  pancreas,  233 

ferrocyanide,     absorption     from 
bowel,  259 
from  air-passages,  257 
from  cellular  tissue,  258 
from  skin,  258 

in  fjeces,  210 

in  red  cells,  327 

in  sweat,  301,  303,  327 

iodide,    al)sorption     from    peri- 
toneum, 259 

salts,  action  on  heart,  52 
muscle,  379 
in  vegetable  food,  663 


Potassium  sulphocyanide,  140 

in  wool,  284 
Priiicipitin,  26 
Praecrucial  gyrus,  dog,  12S 
Pregnancy,  duration  of,  614 
Prehension  of  food,  131 
Principle      of     the     common    path 

(nerves),  411 
Primitive  groove,  600 

streak,  599 
Procestrous  period,  577 
Procestrum,  577 

external  signs  of,  580 
Process  for  calculation  of  amount  of 

blood,  22 
Proamnion,  601 
Proprionic  acid,  207 
Prostate,  secretion  of,  584 
Proteid,  167,  634 

absorption  of,  263 

classification,  636 

in  plasma,  3 

of  serum,  3 

reactions,  641 

separation  from  plasma,  3 

to  peptone,  conversion  of,  168 
Proteoses,  199,  637 

primary,  168 

secondary,  168 
Prothrombin,  18 
Prototheria,  589 
Pseudo-nuclein,  638 
Psychical  powers  (ass,  dog,  elephant, 

horse,  mule),  448 
Pterygoid  muscles,  135,  136 
Ptyalin,  140,  143,  643 
Puberty,  period  of,  585 
Pulmonary  valve,  29 

circulation,  29 
Pulse,  cause  of,  66 

character  of,  83 

explanation  of,  58 

pathological,  83 

rate  and  blood-pressure,  relation, 
71 

rate,  variations  in,  71 

tension,  70 

venous,  67 

wave,  67 

graphic  study  of,  68 
length  of,  67 
sphygmogram,  68 
velocity  of,  67 
Puncture,  diabetic,  230 
Purin  bases,  296 

series,  638 
Purkinje,  cells  of,  31 
Purpura,  27 
Pyloric  glands,  164 
Pyrocatechin,  301,  651 

45 


706     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Quantity  of  blood  in  the  body,  22 

Rabbit,   division  of  cervical  sympa- 
thetic, 75 
heart,  depressor  nerve,  50 
ovary  of,  587 
pancreas  of,  238 
period  of  gestation,  615 
saliva  of,  143 

sympathetic  nervous  system,  447 
time  occupied  by  circuit  of  circu- 
lation, 74 
Rabies,  27 
Radiation,  343 

Rankine  on  daily  work  of  horse,  533 
Ratio  of  heart-beats  to  respiration ,  91 
Reagent,  Barfoed's,  658 
Rearing  of  horse,  532 
Reason  in  animals,  452 

V.  instinct,  452 
Reaction,  biuret,  641 
of  bile,  218 
of  blood,  1 

of  contents  of  stomach,  161 
of  urine,  304 
Reactions,  proteid,  of  Adamkiewicz, 
641 
of  Millon,  641 
of  Piotrowski,  641 
xantlioproteic,  641 
various,  642 
Receptor  organ,  412 
Rectum,  absorj^tion  from,  199 

ether  by,  261 
Recurrent  laryngeal  nerve,  138,  431 
division  of,  126 
sensibility,  399 
Red  corpuscles  of  blood,  4 
Redtenbaclier  on  horse's  daily  work, 

533 
Reduced  haemoglobin,  7 
Reflex  acts,  time  occupied  by,  422 
action,  407 
arc,  411 
Refraction,  errors  of,  in  horses,  470 
Refractory  period  of  heart-beat,  52 
Reiset  on  gases  in  expired  air,  97 
Reissner's  membrane,  499 
Relations  of  the  heart,  31 
Rennin,  165,  620 
Reserve  air,  113 
Residual  air,  113 
Respiration,  84 

abdominal  muscles  in,  89 
absorption  of  gases  in  liquids,  97 
acid,  sarco-lactic,  119 
air,  amount  of,  required,  113 
atmospheric,  95 

moisture  in,  95 
alveolar,  116 


Respiration,  complemental,  113 
reserve,  113 
residual,  113 
tidal,  113 
amount  of  air  respired,  Boussin- 

gault,  116 
apuiua,  107 

aiioplexy  of  lungs,  horse,  120 
argon,  95 

arytenoid  cartilages,  94 
asphyxia,  106 

ass,  amount  of  air  respired,  116 
braying  of,  120 
subepiglottic  sinus  of,  129 
atmospheric  air,  composition  of, 

95 
arytenoideus,  122 
bellowing  of  ox,  129 
laleating  of  sheep,  129 
Ijlood-pressure,  effect  on,  91 
Bohr  on  CO.,  in  blood,  103 
Boussingault's  tables,  116 
'  broken  wind'  in  horse,  120 
bronchitis  in  the  horse,  120 
Bunsen  on  '  partial  pressure,'  98 
carbon  monoxide,  poisoning  by, 
105 
Haldane's  experiments, 
104 
carbonic  acid,  95 

Bohr's  view,  103 
fate  of,  102 
carnivora,    resjiiratory    quotient 

in,  96 
castration,  effect  on  voice,  128 
cat,  larynx  of,  129 

voice  centre  in,  129 
cause  of  first  respiration,  112 
Chauveau  on  larynx,  129 
chemistry  of,  114 
circulation,  effect  on,  91 
Colin's  figures,  chest  of  horse,  85 
experiments,    seventh    pair 
of  nerves,  113 
guttural  pouches,  128 
complemental  air,  113 
composition  of  atmospheric  air, 
95 
inspired,  95 
expired,  95 
coughing,  129 

cow,  amount  of  air  respired,  116 
crico-arytenoideus  lateralis,  122 

posticus,  122 
Dalton  and  Henry's  law,  97 
deficiency  in  oxygen,  105 
diaphragm,  description  of,  86 
horse,  rupture  of,  120 

spasm  of,  120 
in,  89 


INDEX 


707 


Respiration,  diaphragm,  niovemeuts 
of,  85 
diffusion  of  gases,  98 
disassociation,  process  of,  103 
division     of    seventh     pair     of 
nerves,  113 
of  phrenic  nerves,  112 
dog,  amount  of  air  required,  116 
larynx  of,  129 
number  of  respirations,  90 
pressure  in  pleural   cavity, 

96 
respiratory  curves  of,  92 
voice     centre     in    cerebral 
cortex,  128 
dyspncea,  106 
effect  of,  on  circulation,  91 
epiglottis,  125 
expiration,  88 

muscles  of,  89 
expired  air,  composition  of,  95 
external  intercostal  muscles  in, 

89,  99 
facial  sinuses,  91 
false  nostril,  92 
ftetal  lung,  88 

foetus,  first  inspiration  in,  112 
gases,  absorption  of,  in  liquids, 
97 
Dalton  and  Henry's  law,  97 
diffusion  of,  98 
'  partial  pressure '  of  Bunsen, 
98 
glottis,  94,  124 
goat,  larynx  of,  129 

number  of  respirations,  90 
guttural  pouches,  128 
Haldane  and   Lorraine  Smith's 
results,  119 
and  Priestley's  experiments, 
117 
Haldane's  experiments  with  CO, 

104 
heart-beats,  ratio  of,  to,  91 
herbivora,  respiratory   quotient 

in,  96 
hiccough,  130 

horse,   amount  of  air  required, 
113,  116 
apoplexy  of  lungs,  120 
'broken  wind,'  120 
bronchitis,  120 
chest  of,  Colin's  figures,  85 
diaphragm  of,  86 
division  of  phrenic  nerves 

in,  112 
laryngitis  in,  121 
larynx,  121,  122 
neighing  of,  129 
nostril  of,  93 


Respiration,  horse,  number  of  respira- 
tions in,  90 

pleural  cavities  of,  84 

pleurisy  in,  120 

pneumonia,  120 

pressure  in  pleural  cavity, 
90 

roaring,  120 

rupture   of    diaphragm   in, 
120 

spasm  of  diaphragm  in,  120 

voice  production  in,  127 

Zuntz   and  Lehmann's  ex- 
periments, 105 
hydrogen  in  expired  air,  97 
hyperpncea,  105 
influence  of  the  vagus  on,  110 

of  work  on,  117 
inspiration,  84 

cause  of  first,  112 

muscles  of,  89 
inspiratory  centre,  108 

tetanus,  111 
inspired  air,  95 

internal  intereostals  in,  89,  99 
krypton,  95 
laryngitis,  horse,  121 
larynx,  122 

muscles  of,  121 

nervous  mechanism  of,  125 

of  cat,  129 

of  dog,  129 

of  goat,  129 

of  horse,  121,  122 

of  ox,  129 

of  sheep,  129 

ventricles  of,  129 
latissimus  dorsi  in,  89 
law  of  Dalton  and  Henry,  97 
levatores  costarum  in,  89 
liquids,  absorption  of  gases  in, 

97 
lungs,  84 

apoplexy  of,  in  horse,  120 
marsh  gas  in  expired  air,  97 
mechanism,     nervous,     of     the 

larynx,  125 
meth^'lene  blue  experiment,  102 
moisture  in  air,  95 
movements  of  diaphragm,  85 
mule,  subepiglottic  sinus  of,  129 
murmur,  respiratory,  119 

vesicular,  119 
nmscles  of,  89 

nasal  chamber,  olfactory  portion, 
93 
respiratory  portion,  93 
negative     pressure     in     pleural 
cavity,  dog,  90 
horse,  90 

45—2 


70H     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Respiration,    negative     pressure     in 
[)leural  cavity  of  sheep,  90 
neighing  of  horse,  129 
nerves,  dorso-lumbar,  108 
facial,  108 

glosso-pharyngeal,  110 
motor  of,  108 
nasal  of  fifth,  110 
phrenic,  108 

division  of,  112 
recurrent      laryngeal,      of 

vagus,  108,  125 
sciatic,  78 
seventh    pair,    division    of, 

113 
superior  laryngeal,  110,  125 
vaso-constrictor,  77 
nervous     mechanism      of      the 
larynx,  125 
governing  respiration,  108 
nitrogen,  95 
nostrils,  92 

false,  92 
omnivora,    respiratory   quotient 

in,  96 
ox,  bellowing  of,  129 
larynx  of,  129 
number  of  respirations,  90 
oxygen,  95 

deficiency  in,  105 
excess  of,  106 
fate  of,  in  the  tissues,  100 
inhalation    of,    in    disease, 
107 
intramolecular,  101 
pathological,  120 
pathology  of  'roaring,'  126 
'  partial  pressure  '  of  Bunsen,  98 
phonation,  127 

phrenic  nerves,  division  of,  112 
pig,  amount  of  air  respired,  116 

number  of  respirations,  90 
pleurisy,  horse,  120 
pneumonia,  horse,  120 
prfficrucial  gyrus,  dog,  128 
ratio  of  heart-beats  to,  91 
recurrent  laryngeal  nerves,  divi- 
sion of,  126 
Keiseton  gases  in  expired  air,  97 
reserve  air,  113 
residual  air,  113 
respiratory  centre,  108 

changes  in  air  and  blood,  94 
exchange,  104 

influenced  by  age,  104 
by  food,  105 
by  muscular  work, 

104 
by      temperature, 
105 


Respiration,  respiratory  exchange,  in 
lungs  and  tissues,  99 
murmur,  119 
jjassages,    absorption    from, 

257 
quotient,  carnivora,  96 
lierbivora,  96 
omnivora,  96 
ribs  in,  85 
roaring,  horse,  120 

pathology  of,  126 
rumination,  effect  of,  on,  90 
rupture  of  the  diaphragm,  horse, 

120 
sarcolactic  acid,  118 
scaleni  muscles  in,  89 
serratus  anticus  in,  89 
magnus  in,  89 
posticus  in,  89 
shceji,  amount  of  air  required, 
116 
bleating  of,  129 
larynx  of,  129 
number  of  respirations,  90 
pressure  in  pleural  cavity, 

90 
sneezing,  129 
spasm,  diaphragm,  horse,  120 
subepiglottic  sinus,  ass,  129 

mule,  129 
Sussdorf  on  division  of  jihrenics, 

112 
tetanus,  inspiratory,  111 
thyro-arytenoideus,  122 
tidal  air,  113 

transversalis  costarum  in,  89 
triangularis  sterni  in,  89 
vagus,  influence  on,  110 
vesicular  murmur,  119 
voice,    cerebral   centre  for,   cat, 
129 
dog,  128 
effect  of  castration  on,  128 
production,  127 
work,  influence  of,  on,  117 
yawning,  129 

Zuntz   and    Lehmann's    experi- 
ments, 105,  114 
Respirations,  number  of,  30 
Resting  nmscle,  changes  in,  369 
Rete  mirabile,  81 
Reticulum,  174 
Retina,  455,  462 

Retinal  image,  formation  of,  484 
Rhodopsin,  463 
Ribs,  in  respiration,  85 
Rigor  mortis,  379,  631 
Rinderpest,  27 

'  Roaring  '  in  horses,  120,  126,  431 
Roger  on  composition  of  fteces,  210 


INDEX 


709 


Romanes  on  instinct  and  reason,  452 
Rubner's  experiments  on  proteids,  332 
Rumen,  the,  172 
Rumination,  Colin's  observations,  180 

effect  of,  on  res[)iration,  90 

Flourens  on,  ISO 

mechanism  of,  182 
Ruminants,    intestinal  digestion   in, 
199 

stomach  digestion  in,  172 

nervous  mechanism  of,  186 
trouble  in,  217 
Rupture,  217 

of  the  diaphragm,  horse,  120 

of  the  heart,  54 

of  the  stomach,  153 

Saccharose.  657 
Saccule,  499 
Saliva,  139 

amount  of  secretion,  139 

chemical  characters.  140 

Meade  Smith  on,  143 

non-amylolytic    action     of,     in 
herbivora,  143 

physical  characters  of,  140 

salts  of,  140 

secretion  of,  144 

nerve  control  of,  144 

use  of.  142 
Salivary  glands,  changes  in  cells,  147 
Salkowski  on  urine,  303 

composition  of  urine,  307 
Salt  solution,  physiological  6 
Salts,  absorption  of,  263 

in  nutrition,  316 

of  blood,  21 
Sarco-lactic  acid,  653 

in  muscle,  374 
in  respiration,  118 
Sarcolemma,  352 
Sarcomere,  353 
Sarcoplasm,  353 
Sarcosine,  295,  645 
Sarcostyles,  353 
Scala  tym[iani,  499 

vestibuli,  499 
Scaleni  in  respiration,  89 
Schiifer's  views  on  nmscle,  353 
Schematic  eye.  478 
Schmidt,  analysis  of  pancreatic  fluid, 

232 
Sciatic  nerves,  78 
Sclerotic,  455 
Scratch  reflex,  414 
Sea-sickness,  horse,  180 
Sebaceous  secretion,  282 
Sebum,  276,  282 
Secretin,  187,  223,  233 

of  Starling  and  Bajdiss,  233,  264 


Secretion  of  gastric  juice,  162 
nerve  control  of,  169 

of  pepsin,  152 

of  saliva,  144 
Secretory  nerves,  Heidenhain's  view. 

146 
Self- digestion  of  the  stomach,  177 
Semicircular  canals,  497,  502 
Semilunar  valves,  29,  34 
Seminal  vesicles,  secretion  of,  584 
Seminiferous  tubules,  583 
Sensory  areas  of  brain,  441 
Serous  cavities,  243 
Serratus  anticus  in  respiration,  89 

magnus  in  respiration,  89 

posticus  in  respiration,  89 
Serum,  3 

albumin  in  liquor  sanguinis,  3 
in  lymph,  245 
precipitation  of,  3 

globulin  in  liquor  sanguinis,  3 
precipitation  of,  3 

proteids  of,  3 

globulieidal  action  of,  25 
Seventh  pair  cranial  nerves,  428 
Sexual  intercourse,  586 

ova,  592 

season  of  animals,  577 

spermatozoa,  592 
Sheep,  amount  of  air  required,  116 
of  heat  produced,  351 

bile,  action  of,  225 

amount  of,  per  hour,  224 
specific  gravity  of,  218 
sulphur  in,  219 

bleating  of,  129 

blood,  time  of  clotting,  16 

composition  of  body,  314 

fffices  of,  208 

fcetal,  gases  in  blood  of,  613 

generation,  578 

gi-owth  of,  625 

impregnation  in,  593 

intestinal  digestion  in,  201 

iris  of,  458 

larynx  of,  129 

mastication,  134 

milk,  analysis  of,  620 

number  of  respirations,  90 

ojsophagus  of,  138 

cestrous  cycle,  Goodall,  579 

ovary  of,  587 

period  of  gestation,  615 

pressure,    negative,    in     pleural 
cavity,  90 

puberty,  period  of,  585 

pulse-rate  of,  70 

relation     between     blood-     and 
body-weight,  22 

sweating,  277 


710     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Sheep,  temperature  of,  339 
uriue  of,  309 
uterine  glands  in,  614 
Sherrington  on  the  labyrinth,  503 
Shoeing,  physiological,  575 
Shoulder-joint,  512 
'Side-bone,'   cause   of   lameness   in, 

563 
Siedamgrotzky  on  effect  of  clipping, 
349 
on  temperatures,  338 
Sigmoid  valves,  34 
Sight,  454 

aberration,  chromatic,  483 

spherical,  483 
accommodation  (eye),  466 

Helmholtz  on,  467 
angle,  visual,  485 
aqueous  humour,  455 
astigmatism,  457 

horse,  469 
atropin,  cat,  468 
dog,  468 

effect  on  iris,  459 
horse,  408 
Berlin,  eye  measurements,  480 
blind  spot  (retina),  463 
binocular  vision,  473 
cartilago  nictitans,  477 
cat,  atropin,  efi'ect  of,  468 
emmetropia,  470 
iris  of,  458 
choroid,  455,  461 
ciliary  muscle,  horse,  461 

etfect  of  atropin,  horse,  468 
processes  (eye),  458 
zone,  461 
chromatic  alierration,  483 
cocaine,  effect  on  iris,  459 
cornea,  455,  457 
corpora  nigra  (horse),  460 
cow,  refraction,  errors  of,  470 
dioptrics,  480 
dog,  iris  of,  458 

emmetropia  of,  470 
emmetropic  eyes,  468 
eserin,  etfect  on  iris,  459 
Eversbusch  on  iris  of  horse,  459 
eyeball,  movements  of,  470 
muscles  of,  horse,  471 
eyelashes  of  horse,  477 
eye,  schematic,  478 

structure  of  the,  454 
fishes,  sight  of,  468 
focus,  lenses,  482 
gland,  Harderian,  477 

lachrymal,  477 
gland.  Meibomian,  477 
horse,  area  of  acute  vision,  464 
fundus  oculi,  465 


Sight,  horse,  iris  of,  458 
myopic,  468,  481 
refraction,  errors  of,  470 
retina  of,  465 
'wall-eyed,'  458 
hypermetropia,  469 
iris,  Langley  and  Anderson  on, 

457,458 
katoptric  test  (eye),  467 
Lang    and    Barrett    on    ciliary 
muscle  and  atropin,  468 

on  errors  of  refraction, 
470 
lens,  457,  458 
lenses,  passage  of  light  through, 

481 
ligamentum  pectinatum,  456,  460 

inhibitorium  (iris),  460 
membrdua  nictitans,  454 
monocular  vision,  473 
morphia,  effect  on  iris,  459 
myopia,  468 

nerves  of  ocular  muscles,  472 
ophthalmia,  sympathetic,  455 
ophthalmoscope,  464 
optic  disc,  464 
nerve,  454 

decussation  of,  455 
ox,  iris  of,  458 
papilla  (retina),  464 
physiological  optics,  478 
refraction,   errors  of,  in  horses, 

470 
retina,  455.  462 

retinal  image,  formation  of,  484 
rhodopsin,  463 
sclerotic,  455 
sheep,  ii'is  of,  458 
spherical  aberration,  483 
tapetiim  lucidum,  457,  461 
tears,  477 

theory  of  vision,  484 
visual  angle,  485 
purple,  463 
vitreous  humour,  456,  462 
wild     animals,     hypermetropic, 

470 
yellow  spot  (retina),  464 
zonule  of  Zinn,  462,  467 
Silica  in  dandruff,  283 

in  f feces,  210 
Silicon,  634 

Sixth  pair  cranial  nerves,  428 
Skatol,  652 

in  digestion,  189,  199,  209 
pancreas,  236 
urine,  300 
iSkatoxyl-sulphuric  acid,  652 
Skin,  the,  271 

absorption  by  the,  258 


INDEX 


'11 


Skiu,  Arloing  ou  sweating,  281 
atropin  in  sweating,  279 
Bouley  on  varnishing,  284 
calcium  oxalate  in,  283 
cat,  hair  of,  276 

pilocarpin  in,  281 

sweating,  277 

experimental,  279 
'cat-hairs,'  horse,  273 
chlorophyll  in  dandruff,  283 
clipping  horses,  275 
Colin  on  insensible  perspiration, 

277 
composition  of  sweat  of  horse, 

278 
dandruff,  composition  of,  283 
dog,  hair  of,  276 

pilocarpin  in,  281 

sweating,  277 
donkey,  sweating,  277 
Durham,  researches  on  hair  pig- 
ment, 274 
effect  of  varnishing,  284 
EUenberger  ou  varnishing,  284 
Grandeau  on  amount  of  sweat, 

277 
hair,  272 

cat,  276 

dog,  276 

horse,  272 

clipping  of,  275 

permanent,  273 

pigment  in,  274 
horn,  272 
horse,  'cat-hairs,' 273 

dandruff.  283 

hair  of,  272 

pilocarpin  in,  281 

sweat  of,  composition,  273 

sweating,  276 

thrombosis  of  iliac  arteries, 
281 
insensible  perspiration,  277 
lanolin  in  dandruff,  283 
mechanism,  nervous,   of   sweat- 
ing, 279 
melanin,  274 
Mendel's   theories   of    heredity, 

274 
mule,  sweating  of,  277 
nitre  in  veterinary  practice,  282 
nerve  '  sweat  centre,'  28 0 
Newsom's    calculation,    hair    of 

horse,  273 
ox,  sweating  of,  277 
parasitic  disease,  284 
pathological,  284 
perspiration,     insensible,     Colin 

on,  277 
pig,  sweating,  277 


Skin,  pigment  in  hair,  274 

pilocarpin,  action  of,  in  cat,  281 
in  dog,  281 
in  horse,  281 
in  man,  281 
in  sweating,  280 
potash  in  wool,  284 
respiratory  function  of,  284 
sebaceous  secretion,  282 
sebum,  276,  282 
sheep,  sweating,  277 
silica  in  dandruff,  283 
'  suint '  in  wool,  284 
sweat,  276 

amount  of,  daily,  277 
Grandeau  on  amount  of,  277 
horse,  composition  of,  278 
urea  in,  279 
sweating,  Arloing  on,  281 
atropin  in,  279 
nervous  mechanism  of,  279 
pilocarpin  in,  280 
thrombosis   of  iliac   arteries   in 

horse,  281 
tyrosin,  pigment  from,  275 
tyrosinase,  275 
urea  in  dandruff,  283 

in  sweat,  279 
varnishing,  effect  of,  284 
AVarrington  on  potash  in  wool, 
284 
Smell,  485 
Smith,  Meade,  on  saliva,  143 

Sydney,  on  instinct  and  reason, 
452 
Smooth  muscle,  phenomena  of  con- 
traction, 3.80 
Sneezing,  129 
Soaps  in  blood,  20 
Sodium  carbonate  in  blood,  1,  21 
in  pancreas,  233 
cliloride,  action  on  plasma,  3 
in  blood.  2.  21,  '22 
in  digestion,  212,  219 
in  pancreas,  233 
lihos])hate  in  pancreas,  233 

in  blood,  1,  21 
in  blood  plasma,  327 
in  urine,  301 

salts,  action  on  heart,  52 
in  vegetable  food,  603 
Soda,  glycocholate,  221 

taurocholate,  221 
Sole,  550 

use  of,  566 
Solitary  follicles.  255 
Somatoplcure,  600,  610 
Sound,  the  nature  of,  495 
Sounds,  cardiac,  40 
Spasm  of  the  diaphragm,  horse,  120 


712      A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


spavin,  position  of,  510 
Special  centres  in  the  spinal  cord,  424 
Specilic  ,<^ravity,   blood  (dog,    horse, 
ox,  sheep,  pig),  2 
bile,  218 
Spectroscope  test,  hemoglobin,  9 
Spectrum  of  CO  hteraoglobin,  10 

of  haemoglobin,  9 

of  oxy-luemoglobin,  9 

of  lueniatin,  11 
'Speedy-cutting,'  horse,  512 
S])ermatic  fluid,  584 
Spermatoblasts,  583 
Spermatogen,  583 
Spermatozoa,  583 
Spherical  aberration,  483 
Sphincters,  212 
Spinal  accessory  nerves,  434 

cord,  391 

nerves,  392 

function  of,  399 
Splanchnopleure,  601,  608 
Spleen,  266 

enzyme  in,  267 

use  of,  267 
Splenic  artery,  leucocytes  in,  12 

vein,  leucocytes  in,  12 
Stag,  rutting  of,  581 
Staining  of  leucocytes,  13 
Standing,  act  of,  horse,  521 
Stanford  on  locomotion  in  horse,  522 
Stapes,  497 
Starch,  655 

of  plants,  142 

proteid-sparing  action  of,  320 
Starling  on  lymph  production,  249 

on  pancreatic  extract,  235 

and  liayliss  on  peristalsis,  203 

and  Tubby  on  absorption,  259 
Starvation,  329 
Steapsin,  234,  236 
Stearic  acid,  652 
Stearin,  653 

iu  blood,  20 

in  milk,  620 
Stepping  reflex,  410 
Stercobilin,  210,  221 
Sterno-maxillaris  muscle,  136 
Stifle-joint,  description  of,  511 

discussion  of,  508 
Stillman   on  function  of  suspensory 
ligament,  513 

on  locomotion  in  horse,  522 

on  muscles  of  propulsion,  506 
Stokes's  fluid,  9 
Stomach  absorption,  176,  259 

acids,  161 

calculi,  210 

contents,  reaction  of,  161 

digestion  in  dog,  176- 


Stomach,  digestion  in  horse,  150 
ill,  periods  of,  171 
in  pig,  175 
in  ruminants,  172 
gases  of,  178 
movements  of,  184 
nerves  of,  185 
of  dog,  horse,  pig,  ox,  150 
of  llama,  182 
pouch  of  Pawlovv,  1<)6 
rupture  of,  153 
self-digestion  of,  177 
Storage  of  tissue,  328 
Storch,  venous  system  of  horse's  foot, 

573 
Strangulation  of  the  bowels,  215 
Stratum  periostale,  543 

vasculosum,  543 
Strongylus  armatus,  83 
StructiU'e  of  muscle,  352 

of  nerves,  383 
Strychnine  absorbed  by  peritoneum, 
259 
by  pleura,  259 
effect  on  brain,  445 
experiments,  177 
per  rectum,  199 
Stylo-maxillaris  muscle,  136 
Subepiglottic  sinus  of  ass,  129 

of  mule,  129 
Sublingual  gland,  139 
Submaxillary  ganglion,  cat  and  dog, 
427 
gland,  139 

of  dog,  144 
Subsistence  diet,  333 
Succinic  acid,  209 
Succus  entericus,  186 
Sucking,  134 

Suffraginis,  fracture  of,  519 
Sugar  in  blood,  20,  226 
invert,  658 

supply,  how  regulated,  229 
tests  for,  HIittcher's,  661 
fermentation,  661 
Moore's,  661 
picric  acid,  661 
Trommer's,  661 
Sugars,  conversion  of,  228 
'  Suint'  in  wool,  284 
Sulphindigotate  of  soda,  290 
Sulphur,  633 
in  bile,  219 
in  body,  665 
in  hair,  327 
in  nutrition,  316 
Sulpliuretted  hydrogen  in  large  in- 
testines, 208' 
in  stomach,  178 
Suljjhuric  acid,  231 


INDEX 


713 


Summation  of  contractions,  361,  380 

Superior  laryngeal  nerve,  138,  431 

Suspensory    ligament,    function    of, 
513 

Sussdorf  on  division  of  phrenics,  112 
on  proportion  of  blood  to  body- 
\veight,  22 

Suatentacular  cell,  583 

Swallowing  centre,  138 

Sweat,  276 

amount  of  daily,  277 
centres  in  cord,  42-1 
Grandeau  on  amount  of,  277 
horse,  composition  of,  278 
urea  in,  279 

Sweating,  Arloing  on,  281 
atropin  in,  279 
pilocarpin  in,  280 
nervous  mechanism  of,  279 

Swine,  temperature  of,  339 

Sympathetic  system,  nerves,  446 

Synajises,  412 

Synovia,  507 

Syntonin,  168 

Syphon-trap  of  duodenum,  153 

Systemic  circulation,  29 

Systole  of  heart,  35 

Tactile  cells,  391 

sensations,  491 
Tapetum  lucidum,  457,  461 
Tappeiner  on  cellulose,  171 
Tartar  emetic,  ISO 
Taste,  489 

of  Ijlood,  2 

goblets,  M'Kendrick,  489 
Taurine,  222,  379,  645 
Taurocholate  of  soda,  221 
Taurocholic  acid,  645 
Tears,  477 
Teeth,  horse.  132 

ox, 131 

sheep,  131 
Temperature  of  the  blood,  22 

Colin  on,  33S 

normal,  of  animals,  338 
Temporal  muscle,  136 
Tendon  reflexes,  423 
Tension  of  pulse,  69 
Tenth  pair  of  cranial  nerves,  430 
Tereg  on  urines,  308 
Termination  of  nerves,  391 
Testicles,  583 

eflect  of  removal  of,  582 
Test,  Gmelin's,  for  bile,  220 

for  sugar,  661 
Tetanus,  362 

inspiratory.  111 
Texas  fever,  organism  of,  27 
Thalami  optici,  437 


Theories  of  urinary  secretion,  290 

of  vision,  484 
Third  pair  cranial  nerves,  425 
Thirst,  494 
Thoracic  duct,  243 
Thrombin,  18 
Thrombosis  of  iliac  arteries  in  horse, 

281 
Thymus,  269 

gland,  influence  of  castration  on, 
266 
Thyro-arytenoideus  muscle,  122 
Thyroid  gland,  268 
Tidal  air,  113 

Tiger,  intestinal  canal  of,  201 
Tissue  i^roteid  of  Voit,  319 

storage  of,  328 
Tongue,  dog,  horse,  ox,  133 

movements  of,  133 

nerves  of,  133 
Torcy  on  growth  of  calves,  625 
Toughness,  provision  for,  in  foot,  559 
Tracts  in  the  spinal  cord,  401,  403 
Training,  377 

Transfusion,  solution  used  for,  6 
Transversalis  costarum  in  respiration, 

89 
Triangularis  stertii  in  respiration,  89 
Tricuspid  valve,  29 
Trophic  centres  in  cord,  424 
Troplioblast,  601,  604 
Trot  of  horse,  524 
Trypanosomes,  27 
Trypophan,  236 
Trypsin,  187,  234,  643 
Trypsinogen,  187,  234 
Tunica  albuginea,  583 

fibrosa,  588 
Tiirck,  column  of,  403 
Turpentine,   absorption    by    air-pas- 
sages, 257 
Twelfth  pair  cranial  nerves,  434 
Tympanum,  497 
Tympany,  217 
Tyrein,  620 
Tyrosin,  650 

from  proteid,  318 

in  f£eces,  209 

in  spermatic  fluid,  584 

in  urine,  293 

pancreas,  236 

pigment  from,  275 
Tyrosinase,  275 

Umbilical  cord,  608 

Urachus,  607 

Urea,  as  measure  of  work,  294 

description  of,  647 

in  allantoic  fluid,  60S 

in  blood,  20 


714     A  MANUAL  OF  VETERINARY  PHYSIOLOGY 


Urea  in  dandruff,  283 
in  liver,  'I'll 
in  muscle,  379 
in  sweat,  279 
in  urine,  292 
synthesis  of,  (548 
variations  in  amount  of,  295 
Urethra,  312 
Uric  acid,  20,  295,  649 
Urine,  285 

acid,  aspartic,  293 

benzoic,  292 

Liebig  on,  298 

glycuronie,  301,  311 

hippuric,  292,  302 

production  of,  298 

oxalic  in,  301,  302 

plios])liate  of  soda,  305 

I)hospIioric  in,  303 

sulphuric  in,  300-2-3 

uric,  origin  of,  292,  295 
adenine,  296 
amido-bodies,  293 
ammonia  in,  304 

salts  in,  297 
ammonium  carbamate,  293 

carbonate  in,  304 
Bellini,  duct  of,  289 
Bischoff  and    Voit  on  urine    of 

dog,  311 
bladder,  urinary,  311,  312 
blood,  amount  of,  through  kid- 
ney, 289 
Bowman,  capsule  of,  287 
calcium  in,  301 
chlorine  in,  303 
colouring  matter  of,  301 
composition  of,  292 
consistence  of,  306 
creatine,  292 
creatinine,  292,  295 
cresol,  300 

ethereal  sulphate  of,  292 
discharge  of,  311 
dog,  experiments  on  kidney  of, 
285,  290 

kidney,  blood  through,  289 

urine  of,  Bischoff  and   Voit 
on,  310 
excretion,  definition  of,  285 
Fischer,  Emil,  on  purin,  296 
glycine,  292 
glycocoll,  293 
Henle,  ascending  liml)  of,  289 

loop  of,  289 
horse,  salts  in,  301 

colour,  306 

odour,  306 

quantity,  305 

solids,  306 


Urine,  horse,  specific  gravity,  305 
hypoxanthine,  296 
indican,  300 
indol,  300 

inorganic  substances  in,  301 
kidney  glomeruli,  286 

Mali>ighian  tufts,  286 

movements  of,  286 

pathological,  313 

structure  of,  286 

uriiiiferous  tulndes,  286 
leucine,  293 
leucocyth;emia,  296 
magnesium  in,  301,  303 
micturition,  act  of,  313 
Moeller  on,  of  calves,  309 
Munk  on  phosphates  in,  304 

on  ox,  308 
nitrogenous  substances,  292 
oncometer  of  Roy,  286 
phenol,  300 

ethereal  sulphate  of,  292 
pig,  urine  of,  305,  309 
potassium  in,  301,  303 
purin  bases,  296 
pyrocatechin,  301 
reaction  of,  304 
Salkowski  on  (chlorides),  303 

composition  of,  307 
sarcosine,  295 
sheep,  urine  of,  309 
skatol,  300 
sodium  in,  301 
sulphindigotate  of  soda,  290 
Tereg  on  urines,  308 
theories    of    urinary    secretion, 

290 
tyrosine,  293 
urea  as  measure  of  work,  292 

variations  in  amount  of,  295 
urethra,  312 
uric  acid,  295 

formation,  296 
urobilin,  301 
urochrome  in,  301 
vascular  mechanism  of  kidney, 

289 
Wolff,  E.,  on,  306 
xanthine,  296 
Urobilin,  11,  301 
Urochrome,  301 
Uterine  milk,  614 
Uterus,    changes     in,    during     pro- 

cestrum,  581 
Utricle.  499 

Vagina,  absorption  from,  258 
Vagus,    action   on  small   intestines, 
205 
and  secretion  of  gastric  juice,  169 


INDEX 


715 


Vagus  in  neck,  stimulation  of,  45 

influence  of,  on  respiration,  110 

motor  nerve  of  stomacli,  185 
Valves,  aortic,  29 

auriculo-ventricular,  29,  33 

bicuspid,  33 

Cliauveau  on,  39 

mitral,  29,  33 

of  the  heart,  29 
action  of,  39 

of  veins,  57 

pulmonary,  29 

semilunar,  29,  34 

sigmoid,  34 

tricuspid,  29 

use  of  the,  30 
Valvular  disease,  horse,  5-± 
Variations  in  body  temperature,  339 
Varnishing  the  skin,  efi'ect  of,  284, 

346 
Vascular  mechanism  of  foot,  573 
of  kidney,  289 

sole,  545 

wall  (foot),  542 
Vaso-constrietor  centre,  79 
Vaso-dilator  nerves,  76 
Vaso-motor  centre,  50,  75 

centres  in  cord,  424 

subcentres  in  cord,  75 
Veins,  57 

abdominal,  57 

capacity  of,  57 

construction  of,  57 

of  pregnant  uterus,  57 

pulse  in,  66 

without  valves,  57 
Velocity  of  blood,  72 

of  gallop,  533 

of  nerve  impulses,  589 

of  trot,  533 
Vcnte  caviB,  57 
Venous  blood,  20 

plexuses  of  corjnis  cavernosum, 
82 
Vesico-spinal  centre  in  cord,  424 
Vesicular  murnuir,  119 
Vestibule,  497 
Villi,  the,  253 
Vision,  theory  of,  484 
Visual  angle,  485 

jmrple,  463 
Vitellin,  638 
Vitelline  memlirane,  589 
Vitreous  humour,  456,  462 


Voice,  cerebral  centre  for,  cat,  129 
dog,  128 

elfect  of  castration  on,  128 

production,  127 
Voit,  experiments,  bile,  225 

theor}' of  metabolism,  318 
Volkmann's     estimate     of    area     of 
vascular  system,  72 

observations,  velocity  of  blood,  72 
Voluntary  muscles,  352 
Vomiting,  178 

Walk  of  horse,  523 

Wall-secreting  substance  of  foot,  545 

Waller's  co-operative  antagonism, 506 

degeneration,  spinal  nerve,  400 
Warrington  on  potash  in  wool,  284 
Water,  662 

absorption  of,  263 

liy  air-passages,  257 

in  tissues,  328 
Weight,  how  carried  by  foot,  561 

of  the  body,  distribution  of,  515 

which  a  horse  can  carry,  534 
Weissmann  on  polar  bodies,  590 
Wet,  effect  of,  347 
Whartonian  jelly,  610 
AVhite  corpuscles  of  blood,  12 
Wild  animals,  hypermetropia  in,  470 
Wolf,  generation,  578 
Woltf  on  diet,  334 

on  heat  loss,  351 

on  urine,  306 
Wooldridge  on  temperature,  339 
Work  of  the  heart,  43 

influence  on  respiration,  117 

Xanthine  (muscle),  379 

series,  638 

(urine),  296 
Xanthoproteic  reaction,  641 

Vawning,  129 

Yellow  spot,  retina,  464 

Yolk  sac,  600,  605 

Zebra,  period  of  gestation,  614 
Zona  radiata,  589 
Zonule  of  Zinn,  462,  467 
Zuntz  on  muscle  work,  365 

and  Lehmann,  experiments,  res- 
piration, 105, 114 
Zymase,  643 
Zymogen,  644 


Bailliire,  TindrUl  and  Cox,  8,  Henrietta  SlreH,  Covent  Garden. 


CATALOGUE    OF 

William  R-  Jenkins  Co/s 

Works   Concerning 

HORSES,  CATTLE,  SHEEP,  SWINE,  Etc. 

1908 


(*)  Designates  New  Books. 

())  Designates  Recent  Publications . 


ANDERSON.  "Vice  in  tlie  Horse"  and  other  papers 
on  Horses  and  Riding.  By  E.  L.  Anderson.  Size, 
6x9,  clolli,  illustrated 1  75 

ARMSTEAD.      "  The  Artistic  Anatomy  of  the  Horse." 

A  brief  description  of  the  various  Anatomical  Struc- 
tures which  may  be  distinguished  during  Life  through 
the  Skin.  By  Hugh  W.  Armstead,  M.D.,  F.R.C.S. 
With    illustrations    from    drawings  by    the    author. 

Cloth  oblong,  10  x  12^ 3  75 

BACH.  "  How  to  Judge  a  Horse."  A  concise  treatise 
as  to  its  Qualities  and  Soundness ;  Including  Bits  and 
Bitting,  Saddles  and  Saddling,  Stable  Drainage,  Driv- 
ing One  Horse,  a  Pair,  Four-in-hand,  or  Tandem,  etc. 
By  Capt,  F.W.Bach.     Size,  5  x7|,,  clo.,  fully  illus.l  00 

BANHAm.  "Tables  of  Veterinary  Posology  and  Thera- 
peutics," with  weights,  measures,  etc.  By  Geo.  A. 
Banhara,  F.  R.  C.  V.  S.  New  edition.  Cloth,  size 
4x5  1-2,  192  pages 1  00 

BAUCHER.  "Method  of  Horsemanship."  Including 
the  Breaking  and  Training  of  Horses.  By 
F.  Baucher 1  00 

BELL,.     O^'The  Veterinarian's  Call   Book  (Perpetual)." 

By  Roscoe  R.  Bell,  D.V.S.,  editor  of  the  American 
Veterinary  Review.    Completely  revised  1907. 

A  visiting  list,  that  can  be  commenced  at  any  time 
and  used  until  full,  containing  much  useful  informa- 
tion for  the  student  and  the  busy  practitioner. 
Among  contents  are  items  concerning :  Prescription 
writing;  Veterinary  Drugs;  Poisons;  Solubility  of 
Drugs;  Composition  of  Milk, Bile,  Blood,  Gastric 
Juice,  Urine,  Saliva;  Respiration;  Dentition ;  Temp- 
erature, etc.,  etc.  Bound  in  flexible  leather,  with 
flap  and  pocket , , .^^•.^•.^ 1  2& 


BITTING.    *'  Cadiot's  Exercises  in  Equine  Surgery." 

See  ''Cadiof." 

BRADLEY.        "Outlines      of     Yeterinary    Anatomy." 

By  O.  Charnock  Bradley,  Member  of  the  Royal  Col- 
lege of  Veterinary  Surgeons ;  Professor  of  Anatomy 
In  the  New  Veterinary  College,  Edinburgh. 

The  author  presents  the  most  important  facts  of 
veterinary  anatomy  in  as  condensed  a  form  as  possible, 
consistent  with  lucidity.     12mo. 

Complete  In  three  parts. 

Part  I. :      The,  Limbs  (cloth) 1  25 

Part  II. :     The  Trunk  (paper) 1  25 

Part  III. :    The  Head  and  Neck  (paper) 1  25 

The  Set  complete 3  26 

CADIOT.     "  Exercises  in  Equine  Surgery."     By  P.  J. 

Cadiot.  Translated  by  Prof.  A.  W.  Bitting,  D.V.M. 
Edited  by  Prof.  A.  Llautard,  M.D.V.M.  Size,  6  x  9%. 
cloth,  illustrated 2  50 

—  **  Roaring   in    Horses."     Its    Pathology   and    Treatment. 

This  work  represents  the  latest  development  in  oper- 
ative methods  for  the  alleviation  of  roaring.  Each 
step  is  most  clearly  defined  by  excellent  full-page 
illustrations.  By  P.  J.  Cadiot,  Professor  at  the 
Veterinary  School,  Alfort.  Translated  by  Thos,  J. 
Watt  Dollar,  M.R.C.V.S.,  etc.  Cloth,  size  5  1-4  x  7  1-8, 
77  pages,  illustrated 75 

—  "Studies  in  Clinical  Veterinary  Medicine  and  Surgery." 

By  P.  J.  Cadiot.  Translated,  edited,  and  supplemented 
with  49  new  articles  and  34  illustrations  by  Jno.  A.  W. 
Dollar,  M.R.C.V.S.  Cloth,  size  7x9  3-4,  619  pages, 
94  black  and  white  illustrations 5  25 

—(•)"  A  Treatise  on  Surgical  Therapeutics  of  the  Domestic 
Animals."  By  F.  J.  Cadiot  and  J.  Almy.  Translated 
by  Prof.  A.  Llautard,  M,D.,V.M. 

I.  General  Surgery. — Means  of  restraint  of  animals, 
general  anaBSthesia,  local  antothesia,  surgical  anti- 
sepsis and  asepsis,  hematosis,  cauterization,  firing, 

II.  Diseases  Common  to  all  Tissues. — Inflammation, 
abscess,  gangrene,  ulcers,  fistula,  foreign  bodies, 
traumatic  lesions,  complications  of  traumatic  les- 
ions, granulations,  cicatrices,  mycosis,  virulent 
diseases,  tumors. 

III.  Diseases  Special  to  all  Tissues  and  Affections  of 
the  Extremities. — Diseases  of  skin  and  cellular  tis- 
eue,  of  serous  bursae,  of  muscles,  of  tendons,  of 
tendinous  svnovial  sacs,  of  aponeurosis,  of  arteries, 
of  veins,  of  lymphatics,  of  nerves,  of  bones,  of 
articulations. 

Cloth,  si/e  6x9,  580  pages,  118  illustrations 4  50 

CHAPMAN.  "Manual  of  the  Pathological  Treatment 
of  LamenesH  in  the  Horse,"  treated  solely  by 
mechanical  means.  By  George  T.  Chapman.  Cloth, 
size  6x9,  124  pages  with  portrait 2  00 


CLARKE.  "Chart  of  the  Feet  and  Teeth  of  Fossil 
Horses."  By  \V.  H.  Clarke.  Card,  size  9  1-2  x  12.  .   25 

— "Horses'  Teeth.^'  Fourth  edition,  re-revised,  with  second 
appendix.     Cloth,  size  5  1-4  x  7  1-2,  322  pp..  illus.  .2  60 

CLEAVELANiy.        "Pronouncing      Medical     Lexicon." 

Pocket  edition.  By  C  H.  Cleveland,  M.D.  Cloth, 
size  3  1-4  X  i  1-2,  o02  pages 75 

• 
CLEMENT.  "  Veterinary  Post  Mortem  £\amina> 
tions."  By  A.  W.  Clement,  V.iS.  The  absence  in  the 
English  language  of  any  guide  in  making  autopsies 
upon  the  lower  animals,  induced  Dr.  Clement  to 
write  this  book,  trusting  that  it  would  prove  of  prac- 
tical value  to  th«  profession.  Cloth,  size  5x7  1-2,  64 
pages,  illustrated 75 

CO  URTENA  Y.  ( f)  "  Manual  of  the  Practice  of  Veterinary 
Medicine."  Hy  Edward  Courtenay,  V.  S.  Revised  by 
Frederick  T.  G.  Hobday,  F.R.C.V.S.  Second  edition. 
Cloth,  size  5  1-4  x  7  1-2,  .'i73  pages    2  75 

COX.        "  Horses  :     In     Accident    and    Disease."       The 

sketches  introduced  embrace  various  attitudes  which 
have  been  observed,  such  as  in  choking ;  the  disorders 
and  accidents  occurring  to  the  stomach  and  intestines ; 
affection  of  the  brain  ;  and  some  special  forms  of  lame- 
ness, etc.  By  J.  Roalfe  Cox,  F.R.C.V.S.  Cloth,  size 
6  X  9,  28  full  page  illustrations 1  tO 

DALRYMPLE.  (*)"Veterinary  0»)stetrics."  A  compen- 
dium for  th«  use  of  advanced  students  and  Practi- 
tioners. By  W.  H.  Dalrymple,  M.  R.  C.  V.  S., 
principal  of  the  Department  of  Veterinary  Science  in 
the  Louisiana  State  University  and  A.  &  M.  College; 
Veterinarian  to  the  Louisiana  State  Bureau  of 
Agriculture,  and  Agricultural  Experiment  Stations. 
Second  edition  revised.  Cloth,  si-'.e  6x9  1-4,162  pages, 
51  illustrations 2  50 

DALZIEL.  "  Breaking  and  Training  Dogs."  Part  I,  by 
Pathfinder.  Part  II,  by  Hugh  Dalziel.  Cloth, 
illustrated 2  50 

—  "The    Collie."    By  Hugh  Dalziel.     Paper,  illustrated 50 

—  "The  Diseases  of  Dogs."    Causes,  symptoms  and  treatment. 

By  Hugh  Dalziel.  Illustrated.  Paper,  50c.  Cloth,  1  CO 

—  "Diseases  of  Horses."    Paper 50 

—  "  The  Fox  Terrier."    By  Hugh  Dalziel.    Paper,  50;  clo.l  00 

—  "The  Greyhound."   Cloth,  illus 100 

—  "  The  St.  Barnard."    Cloth,  illustrated. , , , , 1  00 


DANA.  "Tables  in  Comparative  Physiology."  By  Prof. 
C.  L.  Dana,  M.D.     Chart,  17  x  17 96 

DANCE.  "Veterinary  Tablet."  By  A.  A.  Dance.  Chart, 
17  X  24,  mounted  on  Itnen,  folded  in  a  cloth  case  for 
the  pocket,  size  3  .S-4  x  6  1-2.  Shows  at  a  glance  the 
synopsis  of  the  diseases  of  horses,  cattle  and  dogs; 
with  their  cause,  symptoms  and  cure 75 

DE  BRUIN.  (♦)"  BoTine  Obstetrics."  By  M.  G  De  Bruin 
Instructor  of  Obstetrics  at  the  State  Veterinary 
School  in  Utrecht.  Translated  by  W.  E.  A.  Wyman, 
formerly  Professor  of  Veterinary  Science  at  Clemson 
A.  &  M.  College,  and  Veterinarian  to  the  South 
Carolina  Experiment  Station.  Cloth,  size  6x9,  382 
pages,  77  illustrations ; 5  00 

Synopsis  of  the  Essential  Features  of  the  Work 

1.  Authorized  translation. 

2.  The  only  obstetrical  work  which  is  up  to  date. 

3.  Written  by  Europe's  leading  authority  on  the  subject. 

4.  Written  by  a  man  who  has  practiced  the  art  a  lifetime. 

5.  Written  by  a  man  who,  on  account  of  iiis  eminence  as 
bovine  practitioner  and  teacher  of  obstetrics,  was  selected 
by  Prof.  Dr.  FrOhner  and  Prof.  Dr.  Bayer  (Berlin  and 
Vienna),  to  discuss  bovine  obstetrics  both  practically  and 
scientifically. 

6.  The  only  work  containing  a  thorough  differential  diat:- 
nosis  of  ante  and  post  partum  diseases. 

7.  The  only  work  doing  justice  to  modern  obstetrical 
surgery  and  therapeutics. 

8.  Written  by  a  man  whose  practical  suggestions  revolu- 
tionized the  teaching  of  veterinary  obstetrics  even  in  the 
great  schools  of  Europe. 

9.  The  only  work  dealing  fully  with  the  now  no  longer 
obscure  contagious  and  infectious  diseases  of  calves. 

10.  Absolutely  original  and  no  compilation. 

11.  The  only  work  dealing  fully  with  the  difficult  problem 
of  teaching  obstetrics  in  the  colleges. 

12.  The  only  work  where  the  practical  part  is  not  over- 
shadowed by  theory. 

...  A  veterinarian,  particularly  if  his  location  brings  him  in 
contact  with  obstetrical  practice,  who  makes  any  pretence  toward 
being  scientific  and  in  possession  of  modern  knowledge  upon  this 
subject,  will  not  be  without  this  excellent  work,  as  it  is  really  a  very 
valuable  treatise.— Pro/.  Roscoe  R.  BeU,  in  the  America7i  Veterina/ry 
Revi^iv. 

In  translating  into  English  Professor  De  Bruin's  excellent  text- 
book on  Bovine  Obstetrics,  Dr.  Wyman  has  laid  British  and  American 
veterinary  surgeons  and  students  under  a  debt  of  gratitude.  The 
works  represents  the  happy  medium  between  the  booklets  which  are 
adapted  for  cramming  purposes  by  the  student,  and  the  ponderous 
tomes  which,  although  usefid  to  the  teacher,  are  not  exactly  suited  to 
the  requirements  of  the  everyday  practitioner  .  .  .  We  can  strongly 
recommend  the  work  to  veterinary  students  and  practitioners.-  The 
Journal  of  Comparative  Pathology  and  Therapeutics. 

DOLLAR.  ^."niseasfs  of  Cattle,  Sheep,  Goats  and 
Swine."  By  G.  Moussu  and  Jno.  A.  W.  Dollar, 
M.K.C.  V.S.  "Size  6x9  1-2,  7«5  pages,  329  illustrations 
in  the  text  and  4  full  page  plates 8  75 

—  (t)"A  Hand-book  of  Horse-Shoeing',"  with  introductory 
chapters  on  the  anatomy  and  physiology  of  the 
horse's  foot.  By  Jno.  A.  W.  Dollar,  M.RCV.S., 
with  the  collaboration  of  Albert  Wheatley,  F.R.C.V.S. 
Cloth,  size  6x8  1-2,  433  pages,  406  illustrations  .  .4  75 


DOLLAR  (continued) 

—  (fj "Operative  Technique."     Volume  1  of  "The  Practice  of 

Veterinary  Surgery."  Cloth,  size  6  3-4  x  10,  264  pages, 
272  Illustrations 3  75 

—  *'  General  Surgery.*'     Volume  2  of  "  The  Practice  of  Veter- 

Inary  Surgeiy."     In  preparation. 

—  (t)"  Regional  Veterinary  Surgery."     Volume  3  of  "The 

Practice  of  Veterinary  Surgery."  By  Drs.  Jno.  A. 
W.  Dollar  and  H.  MoUer.  Cloth,  size  6  1-2  x  10  853 
and  xvi  pages,  315  illustrations 6  25 

—  "Caiiot's  Clinical  Veterinary  Medicine  and  Surgery." 

See  "  (Jadiot." 

—  "  Cadlot's  Roaring  in  Horses."    See  "  Cadiot." 

DUN.     "Veterinary  Medicines,  their  Actions  and  Uses." 

By  Finlay  Dun,  V.S.,  late  lecturer  on  Materia 
Medica  and  Dietetics  at  the  Edinburgh  Veterinary 
College,  and  Examiner  in  Chemistry  to  the  Eoyal 
College  of  Veterinary  Surgeons.  Edited  by  James 
Macqueen,  F.E.C.  V.S.  Tenth  revised  English  edition. 
Cloth,  size  6x9 3  75 

FLEMING.  •'  The  Contagious  Diseases  of  Animals."  Their 
influence  on  the  wealth  and  health  of  nations  and  how 
they  are  to  be  combated.  Paper,  size  5x7  1-2, 
30  pages 36 

—  "  Human  and  Animal  Variolae."    A  Study  in  Comparative 

Pathology.      Paper,  size  5  1-2x8  1-2,  61  pages. . .     25 

—  "  Parasites  and  Parasitic  Diseases  of  the  Domesticated 

Animals."  By  L.  G.  Neumann.  Translated  by 
Dr.  Fleming.     See  "  Neumann." 

—  "Operatiye  Veterinary  Surgery."     Vol.    I,    by  Dr.  Geo. 

Fleming,  M.R.O.V.S.  This  valuable  work,  one  of  the 
most  practical  treatises  yet  issued  on  the  subject  in 
the  English  language,  is  devoted  to  the  common  opera- 
tions of  Veterinary  Surgery ;  and  the  concise  descrip- 
tions and  directions  of  the  text  are  illustrated  with 
numerous  wood  engravings.     Cloth,  size  6x9  1-4,  285 

and  xviii  pages,  343  illustrations 2  75 

(•)Vol.  II,  edited  and  passed  through  the  press  by 
W.  Owen  Williams,  F.R.C.V.S.  Cloth,  size  6x9  1-4, 
430  and  xxxvii  pages,  344  illustrations 3  25 

—  "  Roaring     in     Horses."         By    Dr.     George      Fleming, 

F.R  C.V.S.  Its  history,  nature,  causes,  prevention 
and  treatment.  Cloth,  size  5  1-2x8  3-4,  160  pages,  21 
engravings,  1  colored  plate 1  50 

—  "Veterinary  Obstetrics."    Including  the  Accidents  and  Dis- 

eases incident  to  Pregnancy,  Parturition,  and  the  Early 
Age  in  Domesticated  Animals.  By  Geo.  Fleming. 
F.R.C.V.S.     Cloth,  size  6x8  3-4,  758  pages,  illu8.6  25 


GOTTHIEL.     r)"A    Manual    of    General    Histology. 

By  Wm.  S.  Gottheil,  M.D.,  Professor  of  Pathology  in 
the  American  Veterinary  College,  New  York;  etc.,  etc. 
Histology  is  the  basis  of  the  physician's  art,  as 
Anatomy  is  the  foundation  of  the  surgeon's  science. 
Only  by  knowing  the  processes  of  life  can  we  under- 
stand the  changes  of  disease  and  the  action  of 
remedies ;  as  the  architect  must  know  his  building 
materials,  so  must  the  practitioner  of  medicine  know 
the  intimate  structure  of  the  body.  To  present  this 
knowledge  in  an  accessible  and  simple  form  has 
been  the  author's  task.  Second  edition  revised. 
Cloth,  size  5  1-2  x  8,  152  pages,  G8  illustrations. .  .1  00 

GRESSWELL.    "  The  Bovine  Prescriber."    For  the  use 

of  Veterinarians  and  Veterinary  Students.  Second 
edition  revised  and  enlarged,  by  James  B.  and  Albert 
Gresswell,  M.R.C.V.S.  Cloth,  size,  5x7  1-2,  102 
pages 75 

—  "Tlie  Equine  Hospital  Prescriber."    For  the  use  of  Veter- 

inary Practitioners  and  Students.  Third  edition  re- 
vised and  enlarged,  by  Drs.  James  B.  and  Albert 
Gresswell,  M.R.C.V.S.  Cloth,  size  5x7  1-2,  165 
pages 76 

—  "  Diseases  and  Disorders  of  tlie  Horse."     A  Treatise  on 

Equine  Medicine  and  Surgery,  being  a  contribution  to 
the  science  of  comparative  pathology.  By  Albert, 
Jas.  B.  and  Geo.  Gresswell.  Cloth,  size  5  3-4  x  8  3-4, 
227  pages,  illustrated 1  75 

—  Manual  of  "The  Theory  and  Practice  of  Equine  Medicine." 

By  James  B.  Gresswell,  F.R.C.V.S.,  and  Albert 
Gresswell,  M.R.C.V.S.  Second  edition  revised. 
Cloth,  size  5  1-4  x  7  1-2,  539  pages 2  75 

—  (t)  "Veterinary  Pliarniacopaeia  and  Manual  of  Comparative 

Tlierapy."  By  George  and  Charles  Gresswell,  with 
descriptions  and  physiological  actions  of  medicines, 
by  Albert  Gresswell.  Second  edition  revised  and 
enlarged.     Cloth,  6x8  3-4,  457  pages 3  60 

HASSLOCH.  "  A  Compend  of  Veterinary  Materia  Medica 
and  Therapeutics."  By  A.  C.  Hassloch,  V.S., 
Lecturer  on  Materia  Medica  and  Therapeutics,  and 
Professor  of  Veterinary  Dentistry  at  the  New  York 
College  of  Veterlnarv  Surgeons  and  School  of  Compa- 
rative Medicine,  N.  Y.  Cloth,  size  5  1-4x7  1-2,  225 
pages 150 

HEATLEV.  "  The  Stock  Owner's  Guide."  A  handy  Medi- 
cal Treatise  for  every  man  who  owns  an  ox  or  cow. 
Bv  George  S.  Heatley,  M.R.C.V.S.  Cloth,  size 
5  1-4x8,  172  pages 1  26 


HILL.  (t)"The  Diseases  of  the  Cat."  By  J.  Woodroffe 
Hill,  F.K.C.V.S.  Cloth,  size  5  1-4x7  1-2,  123  pages, 
illustrated 1  25 

Written  from  the  experience  of  many  years'  prac- 
tice and  close  pathological  research  into  the  maladies 
to  which  our  domesticated  feline  friends  are  liable— a 
subject  which  it  must  be  admitted  has  not  found  the 
prominence  in  veterinary  literature  to  which  it  is 
undoubtedly  entitled. 

—  "The   Management   and   Diseases    of  the   Dog"    By  J. 

Woodroffe  Hill,  F.R.C.V.S.  Cloth,  size  5x7  1-2, 
extra  fully  illustrated. 

HINEBAUCH,    "Veterinary  Dental  Surgery."     By  T.  D. 

Hinebauch,  M.S.V.S.  For  the  use  of  Students,  Prac- 
titioners and  Stockmen.  Cloth,  size  5  1-4  x  8,  256 
pages,  illustrated 2  uu 

HO  ARE.  n"A  Manual  of  Teterinary  Therapeutics  and 
Pharmacology."  By  E.  Wallis  Hoare.  F.R.C.V.S. 
Cloth,  size  5  1-4x7  1-4,  xxvi  plus  786  pages 4  75 

HOBDAY,  (t)"  The  Castration  of  Cryptorchid  Horses  and 
the    Oyariotoniy    of    Tronblesome    Mares."     By 

Frederick  T.  G.  Hobday,  F.R.C.V.S.  Cloth,  size 
6  3-4  X  8  3-4,  116  pages,  34  illustrations 1  76 

HUKTINO.  (f)  The  Art  of  Horse-shoeing.  A  manual 
for  Horseshoers.  By  William  Hunting,  F.R.C.V.S., 
ex-President  of  the  Royal  College  of  Veterinary  Sur- 
geons. One  of  the  most  up-to-date,  concise  books  of 
its  kind  in  the  English  language.  Cloth,  size  6x9  1-4. 
126  pages,  96  illustrations 1  00 

JENKINS.  (*)"  Anatomical  and  Physiological  Model  of 
the  Cow."  Half  life  size.  Composed  of  superposed 
plates,  colored  to  nature,  showing  internal  organs, 
muscles,  skeleton,  etc.,  mounted  on  strong  boards, 
with  explanatory  text.  Size  of  Model  opened, 
10  ft.  X  3  ft.,  closed  3  ft.  x  li  ft 12  00 

—  ^'Anatomical  and  Physiological   Model    of  the   Horse." 

Half  life  size.     Size  of  Model  38  x  41  in 12  dO 

These  models  may  also  be  obtained  in  smaller 
sizes  together  with  Models  of  the  Dog,  Sheep  and 
Pig. 

JONES.     n**The   Surgical   Anatomy    of  the    Horse." 

By  Jno.  T.  Share  Jones,  M.R  C.V.S.  Part  I.  To  be 
completed  in  four  parts.  Each  part — paper,  $4.25 ; 
cloth,  $5.00.  Subscriptions  for  the  four  parts,  pay- 
able in  advance,  paper,  $15.00;  cloth,  $17.50. 


ROBERT.  "Practical  Toxicology  for  Physicians  and 
Students  "  By  Professor  Dr.  Rudolph  Kobert, 
Medical  Director  of  Dr.  Brehmer's  Sanitarium  for 
Pulmonary  Diseases  at  Goerbersderf  in  Silesia  (Prus- 
sia), late  Director  of  the  Pharmacological  Institute, 
Dorpat,  Kussia  Translated  and  edited  by  L.  H. 
Friedburg,  Ph.D.  Authorized  Edition.  Practical 
knowledge  by  means  of  tables  which  occupy  little 
space,  but  show  at  a  glance  similarities  and  differ- 
ences between  poisons  of  the  same  group.  Also  rules 
for  the  Spelling  and  Pronunciation  of  Chemical  Terms, 
as  adopted  by  the  Amerif^an  Ast-ociation  for  the  Ad- 
vancement of  Science.     Cloth,  6  1-2  x  10,  201  pp.. 2  60 

KOCH.  "Etiology  of  Tuberculosis."  By  Dr.  R.  Koch. 
Translated  by  T.  Saure.  Cloth,  size  6x9  1-4,  97 
pages , 1  00 

LAMBBRT.       "The      Germ     Theory       of      Disease." 

Bearing  upon  the  health  and  welfare  of  man  and  the 
domesticated  animals.  By  James  Lambert,  F.R.C.V.S. 
Paper,  size  5  I-*  x  8  1-4,  26  pages,  illustrated 25 

LAW.  "Farmers'  Veterinary  Adviser."  A  Guide  to  the 
Prevention  and  Treatment  of  Disease  in  Domestic 
Animals.  By  Prof.  James  Law.  Cloth,  size 
5  1-4x7  1-2,  illustrated 3  00 

LIAUTARD.  (f)" Animal  Castration."  A  concise  and 
practical  Treatise  on  the  Castration  of  the  Domestic 
Animals.  The  only  work  on  the  subject  in  the 
English  language.  By  Alexander  Liautard,  M.D.,V.S. 
Having  a  fine  portrait  of  the  author.  Tenth  edition 
revised  and  enlarged.  Cloth,  size  6  1-4x7  1-2,  165 
pages,  45  illustrations 2  00 

.  .  .  The  most  complete  and  comprehensive  work  on  the 
subject  in  English  veterinary  literature.— J.merican  Agri- 
culturist. 

—  *'Cadiot's  Exercises  in  Equine  Surgery."     Translated  by 

Prof'.  Bitting  and  edited  by  Dr.  Liautard. 
See  "  Cadiot." 

—  "  A  Treatise  on  Surgical   Therapeutics  of  the  Domestic 

Animals."  By  Prof.  Dr.  P.  J.  Cadiot  and  J.  Almy. 
Translated  by  JProf.  Liautard.     See  "  Cadiot." 

—  *'  How  to  TeU   the   Age   of  the  Domestic  Animal."    By 

Dr.  A.  Liautard,  M.D.,  V.S.  Standard  work  upon 
this  subject,  concise,  helpful  and  containing  many 
illustrations.  Cloth,  size  5x7  1-2,  35  pages,  42 
illustrations , 50 

—  "Lameness  of  Horses  and  Diseases  of  the    Locomotory 

Apparatus."  By  A.  Liautard,  M.D., V.S.  This  work 
is  the  result  of  Dr.  Liautard's  many  years  of  experi- 
ence.   Cloth,  size  5  1-4x7  1-2,  314  pages .  .2  60 


LIAUTARD  (continued). 

—  (•)**  Manual   of  Operative  Veterinary  Surgery  "     By   A. 

Liiautard,  M.D.,  V.M.  Engaged  for  years  in  the  work 
of  teaching  this  special  department  of  veterinary 
medicine,  and  having  abundant  opportunities  of 
realizing  the  difficulties  which  the  student  who 
earnestly  strives  to  peifect  himself  in  his  calling  is 
obliged  to  encounter,  the  author  formed  the  deter- 
mination to  facilitate  his  acquisition  of  knowledge, 
and  began  the  accumulation  of  material  by  the  com- 
pilation of  data  and  arrangement  of  memorandum, 
with  the  recorded  notes  of  his  own  experience,  the 
fruit  of  a  long  and  extended  practice  and  a  careful 
study  of  the  various  authorities  who  have  illustrated 
and  organized  veterinary  literature.  Revised  edition, 
with  complete  index.  Cloth,  size  6  1-4  x  9,  xxx  and  803 
pages,  563  illustrations 5  00 

—  "Pellerin's    Median    Neurotomy    in    the    Treatment   of 

Chronic  Tendinitis  and  Periostosis  of  the  Fetlock." 

Translated  by  Dr.  A.  Liautard.     See  "  Pellerin." 

—  "Vade  Mecum    of  Equine   Anatomy/*     By  A.  Liautard, 

M.D.V.S.  For  the  use  of  advanced  stuients  and 
veterinary  surgeons.  Third  edition.  Cloth,  size 
5x7  1-2,  30  pages  and  10  full  page  illustrations  of 
the  arteries 2  00 

—  Zundel's  "The  Horse's  Foot  and  Its  Diseases." 

See  "  Zundel." 

LONG.  "  Booli  of  the  Pig-."  Its  selection.  Breeding, 
Feeding  andManagement.    Cloth 4.00 

LOWE.  (t)"  Breeding:  Racehorses  by  the  Figure 
System."  Compiled  by  the  late  C.  Bruce  Lowe. 
Edited  by  William  Allison,  "  The  Special  Commis- 
sioner," London  Sportsman,  Hon.  Secretary  Sporting 
League,  and  Manager  of  the  International  Horse 
Agency  and  Exchange.  With  numerous  fine  illustra- 
tions of  celebrated  horses.  Cloth,  size  8  x  10,  262 
pages • 7  50 

LUDLOW.  "Science  in  the  Stable";  or  How  a  Horse 
can  be  Kept  in  Perfect  Hnaith  and  be  Used  Without 
Shoes,  in  Harness  or  under  the  Saddle.  With  the 
Reason  Why.  Second  Edition.  By  Jacob  R.  Ludlow, 
M.D.  Late  Staff  Surgeon,  U.  S.  Army.  Paper,  size 
4  1-2x5  3-4,  166  pages 50 

LUPTON.  "Horses:  Sound  and  Unsound,"  with 
Law  relating  to  Sales  and  Warranty.  By  J  Irvine 
Lupton.  F.R.C.VS.  Cloth,  size  6  3-4  x  7  1-2,  2]7 
pages,  28  illustrations 1  25 


M'FADTEAN.  (f)  "  Anatomy  of  the  Horse."  Second 
edition  completely  revised.  A  Dl.'ssectlon  Guide. 
By  John   M'Fadyean,   M.B.,  B.8c.,  F.R.S.E.      Cloth, 

size  (5x834,  388  pages,  illustrated S  50 

This  book  is  intended  for  Veterinary  students,  and 
offers  to  them  in  its  48  full-page  colored  plates, 
54  illustrations  and  excellent  text,  a  valuable  and 
practical  aid  in  the  study  of  Veterinary  Anatomy, 
especially  in  the  dissecting  room. 

—  "  Comparatiye  Anatomy  of  the   Domesticated  Animals." 

By  J.  M'Fadyean.     Profusely  illustrated,  and  to  be 

issued  in  two  parts. 

Part  I— Osteology,   ready.      Size  5  1-2x8  1-2,   166 

pages,  132  illustratione.     Paper,  2  50;  cloth 2  75 

(Part  II  in  preparation.) 

MAGNEB.    ♦*  Standard   Horse    and   Stocli   Book."      By 

D.  Magner.  Comprising  over  1,000  pages,  illustrated 
with  1756  engravings.     Leather  binding 6  (0 

MILLS.    "How  to    Keep    a    Dog    in     the    City."    By 

Wesley  Mills,  M.D.,  D.V.S.  It  tells  how  to  choose, 
managej  house,  feed,  educate  the  pup,  how  to  keep  him 
clean  and  teach  him  cleanliness.  Paper,  size  5x7 1-2, 
41)  pages 25 

MOHLEM.  "  Handbook  of  Meat  Inspection."  By  Eobert 
Ostertag,  M.D.  Translated  by  Earley  Vernon 
Wilcox,  A.M.,  Ph.D.  With  an  introduction  by 
John  R.  Mohler,  V.M.D.,  A  M.     See  "  Ostertag." 

MOLLER  —  DOLLAR.       (f)  "  Regional       Veterinary 

Surgery."  See  ''  Dollar." 
MOSSELMAJS-LIEXAUX.  *' Manual  of  Veterinary 
Microbiology,"  By  Professors  Mosselman  and 
Lienaux,  Nat.  Veterinary  College,  Cureghem,  Belgium. 
Translated  and  edited  by  R.  R.  Dinwiddle,  Professor 
of  Veterinary  Science,  College  of  Agriculture,  Arkansas 
State  University.  Cloth,  size  6  12x8,  342  pages, 
illustrated 2  00 

MOUSSXJ.  (•)"  Diseases  of  Cattle,  Sheep,  Goats  and 
Swine."     See  "  Dollar." 

NEUMANN.  (*)"A  Treatise  on  Parasites  and  Parasitic 
Diseases   of  the  Domesticated  Animals"     A  work 

to  which  the  students  of  human  or  veterinary  medi- 
cine, the  sanitarian,  agriculturist  or  breeder  or  rearer 
of  animals,  may  refer  for  full  information  regarding 
the  external  and  internal  Parasites — vegetable  and 
animal — which  attack  various  species  of  Domestic 
Animals.  A  Treatise  by  L.  G.  Neumann,  Professor 
at  the  National  Veterinary  School  of  Toulouse. 
Translated  and  edited  by  Geo.  Fleming,  C.B.,  LL.D.. 
F.R  O.V.S.  Second  edition,  revised  and  edited  by 
James  Macqueen,  F.R.C.V.S.,  Professor  at  the  Royal 
Veterinary  College,  London.  Cloth,  size  6  34  x  10, 
xvi  -t-  698  pages,  365  illustrations 6  76 


NOCARD.  "  The  Animal  Tuberculoses,  and  their  Relation 
to  Human  Tuberculosis."  By  Ed.  Nocard,  Prof,  of  the 
Alfort  Velerinary  College.  Translated  by  H.  Scurfield, 
M.D.  Ed.,Ph.  Camb.  Cloth,  5  x  7  1-2, 143  pages..!  OU 
Perhaps  the  chief  interest  to  doctors  of  human 
medicine  in  Professor  Nocard's  book  lies  in  the 
demonstration  of  the  small  part  played  by  heredity, 
and  the  great  part  played  by  contagion  in  the  propa- 
gation of  bovine  tuberculosis. 

NUyK.  (*)"  Veterinary  Toxicology."  By  Joshua  A.  Nunn, 
F.R.C.V.S.  The  study  of  toxicology  is  intimately 
blended  with  other  biological  sciences,  particularly 
physiology  and  chemistry,  both  of  which  it  on  many 
occasions  overlaps.  A  carefully  arranged  and  com- 
plete index  is  given  in  the  front  of  the  volume. 
Cloth,  size  Gx83-4,  vii  +  191  pages 1  75 

OSTEBTAG,    C)  "  Handbook  of  Meat  Inspection."      By 

Robert  Ostertag,  M  D.  Authorized  Translation  by 
Earley  Vernon  Wilcox,  A.M.,  Ph.D.  With  an  intro- 
duction by  John  R.  Mohler.  V.M.D.,  A.M.  The  work 
is  exhaustive  and  authorative  and  has  at  once  become 
the  standard  authority  upon  the  subject  Second 
edition,  revised.  Cloth,  size  6  3-4x9  3-4,  920  pages, 
260  illustrations  and  1  colored  plate. 7  60 

PAL  LIN,    (•)  **  A  Treatise  on  Epizootic  Lymphangitis."  By 

Capt,  W.  A.  Pallin,  F.R.C.V.S.  In  this  work  the 
author  has  endeavored  to  combine  his  own  experience 
with  that  of  other  writers  and  so  attempts  to  give  a 
clear  and  complete  account  of  a  subject  about  which 
there  is  little  at  present  in  English  veterinary  litera- 
ture. Cloth,  size  5  3-4x8  1-2,  90  pages,  with  17  fine 
full  page  illustrations 1  25 

PEGLER.    "  Goat  Keeping  for  Amateurs."     Paper,  5x7i, 
77  pages,  illustrated 50 

PELLERIN.  "Median  Neurotomy  in  the  Treatment 
of  Chronic  Tendinitis  and  Periostosis  of  the  Fetlock." 

By  C.  Pellerin,  late  repetitor  of  Clinic  and  Surgery  to 
the  Alfort  Veterinary  School.  Translated,  with  Addi- 
tional Facts  Relating  to  It,  by  Prof.  A.  Liautard,  M.D., 
V.M.  Having  rendered  good  results  when  performed 
by  himself,  the  author  believes  the  operation,  which 
consists  in  dividing  the  cubito-plantar  nerve  and  in 
•xcising  a  portion  of  the  perlpherical  end,  the  means 
of  improving  the  conditions,  and  consequently  the 
values  of  many  apparently  doomed  animals.  Agricul- 
ture in  particular  will  be  benefited. 

The  work  is  divided  in'o  two  parts.  The  first  covers 
the  study  of  Median  Neurotomy  itself;  the  second, 
the  exact  relations  of  the  facts  as  observed  by  the 
author.    Boards,  6x9  1-2,  61  pages,  Illustrated . .  1  00 


PETERS.  "  A  Tuberculous  Herd-Test  with  Tuber- 
culln."  By  Auslln  Peters,  M.  K.  C.  V.  S.,  Chief 
Inspector  of  Cattle  for  the  New  Yoik  State  Board  of 
Health  during  the  winter  of  l>592-93.     Pamphlet 25 

REYNOLDS.  "An  Essay  on  the  Breeding-  and  Manage- 
ment of  Draught  Horses."  By  R.  S.  Reynolds, 
M  R.C.  V.S.     Cloth,  size  6  1-2x8  3-4,  104  pages.  .1  40 

ROBERGE.  "The  Foot  of  the  Horse,"  or  Lameness 
and  all  Diseases  of  the  Feel  traced  to  an  Unbalanced 
Foot  Bone,  prevented  or  cured  by  balancing  the  foot. 
By  David  Roberge.  Cloth,  size  6x9  1-4,  308  pages, 
H I  ustrated 6  00 

SESSIONS.  (*)*'  Cattle  Tuberculosis,"  a  Practical  Guide  to 
the  Agriculturist  and  Inspector.  By  Harold  Sessions, 
F.R,C.V.S.,  etc.  Second  edition.  Size  5x7  1-4,  vi  + 
120  pages 1  00 

The  subject  can  be  understood  by  those  who  have 
to  deal  p>irticularly  with  it,  yet  who,  perhaps,  have 
not  had  the  necessary  training  to  appreciate  technical 
phraseology. 

SEWELJj.     "The   Examination  of  Horses   as  to  Sound- 
ness and  .Selection  as  to  Purchase."     By  Edward 
Sewell.  M.  R.C.  V.S      Paper,  size  5  1-2  x  8  1-2,  86  pages, 
illustrated  with  8  plates  in  color 1  60 

It  is  a  great  advantage  to  the  business  man  to 

know  something  of  the  elements  of  law,  and  nobody 
ought  either  to  buy  or  own  a  horse  who  does  not  know 
something  about  the  animal.  That  som^'thiug  this  book 
gives,  and  gives  in  a  thoroughly  excellent  way 

SMITH.    (•)"  A  Manual  of  Veterinary    Physiology."     By 

Vet.  Capt.  F.  Smith,  C.M.S.,  M  K.C.V  S.,  Examiner  in 
Physiology,  Royal  College  of  Veterinary  Surgeons, 
author  of  "A  Manual  of  Veterinary  Hygiene."  A 
completely  revised  and  enlarged  edition  just  pub- 
lished.    Cloth,  6  X  8  3-4,  720  pp,  1U2  illust'ns 4  25 

The  whole  book  has  been  carefully  revised  and 
brought  up  to  date.  All  the  important  advances  of  the 
last  few  years  have  been  embodied.  The  chapter  on 
the  nervous  system  has  been  specially  revised  by  Prof. 
Sherrington,  whose  remarkable  work  on  the  "spinal 
dog"  has  been  introduced.  A  special  point  is  made 
of  the  bearing  of  physiology  on  pathology,  and  the 
utilization  of  physiology  to  the  better  understanding  of 
every-day  practice.  The  book  is  written  by  a  veterin- 
ary surgeon  for  veterinary  practitioners  and  students, 
and  is  the  only  work  in  the  English  language  which 
can  claim  to  be  purely  veterinary. 
—  (*)"  Manual  of  Veterinary  Hygiene."  Third  edition  revised. 
Cloth,  size  5  1-4x7  1-2,  xx  +  1036  pages,  with  255 
illustrations 4  75 

Recognizing  the  rapid  advance  and  extended  field 
of  the  .subject  since  the  previous  issue,  the  author 
has  entirely  re-written  the  work  and  enlarged  its 
scope,  whieh  is  brought  thoroughly  up  to  date.  Con- 
tains over  600  more  pages  than  the  second  edition. 


STRANGEWAY.  (t)"Teterinary  Anatomj."  Edited  by 
1  Vaughan,  F.L.S.,  M  R.C  V.S.  New  edition  revised. 
Cloth,  size  6  l-l  x  9  1-2,  625  pages,  224  illus 5  00 

SUSSDORF.    " Six  Large  Colored  Wall  Diagrams."    By 

Prof.  Sussdoif,  M.D.  (of  Gottiugen).  Text  translated 
by  Prof.  W.  Owen  Williams,  of  the  New  Veterinary 
College,  Edinburgh.     Size,  44  inches  by  30  inches. 

1. — Horse.  4.— Ox. 

2.    Mare.  5.— Boar  and  Sow. 

3.— Cow.                       e.^Dog  and  Bitch. 
The  above  are  printed  in  eight  or  nine  colors. 
Showing  the    position    of   the    viscera  in  the  large 
cavities  of  the  body. 
Price,  unmounted 1  75  each 

"      mounted  on  linen,  with  roller 3  50     " 

THOMPSON,  if)*' Elementary  Lectures  on  Veterinary 
Science."  For  agrlfuliural  siudeuts,  farmers  and 
stock  keepers.  By  Henry  Thompson,  M.R.C.V.S., 
lecturer  on  Veterinary  Science  at  the  Aspatria  Agri- 
cultural College,  England.  It  is  complete  yet  concise 
and  an  up-to-date  book.     Cloth,  397  pp.,  51  illus.. 3  75 

VAN  MATER.  "  A  Text  Book  of  Yeterinary  Oph- 
thalmology."  By  George  G.  Van  Mater,  M.D., 
D.V.S.,  Professor  of  Ophthalmology  in  the  American 
Veterinary  College;  Oculist  and  Auristto  St.  Martha's 
Sanitarium  and  Dispensary ;  Consulting  Eye  and  Ear 
Surgeon  to  the  Twenty-sixth  Ward  Dispensary ;  Eye 
and  Ear  Surgeon,  Brooklyn  Eastern  District  Dispen- 
.sary,  etc.  Illustrated  by  one  chromo  lithograph  plate 
and  71  engravings.      Cloth,  6x9  1-4,  151  pages.. .3  00 

,  .  .  We  intend  to  adopt  this  valuable  work  as  a  text 
book.— E.  J.  Creelij,  D.V.S.,  Dean  of  the  San  Francisco 
Veterinary  College. 

VETERINARY  DIAGRAMS  in  Tabular  Form. 
Size,  28^  in.  x  22  inches.     Price  per  set  of  five. . .  4  00 

Mounted  and  folded  in  case  7  60 

•  Mounted  on  roller  and  varnished 10  00 

No.  1.  "The  External  Form  and  Elementary  Ana- 
tomy oT  the  Horse."  Eight  colored  illustrations — 
1.  External  regions  ;  2.  Skeleton  ;  3.  Muscles  (Superior 
Layer);  4.  Muscles  (Deep  Layer);  5.  Respiratory  Ap- 
paratus; 6.  Digestive  Apparatus ;  7.  Circulatory  Ap- 
paratus ;  8.  Nerve  Apparatus ;  with  description 1  25 

Mounted  on  roller  and  varnished 2  25 

No.  2.  "The  Age  of  Domestic  Animals."  Forty-two 
figures  illustrating  the  structure  of  the  teeth,  indicat- 
ing the  Age  of  the  Horse,  Ox,  Sheep,  and  Dog,  with 

full  description 75 

Mounted  on  roller  and  varnished 2  00 


VETERINARY  DIAGRAMS  rcontlnued). 

No.  3.    "The  Unsoundness  and  Defects  of  the  Horse." 

Fifty  figures  illusstiating — 1.  The  Defects  of  Confor- 
mation ;  2.  Defects  of  Position  ;  3.  Infirmities  or  Signs 
of  Disease ;  4.  Unsoundnesses ;  6.  Defects  of  the  Foot ; 

with  full  description 75 

Mounted  on  roller  and  varnished 2  00 

No.  4.    "The  Shoeing  of  the  Horse,  Mule  and  Ox." 

Fifty  figures  descriptive  tif  the  Anatomy  and  Physio- 
logy of  the  Foot  and  of  Horse-shoeing 76 

Mounted  on  roller  and  varnished 2  00 

No.  5.  "The  Elementary  Anatomy,  Points,  and  But- 
cher's Joints  of  the  Ox."  Ten  colored  illustrations 
—  1.  Skeleton;  2.  Nervous  System;  3.  Digestive 
System  (Right  Side) ;  4.  Respiratory  System  ;  5.  Points 
of  a  Fat  Ox  ;  6.  Muscular  System ;  7.  Vascular  System; 
8.  Digestive  System  (Left  Side) ;  9.  Butcher's  Sections 
of  a  Calf ;   10.  Butcher's  Sections  of  an  Ox ;  with  full 

description 1  25 

Mounted  on  roller  a  nd  varnished 2  25 

If  ALLEY.    "A  Practical  Guide  to  Meat  Inspection."    By 

Thomas  Walley,  M.R.C.V.S.,  late  principal  of  the 
Edinburgh  Royal  (Dick)  Veterinary  College;  Pro- 
fessor of  Veterinary  Medicine  and  Surgery,  etc. 
Fourth  Edition,  thoroughly  revised  and  enlarged 
hy  Stewart  Stockman,  M.R.C.V.S.,  Professor  of 
Pathology,  Lecturer  on  Hygiene  and  Meat  Inspection 
at  Dick  Veterinary  College,  Edinburgh.  Cloth,  size 
5  1-2x8  1-4,  with  45  colored  illus.,  295  pages 3  00 

An  experience  of  over  30  years  in  his  profession 
and  a  long  official  connection  (some  sixteen  years) 
with  Edinburgh  Abattoirs  have  enabled  the  author  to 
gather  a  large  store  of  information  on  the  subject, 
which  he  has  embodied  in  his  book. 

While  Dr.  Stockman  is  indeed  indebted  to  the 
old  for  much  useful  information,  this  up-to- 
date  woik  will  hardly  be  recognized  as  the  old 
"  Walley's  Meat  Inspection." 

fVILCOX.  (*)<*  Handbook  of  Meat  Inspection."  By  Robert 
Ostertag,  M.D.     See  "  Ostertag." 

IFILLIAMS.  "Principles  and  Practice  of  Yeterinarj 
Medicine."  Author's  edition,  entirely  revised  and 
illustrated  with  numerous  plain  and  colored  plates. 
By  W,  Williams,  M.R.C.V.S.  Cloth,  size  5  3-4x8  3-4, 
86S  pages 7  50 

—  *'  Principles    and    Practice    of     Yeterinary     Surgery." 

Author's  edition,    entirely    revised    and    illustrated 
with    numerous    plain    and    colored  plates.     By  W 
Williams,  M.R.C.V.S.      Cloth,  size  6  1-2x9  1-4,  756 
pages 7  50 


THE  MOST  COMPLETE,  PROGRESSIVE  AND 
SCIENTIFIC  BOOK  ON  THE  SUBJECT  IN 
THE  ENGLISH  LANGUAGE 

(•) WINSLOW.  "Veterinary  Materia  Medica  and  Therapeu- 
tics." By  Kenelm  Winslow.  B.A.S.,  M.D.V.,  M.D., 
(Haiv.) ;  formerly  Assistant  Professor  of  Therapeutics 
in  the  Veterinary  School  of  Harvard  University ; 
Fellow  of  tiie  Massachusetts  Medical  Societj' ;  Surgeon 
to  the  Newton  Hospital,  etc. 

Fifth  Edition,  Revised  and  Enlarged 

Cloth,  size  6  1-4  x  9  1-i,  x  +  804 pages ...6  00 

Til  accordance  with  the  hitherto  expressed  desire  of  the  author  and 
publishers  to  keep  this  -nork  at  its  highest  point  of  efficiency,  it  has 
been  deemed  incumbent  upon  them  to  again  present  a  new  and  revised 
edition— the  fourth  edition  of  19fK)  being  exhausted. 

In  tlie  present  revision  the  most  notable  feature  is  the  substitution 
of  a  section  on  Condensed  Treatment  of  Diseases  of  the  Domestic 
Animals  for  the  Index  of  Diseases  and  Remedial  Measui'es,  at  the  end 
of  the  book.  In  the  preparation  of  this  matter,  very  considerable  time 
and  pains  have  been  taken  to  render  this  section  a  reflection  and  epi- 
tome of  all  that  is  most  modern  and  progressive  in  veterinary  thera- 
peutics. 

Special  indications  for  treatment,  including  drugs  and  therapeutic 
agents  other  than  drugs,  in  the  difl:erent  phases  and  stages  of  all  the 
important  diseases  of  tlie  domestic  animals  are  to  be  found.  These  dis- 
eases embrace  not  only  medical  and  sui'gical  disorders,  but  those  of  the 
EYK,  SKIN  and  EAR.  If  the  attempt  has  been  in  any  degree  successful, 
this  ne^v  edition  to  the  book  should  prove  one  of  its  most  valuable 
features  both  to  practitioners  and  students. 

Moreover,  many  changes  have  been  made  in  the  text  in  consonance 
with  recent  advances  in  our  knowledge  of  the  action  of  drugs. 


WYMAN.    (*)" Bovine  Obstetrics."     By  M.  G.  De  Bruin. 
Translated  by    W.  E.  A.  Wyman,  M.D.V.,V.S. 
See  also  '' De  Bruin." 

—  (*)"Cateeliism  of  the  Principles  of  Veterinary  Surgery." 

Bv  W.  E.  A.  Wyman,  M.D.V..V.S.      Cloth,  size  6x9, 
321  pages 3  50 

Concerning;  ibi^  new  work  attention  is  called  to  the 
follo^ving  points: 

1.— It  discusses  the  subject  upon  the  basis  of  veterinary  investigations. 

2.— It  does  away  with  works  on  human  pathology,  histology,  etc. 

3.— It  explains  each  question  thoroughly  both  from  a  scientific  as  well 

as  a  practical  point  of  view. 
4.— It  is  writen  by  one  knowing  the  needs  of  the  student. 
5.— It  deals  exhaustively  with  a  chapter  on  tumors,  heretofore  utterly 

neglected  in  veterinary  pathology. 
6.— The  only  work  in  Englisli  specializing  the  subject. 
7.— The  only  work  thoroughly  taking  into  consideration  American  as 

well  as  European  investigations. 
8.— Offering  practical  hints  which  have  not  appeared  in  print,  the 

result  of  large  city  and  country  practice. 


WYMAN  (Continued) 


—  (t/'The    Clinical  Diagnosis  of  Lameness  in  the  Horse." 

Hy  W.  E.  A.  Wynian,  D.V.S.,  formerly  Professor  of 
Veterinary  ycience,  Clemson  A.  &  M.  College,  and 
Veterinarian  to  the  South  Carolina  Experiment 
Station.     Cloth,  size  6x9  1-2,  182  pp.,  32  illus. . .  .2  50 


—  (t)*'Tibio-peroneal   Neurectomy  for  tlie  Relief  of  Spavin 
Lameness."    By  W.  E.  A.  Wyman,  M.D.V.,  V.S. 
Boards,  size  6  x  9,  30  pages,  illustrated 50 

Anyone  wanting  to  perform  this  operation  should  procure 
this  little  treatise ;  he  will  find  it  of  considerable  help.— The 
Veterinary  Journal. 


ZUILL,  "Typhoid  Feverj  or  Contagions  Inflaenia 
in  the  Horse."  By  Prof.  W.  L.  Zuill,  M.D„D.V.S. 
Pamphlet,  size  6x9  1-4,  29  pages 25 


ZUNDEL.      "The   Horse's   Foot   and   Its   Diseases."    By 

A.  Zundel,  Principal  Veterinarian  of  Alsace  Lorraine. 
Translated  by  Dr.  A.  Liautard,  V.S.  Cloth,  size 
5x7  3-4,  248  pages,  illustrated 2  00 


Any  book  sent  prepaid  for  the  price 

WILLIAM  R.  JENKINS  CO. 

851  and  853  Sixth  Avenue,  NEW  YORK. 


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